Tag Archives: battery pack

A matter of voltage

Posted 20 March 2018

I think it is important that Wall-E2 have an accurate measurement of battery voltage, so that he knows when he should be looking for his next charging fix, and more importantly, so he can stop and yell for help if the battery voltage gets dangerously low.  In addition, I would like to monitor the battery voltage during charge, so Wall-E2 can report & display charging progress to any interested humans (like me). ;-).

From what I’ve read, it appears a LiPo cell can go down to about 3V without damage, or 6V for my 2-cell stack.  So, my operating voltage range is from full charge (approx 8.4V) to empty (6.0V).  My first cut at battery voltage monitoring was a simple 1/3 – 2/3 resistive voltage divider tied to an analog input; simply measure the voltage, multiply by 3, and voila – battery voltage!

Only it didn’t work that way; once the battery voltage dropped below about 7V, the drop across the Arduino Mega’s voltage regulator wasn’t sufficient to maintain regulated 5V, so the Mega’s bus voltage began to drop.  At Vbatt = 6V, the Mega was still running OK, but the bus voltage was down to 4V, and the A/D reference was no longer what it should be – rats!

In addition, once I started looking at this issue, I realized I was throwing away most of the A/D dynamic range with the divider idea.  with a 5V A/D reference and a 1/3 divider, the A/D input voltage only varies between 2.0 and 2.8V for an input range between 6 and 8.4V.  In other words, I’m only using 0.8V of the available 5V range or about 16%.

So, I thought that maybe I should implement a level shifter, so the sensed voltage varies from 0 to 2.4 as the battery voltage varies from 6 to 8.4 – and then use the Mega’s internal 2.56V reference for the A/D operation.  This would mean an immediate increase in dynamic range usage from 16% to almost 94%, and would increase resolution from about 15mV/count to about 2.5mv/count.  To do the level shifting, I’ll need a 6V zener, such as the 1N5233B, available from Mouser for a few pennies each.

One last voltage issue to be addressed is the problem with the Mega’s onboard regulator dropping out for battery voltages between 7 and 6V – this is almost half of the available voltage range.  Eventually I decided to address this problem by replacing (or rather, bypassing) the onboard regulator with a low dropout (LDO) regulator such as the LF50CV-DG , available from Mouser for less than $1 each.  The LF50CV-DG can maintain 5V output down to well below my 6V battery voltage cutoff limit, so it is a good match.

23 March 2018 Update:

I just received the LF50CV-DG regulator and 1N5233B parts from Mouser, so I’m in the process of installing them onto Wall-E2.  The regulator will take the place of the MOSFET low-drop diode I installed on the Pololu Wixel Shield some time ago as part of my old PB1000C-based charging subsystem, and is now no longer needed.  The following photos show the installation:

Wixel shield showing MOSFET diode to be replaced by LDO 5V regulator

LF50CV-DG LDO 5V regulator and 1N5233B 6V Zener diode installed on Wixel shield

Rear of Wixel shield showing regulator output connection to +5V bus

25 March Update:

While testing the above arrangement, I managed to somehow kill my Mega 2560 SBC (I think my old power supply did it in, but I’m not sure).  So, in the process of recovering from this mess, I also decided to replace my old Wixel shield for the latest version (v1.1) with updated level-shifting circuits and carry-throughs for the added pins on the UNO R3, Mega, and cousins.  The new layout is shown below

Updated Wixel shield board with LDO 5V regulator and level-shifter circuit installed

Once I got everything back together, I started over with testing the LDO 5V regulator and level shifter performance, and ran into another problem.  The original idea was to use the 1N5233B 6V zener to level shift 6-8.4V to 0-2.4V so the range would fit into the range obtainable using the Mega’s internal 2.56V ADC reference.  This worked almost perfectly, but the combination of a slightly lower Vz (5.84 vs 6.00V) and a slightly lower Vref (2.42 vs 2.56V) caused the ADC to hit full scale (1023 counts) at about 8.26V (2.44Vref + 5.84Vz = 8.26V).   Most unfortunate, as I really needed to accurately measure Vbatt to at least 8.4V, nominal end-of-charge voltage for a 2-cell LiPo stack.

So, I needed to expand the measurable voltage range at least a little bit on the top end.  With the installation of the LF50CV LDO 5V regulator, I could now do that by reverting to the internal 5V reference, as the LDO easily maintains 5V output all the way down to 6V, the cutoff voltage for my battery stack.  But, this wastes half the available ADC range, as the ADC input voltage for Vbatt = 8.4 is only 8.4-5.84 = 2.56V.  So, after some more Googling through Arduino-space, I realized I could tie the Mega’s AREF pin to the Mega’s 3.3V output line and use ‘analogReference(EXTERNAL)’ to obtain an ADC range from 0-3.3V, corresponding to a Vbatt range of 0 to (5.84+3.3)= 9.14V – perfect!

After making this change I ran some measurements to verify the input range and accuracy, as displayed in the following Excel plot

Measured vs Calculated Vbatt, with raw ADC values

As can be seen in the above plot, the measured and calculated voltage plots are almost perfectly overlaid, and well within the accuracy requirements for effective battery management.

Summary:

As usual, what started out as a simple plan (in this case, to accurately measure the battery voltage) rapidly metastasized into a full-blown hardware and software project, complete with howls of anguish and gnashing of teeth.  The first idea was to use a simple 1/3 resistive voltage divider input to a ADC port referenced to 5V. This worked OK, but failed at battery voltages below 7V because the Mega’s onboard voltage regulator requires an approximately 2V input-output offset.  Since I needed to measure Vbatt down to 6V, this was never going to work.  In addition, the available measurement accuracy sucked because the 2.5V range of interest was being compressed into 2.5/3 = 0.833V, and with a 5V reference I was using less than 20% of the available ADC counts. The next idea was to replace the onboard regulator with the LF50CV LDO regulator, and use a 6V zener to level shift the range of interest to under 2.56V so that the Mega’s internal 2.56V reference could be used.  This almost worked, but I ran out of ADC counts before I ran out of battery voltage – oops.  The third (and last, I hope) idea was to change the ADC reference from internal 2.56V to external 3.3V using the AREF pin tied to the Mega’s 3.3V regulated output.  This allowed the top voltage to go to a little over 9V, just about perfect for this application.

 

 

Stay tuned,

Frank

 

 

New TP5100-based Battery Pack for Wall-E2

Posted 13 March 2018

In a recent post, I described my study of the widely available and dirt-cheap TP5100 1/2-cell LiPo battery charger as a possible replacement for my current Adafruit PB1000C-based battery charger.  Based on the results of this study, it was clear the TP5100-based system was superior in all respects to my home-brew system:

  • Twice the charge current (2A vs 1A) means significantly shortened charge times
  • Much smaller and simpler
  • Charger current path independent of load path – much lower IR drop
  • Battery always connected to the system, so no requirement for ultra-low-drop MOSFET diode
  • Much simpler software – no requirement to monitor status of two separate chargers
  • No electromechanical relay to screw up.

I constructed a small charger module using some perfboard and a couple of 2-place screw terminals, as shown below (with the previous module shown for size comparison).

New TP5100-based charger module, with previous Adafruit PB1000C-based module below for size comparison. The orange box contains 4 Panasonic 18650 cells.  Note the separate charge & load circuits

The following figures show the old and new schematics:

Old battery pack schematic

New battery pack schematic

Now that the load current doesn’t have to go through the charging module, I was able to replace all main battery wiring with #20 wire for lower IR drops, as shown below

Power wiring replaced with #20 wiring, and 2-pin Deans connectors

#20 wiring to main battery buss. Note in-line safety disconnect

 

The change to the new battery pack also considerably simplified the system hardware and software.  The changes to the system schematic are shown below:

Old system schematic. Note the ultra-low-drop MOSFET diode required to keep Arduino Mega alive during charge. and the number of pins required for charge monitoring.

New system schematic. No requirement for diode, as full battery voltage is available at all times. Also, only two pins are required for charge monitoring

The operating system software has also been simplified.  Now, instead of monitoring both cell voltages and four different status lines, only two lines have to be monitored.  Also, there is now no requirement to correctly sequence the ‘Charger Connect’ and ‘Coil Enable signals in order to accomplish correct charging station connect-disconnect behavior.  Now the system simply shuts off the motors when the robot connects to the station, and turns them back on again to disconnect.  As an added benefit, the six charge status LEDs have been repurposed to show a crude approximation (based on battery voltage only for the moment) of charge status.

All these changes have caused one minor hiccup in the implementation of the charging station; the new charging voltage is +12V vs +5V as before.  As you may recall, the charging station implements a square-wave modulated IR signal, and this signal is produced by a Teensy 3.2 and some associated circuitry, all of which expect +5V.  This will require either a dual-output supply, or the addition of an on-board 12-to-5V regulator. This is still up in the air, but I suspect it will land on a simple 3-pin regulator.

So far, all the hardware changes (except for the charging station changes) have been accomplished, but the software changes have yet to be implemented and tested. Stay tuned!

Frank

 

TP5100 2-cell LiPo Charger Module Study

Posted 24 February 2018

I have been working on Wall-E2, my autonomous wall-following robot, for almost three years now, and it seems like I have been struggling with the battery and charger arrangement for that entire time.  I started out with 4 AA batteries, but quickly moved on to a pair of Sparkfun 2000mAh ‘flat-pack’ cells with Sparkfun chargers, with a relay to switch the batteries from series (RUN) to parallel (CHG) wiring.  This worked, but not very well.  The flat-pack batteries weren’t a good match for motor control, and I kept burning up charger modules as well.  After struggling with this through several iterations, I finally abandoned it entirely in favor of a 7.4V 20C LiPo RC battery and an external charger. This worked much better, but forced me to manually disconnect the battery from the robot and charge it externally – not at all what I wanted.  Later on I made another run at the 2-cell series/parallel switching strategy for charging, this time using Adafruit Powerboost 1000C charge modules, each capable of 1A charge rates. Again this worked (actually quite well), but I recently discovered that it has a fatal flaw – this design imposes significant IR drops on the way from the battery terminals to the motors.

So, I have once again been searching for a solution to the battery/charger problem.  While wandering through the Googleverse the other day, I ran across a mention of the 1/2-cell TP5100-based charger module (about 9:50 from start), available for next to nothing on eBay.

Unfortunately, the available technical information on this module is also next to nothing, and what does exist is all in Chinese.  Still, this module has the potential for vastly simplifying my charger setup, so I thought it was worth the effort to perform a thorough study.

In a previous post, I described an Arduino controlled charge/discharge test setup for testing operation of my 2-cell parallel/series switched setup, so I decided to modify it for evaluating the TP5100 module, as shown below

View of TP5100 module showing RUN & CHG indicator connections

Charger test setup, in discharge mode (note 1.1A discharge current)

Charger test setup, in charge mode (note 1.8A charge current)

TP5100 Module Test Circuit

Using this setup, I was able to cycle the battery between a 7.5Ω load and the TP5100 charge module.  In order to keep the cycle times down to a dull roar, I set the software to switch to charge when the battery voltage dropped below 7.5V, resulting in the plots shown below.

In this case, the discharge current was about 1.1A, and the observed charge current was about 1.8A. The TP5100 modules seems to work as advertised – with a 12V 5A power supply and a partially charged battery, it successfully charged my 2-cell LiPo pack terminated the charge at about 8.4V (I’m not sure if it is terminating based on current or voltage).

Over the next couple of days, I performed three complete charge/discharge cycles using this same setup.  Discharge was terminated at 6V, and charge was terminated when the TP5100 ‘complete’ output changed from open-circuit to active-low. As can be seen in these charts, performance was very consistent – almost 6 hours run time into a 7.5Ω load, and about 4 hours for a complete recharge.

 

So here’s what I know now about the TP5100 module

  • When the ‘1-cell/2-cell’ jumper selector is shorted to select 2-cell, the output voltage stabilized at 8.4 with a 10-15V DC input (I used a 12V 5A supply for the tests).  Below about 10V, the output voltage falls below 8.4V
  • with a partially charged battery stack, output current was about 1.8A at the start, tapering to below 200mA at termination
  • There is an onboard Red/Blue LED and solder holes for an external bi-color LED.  The onboard LED states are:
    • RED = Charging
    • Blue = Finished
  • Both the onboard and external LED connections are tied to +V via the same 1K current limiting resistor.  This resistor is routed to the center hole of the 3-hole external LED breakout.  The rightmost hole is tied to an open-collector gate that goes LOW upon charge termination, and the leftmost hole is tied to an open-collector gate that goes LOW upon charge initiation.  In my testing circuit above, these lines are labelled ‘Fin’ and ‘Chg’ respectively and were routed to digital inputs with 20K pullups on the Arduino UNO.
  • This is NOT a balance charger – so there may be differences in cell voltages over time.  If this is a potential issue, then separate cell protection modules like these should be installed.

Here’s an annotated photo showing the pertinent features:

So, it looks like this TP5100 module will work fine for my 2-cell LiPo application, with the addition of an external 2-cell protection module like the one noted above.  Not only will this solve my original IR drop problem, but it is much smaller and simpler too, as shown in the following size comparison shot.  Oh well, at least I had a lot of fun building up and testing the original charger module ;-).

charger module and TP5100 size comparison

Stay tuned!

Frank

 

 

 

The operation was a success, but …

Posted 21 February 2018

In early January of this year I posted about finishing the integration of my new-improved battery charger & battery pack into my new-improved robot chassis.  Between then and now I have been working on getting the new robot chassis mated up with the charging station (the new robot chassis is wider, and I also changed to larger diameter wheels) in preparation for renewed field testing.

Unfortunately, just as I was getting ready to move into field testing, my robot started acting funny.  About half the time, it wouldn’t disengage from the charging station and instead would reboot.  At first I thought the added weight of the new battery pack and robot chassis was causing the motors to stall, so I changed the code to have the robot disconnect at full motor speed rather than 1/2 as before.  This made the problem even worse; now not only wouldn’t Wall-E2 disengage from the charger, it wouldn’t even move forward or backward under it’s own power!  Clearly something was badly wrong, but I had no clue what it was.

Applying my time-honored troubleshooting – I simply put Wall-E2 aside for a few days and let my subconscious work backwards through all the changes since Wall-E2 had last worked properly.  After enough time had elapsed, my subconscious reported back and said:

“You are an idiot.  All of the complexity you added in your quest for an on-board charging system has placed that wonderful high-capacity battery pack at the far end of a long series of (relatively) high resistance circuitry, and the IR drop caused by full-speed motor currents is killing you!”  “Oh, and by the way, you’re ugly too!”

Well, my subconscious is almost never wrong, and it only took me a little bit of testing to confirm it’s theory.  I set the code up to go forward and backward at full speed, and monitored the CPU’s 5V regulated output line with my trusty oscilloscope.  As soon as the motor command was executed, the 5V line drooped to less than 3V, and the CPU rebooted – oops!

So now I knew what was happening, and I (or my subconscious anyway) had a good idea why.  To confirm the why, I bypassed all the charge-management circuitry and wired my 7.4V 7200mAh battery pack directly into the main robot power line, as shown in the photo below

7.4V 7200mAh battery pack wired directly into robot power

With this setup, the robot not only was able to move forward and backward at full speed, the thing damn near took my arm off when I tried to stop it – whoa!

So, the bottom line is that all the work I put in designing and implementing a really cool on-board dual-cell charge management system had the ultimate effect of making the battery unusable.  The operation was a success, but the patient died! ;-).

So, where to go from here?  It appears that I have to completely revise my thinking about battery charging and maintenance for Wall-E2.  Instead of being in series with the battery, any charging/maintenance system must operate in parallel, and be completely out of the path between the battery pack and the load when the robot is running.  Now I realize this is the reason most RC/Hobbyist multi-cell battery packs have a balance charging cable in addition to the main power cable; charging is done completely independently of the output path.

When I first started the charger project, my original goal was to avoid having to remove the battery from the robot to charge it; I wanted Wall-E2 to  connect to power and charge itself without human intervention.  At the time, I felt the only way to do this was to have the charging circuitry on board, so that only a single DC connection from the charging station was required.  I thought the only way to make this happen was to use two of the Adafruit SBC1000 charger modules to charge each of the two cells independently.  Unfortunately, the SBC1000’s grounds aren’t isolated, so this meant that I had to disconnect the two battery pack cells from each other to charge them independently and then switch them back together again to run the robot after charging.  This worked (rather elegantly if I do say so myself), but had the unintended side-effect of putting too much high-loss circuitry and wiring between the battery pack and the motors.

Now that it is clear that I can’t interfere with the current path to the motors, I know I have to abandon the current charging module design, but what are the alternatives?

  • The TP5100 is a little module that can balance charge a 2-cell LiPo stack at 2A.  It has a dual-color LED output that I might be able to use for charge termination.  Unfortunately, the specs are all in Chinese, so it may take some experimentation to figure out.
  • I can use an external balance charger like the EV Peak e4 ‘cube’ automatic balance charger, and feed the three required wires (ground, B1+, B2+) out through the front of the robot to the charging station. This solves the problem of carrying the charger around, but significantly complicates the interface to the charging station.

Stay tuned!

 

Wall-E2 battery charger module integration

Posted 01 January 2018

What a way to start off the new year!  The battery charger module for my autonomous wall-following robot Wall-E2 has been completed and tested, and now has been integrated into the robot – yay!!

If you have been following this saga, you will recall that I started working on an internal charging module for Wall-E2 well over a year ago, back in November 2016 with this post.  Since then I have gone through several iterations, revisions, and mis-steps (including a semi mind-boggling deep-dive into the details of the Adafruit PowerBoost 1000C specifications in this post).  Last month I finally got a complete system (two PowerBoost 1000C’s integrated onto a single PCB with appropriate control and battery switching circuitry) working, and was able to run extensive charge/discharge cycle testing using a simple test circuit and an Arduino Uno to run it. So, now all I had to do was stuff the whole thing back into the robot.  This task was made possible by my earlier decision to upgrade Wall-E2’s ride to a slightly larger chassis, so instead of trying to cram 2Kg of battery/charger into a 1Kg space, I now had the pleasure of fitting 2Kg into a 3Kg space – nice!   Here are some photos of the integration process.

Battery module shown in the ‘maintenance’ configuration.

Another shot of Battery module in the ‘maintenance’ configuration.

Front cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Front cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Rear cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Rear cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Now that the battery/charger module has been integrated into the robot chassis, I will have to make some minor changes to the robot operating system to accommodate changes I have made along the way, but these should be easy and straightforward.  Then, it will be back to field testing, I hope.

Stay tuned!

Frank

 

 

Wall-E2 battery charger module testing, Part II

Posted 22 December 2017

After having worked out (hopefully) all the bugs in the Arduino Uno program and associated test hardware, I have moved on from testing just one of the Adafruit PowerBoost 1000C  charging module in isolation to testing the entire 2-cell battery pack system, albeit with the smaller 2500mAh LiPo’s rather than the full-up 18650 stacks.

Full-up test of the 2-cell charging system using 2500mAh LiPo cells

Full charge of 2500mAh cells

After several days of testing, I never really got consistent results with this setup – I seemed to be always chasing intermittent problems of one sort or another.  Then I finally figured it out (I think) – the problem wasn’t my setup, it was the proto plug-board I was using.  This board was a cheap no-name plugboard I got off eBay, and apparently I got exactly what I paid for! ;-(.  So, I reverted to my tried-and-true (but HUGE!) AP Products A.C.E. 236 plugboard that I have had around the lab for a couple of decades, at least.  When I transferred the setup to this plugboard, everything started working better.  In particular, the battery charging current went from about 0.5A to about 1.5A – a much more believable (and practical) figure than before.  Here’s the new test setup.

New test setup with my trusty AP ACE 236 plugboard. Note charging current shown on current meter

After getting the hardware problems squared away, I started getting reliable charge/discharge cycle data, as shown in the curves below

Two complete charge/discharge cycles. Note the time axis is in minutes here

The next step in the process will be to replace the PKCell 2500mAh flat-pack LiPo cells with the 6800mAh pack (two series banks of 2ea Panasonic 18650 3400mAh cells in parallel) to be used in the robot.

26 December Update:

Chg/Dischg testing with the Panasonic 18650 packs from the robot.

As shown above, I switched out the 2500 mAh flatpacks for the Panasonic 3400 mAh cells from the Wall-E2 robot.  Initially I was somewhat disappointed with charging performance, as I was only seeing about 1 – 1.5A initial charge current for both 6800 mAh cells, instead of the 2+ A I expected to see.  Eventually I narrowed the problem down to stray resistance in the testing circuit itself, through the plugboard, the plugboard wiring, and the relay being used to switch charging power to the battery module.  When I bypassed these elements and connected the external MeanWell 5V power supply to the battery module terminals, the max current increased to slightly over 2A, as expected.  This is actually very good news, as it means that the resistance of the PCB traces supplying power to the PowerBoost 1000C modules is low enough to not materially affect the max charging current – yay!

Still, even with the max charging current up at 1A/cell and with the PB 1000C’s PROG3 charge termination resistor reduced from 100K to 33K, it takes every bit of 10 hours to charge the battery pack from 3V to 4.2V, as shown below.

Panasonic 18650 6.8AH robot battery pack. Initial charge rate approx 1A per 2-cell parallel stack

Panasonic 18650 6.8AH robot battery pack discharge, 15 ohm load

28 December 2017 Update:

While testing these battery packs for charge and discharge performance, I came to realize that I had over-complicated the test circuit.  The original test circuit is shown below:

This circuit uses a relay to switch +5V power to the charger modules in the battery pack module, and also to switch the load in and out.  As I gained more experience, I realized the relay contact resistance was substantially reducing the charge current, so I wired +5V directly to the battery pack; this worked because the Arduino running the test circuit keeps the battery pack parallel/series relay disengaged (meaning the battery cells are arranged in series) until the load voltage drops below a set threshold.  So, this realization resulted in the following updated circuit schematic.

As shown above, I added the connections to the battery charger module, and the LED displays.

However, I have now come to realize that the other half of the relay isn’t necessary either, as the ‘Robot +’ line isn’t connected to anything until the test manager computer (Arduino Uno) recognizes the end-of-charge condition and changes the Coil Enable signal from HIGH to LOW, disabling the battery pack’s internal relay and changing the battery configuration from parallel (CHARGE) to series (RUN).  So, now the test circuit can be reduced to just the LEDs and the load resistors, as shown below.

While I was making the other changes, I also cut the load resistance in half, from 15Ω to 7.5Ω to more accurately simulate the actual robot motor loads, and (finally!) managed to capture a complete discharge cycle, as shown below.

Complete Robot Battery Pack Discharge Curve, 7.5-ohm load.

The 7.5Ω load for this run provides a good approximation for the maximum current drain experienced by the robot under most operating conditions, so it is now safe to say that the robot should be able to run at least 6 hours on a charge, and that a full charge will take about 10 hours.  So, the robot will spend more time on the charger than on the road, but that’s life in the robot lane.

Stay tuned!

Frank

 

 

 

Wall-E2 battery charger module testing, Part I

Posted 15 December 2017

For the last couple of months I have been working on the battery subsystem for my Wall-E2 autonomous wall-following robot.  Along the way I upgraded the robot chassis to provide more room for the on-board charger & battery pack, and created a PC board design for the module that charges the 2-cell LiPo stack in parallel and then switches to a serial configuration to run the motor.  This post describes the first part of a testing program to validate the performance of the Adafruit PowerBoost 1000C charger module, and the idea of switching seamlessly from charging to powering a load.

By way of background for this post, I ran into some difficulties when I tried to integrate the full-up 2-cell, 2-PB1000C battery pack/charger combination into my Wall-E2 robot, and troubleshooting the problems using the complete operating system software proved to be somewhat tedious, I decided to start at the other end with a very simple test program running on an Arduino Uno, and a single PB1000C charger.

The Arduino test program is very simple; when it is initialized, it checks the battery voltage to see if is above or below a preset threshold.  If above, it energizes a DPDT relay to connect the battery to a 15Ω load; if below, it de-energizes the relay, which disconnects the battery from the load and also connects 5V to the ‘USB’ (5V input) terminal of the PB1000C, thereby charging the battery.  After that, the program alternately switches the battery between the load and charging states, recording the battery voltage every four seconds.

Battery Testing Hardware

Arduino Uno and test circuit.  Note the yellow ‘charging’ LED on PowerBoost 1000C shown at middle right

After working the inevitable bugs out of the Arduino program, I got some reasonable battery cycling data, as shown in the following plots.  The battery used in all these tests was a PKCell LP795060 LiPo cell, rated for 3.7V and 2500mAH.  It is rated for about 5hrs at 0.2C discharge, but I was only seeing about 3hrs at 0.1C, most probably because I was terminating the discharge at 3.4V rather than the 2.75V used in the discharge tests.

Two complete chg/discharge cycles

Chg/Dischg cycles using PB2 (same battery as previous tests). Short cycles may imply battery wear-out

After the last test above, I was concerned that either the PowerBoost 1000C was not operating properly, or the battery itself was failing, or both.  So, I changed the discharge termination threshold from 3.4 to 3.0V to be more consistent with the 2.75V cutoff used by the battery manufacturer.  After making this change only, I got the following plot

Charge/discharge cycles after changing the discharge termination threshold from 3.4 to 3.0V

As can be seen from the above plot, the discharge and charge times were extended dramatically.  Discharge (at approximately 220 mA or 0.1C) took well over 3hrs, and charge took about the same amount of time.  So, it appears that both the PowerBoost 1000C modules are still working OK even after the abuse I put them through by plugging them into a PCB with wiring errors ;-).

 

 

 

Wall-E2 Battery Charger PCB Part II

Posted 1 November 2017

In my last post on this subject, I described my efforts to create a PCB for the charging module in my wall-following robot(s).  I showed the hand-wired original, and the first (of three, so far) PCB versions. My circuit-checking skills are a bit deficient, so I keep finding ‘issues’ with the PCBs I received from Bay Area Circuits.  It’s not their fault as they produced PCB’s exactly like I told them – apparently they don’t know about the RDM (Read Designer’s Mind) checkbox on their order form! 😉

In any case, the major problems I discovered on the first two revs are, kind-of, understandable for a PCB newbie like myself; I had the JST battery connectors switched, so Batt+ showed up on the PCB as Batt-, and the IRF-510 transistor holes were reversed as well.  A bit of cut-and-jumper work solved the first problem, and simply turning the transistor around solved the second one.  However, I really didn’t like leaving that as the ‘final solution’ so I splurged another $30 on a third rev with these problems fixed.  These boards have not yet arrived, so in the meantime I continued testing on the current setup.

As the photo below shows, the finished and populated PCB has almost exactly the same dimensions as the hand-wired perfboard model.  Since the perfboard version had several components mounted on the wiring side, arriving at the same dimensions with everything on one side is a definite plus!

comparison between the hand-wired perfboard version and the PCB

After getting the PCB populated, I did some electrical testing, and somewhere along the way I think I managed to damage the boost regulator chip on one of the PowerBoost 1000C modules.  And this is where I discovered my next major booboo – I had hard-wired the PB1000C’s to the PCB, and so now I couldn’t replace the damaged part without destroying the part, and probably the PCB too – oops!

07 December 2017 (Pearl Harbor Day) Update:

I received the latest (and hopefully, last!) batch of battery charger module PCB’s from Bay Area Circuits a week ago, but have been too busy with other things (mostly dealing with the new V3 robot chassis) to do much with it. So, today I started populating and testing the new charger module.  As I mentioned before, one of the bigger mistakes I made on the previous version was to hard-wire the PowerBoost 1000C’s onto the PCB, making it almost impossible to replace if it failed (and, as I learned from the Adafruit forums they can be easily damaged if they are powered up without a battery attached).  So, this time I soldered female headers onto the PCB and male headers onto the charger modules, making them easy to replace if needed.

New charger module with some test batteries attached

Charger module showing PowerBoost 1000C header arrangement

The next step will be to test the module’s ability to manage the robot’s battery stack.  I plan to simulate the robot load with a 500 mA resistive load, and simulate the actual battery stack with the two smaller 2500 mAH batteries shown in the above photos.  I’ll simulate the robot with an Arduino, programmed to print out time-stamped readings of the robot power voltage (actually 1/3 of the robot power voltage, but what’s a scaling factor between friends?).

Stay tuned!

 

 

A New Chassis For Wall-E2, Part I

Posted 06 November 2017

Back in May of this year I came to the conclusion that I was never going to get my new four-cell battery pack (4ea 18650 3600mAH 3.7V LiPo cells connected as 2ea 7400mAH 3.7V stacks) and its companion charger module to comfortably fit into Wall-E2’s current chassis.  It all fits, but only with a considerable amount of pushing and shoving which has invariably resulted in damage to something – a connector, a wire, or something else vital.

Bottom side view of 4WD robot showing battery packs and charging module

Bottom rear view of 4WD robot showing battery packs and charging module

So, I spent some quality time online looking for a new, larger home for Wall-E2, and found this chassis

Lightweight 4WD Drive Aluminum Mobile Dolly Car Robot Platform for Arduino

This chassis has an internal cavity width of about 14cm compared to 10.5cm for Wall-E2’s current ride.  This extra inch or so make all the difference in the world for comfortable installation of the battery pack and charger.

After getting this chassis on order, I basically forgot about it while I was working on the square-wave modulated IR homing project.  Then when my grandson Danny and his family visited in August, we dug out the kit and assembled it.  Danny wasn’t all that impressed with the quality (well, neither was I, but I didn’t expect all that much for $30 either).  After looking at both chassis (Wall-E2’s current ride and the new one), Danny suggested that maybe we could transplant the motors from the new chassis into the old one and get enough additional space from the different form factors to solve the battery problem.  At the time I pooh-poohed the idea, and put the new chassis back on the shelf to be forgotten again.

However, after finishing up the IR homing project last month, I decided I would actually try this trick and see what happened.  So, I laboriously swapped all four right-angle motors from the new chassis to the old one, and … RATS!!  As the photos below show, no real change in the available room for the charger/battery pack combination.  Well, at least I tried ;-).

So, now I’m back to swapping chassis (wow – plural form of ‘chassis’ is ‘chassis’ – go figure) instead of motors.

One of the major shortcomings in the new model was the cheapness of the threaded holes in the frame components.  The frame metal is so thin that instead of drilling and tapping the material, the holes were punched in a way that left the punched-out metal in the hole, and this material was tapped with the machine thread.  Needless to say, this lasted for about one (or fewer) screw/unscrew cycle before stripping out – bad design!.  Fortunately I knew how to fix this problem, with the help of McMaster-Carr.  I went to their site and ordered a bunch of press-fit nuts (also called PEM-nuts for historical reasons)

Press-fit nuts for my new robot chassis

In case you have never dealt with McMaster-Carr, they are incredibly quick.  I normally tell people that once I click on the ‘order’ button, I get up, walk to my front door, open it, and get hit in the chest by the shipped order! ;-).

Once I got the nuts, I replaced all the threaded holes on the new chassis with these wonderful little gadgets.  I used a #16 number drill to drill out the holes, and pressed the nuts in the new holes with a pair of cheap gas pliers – done!

As seen below, there is a lot more room in the new model

In fact, there is so much room that now I have to figure out how to keep the batteries and charging module from sloshing around in there.  Fortunately I have a fertile imagination, TinkerCad, and a 3D printer – so I designed and printed up a battery box, and a prototype stand-off design for the PCB, as shown below.

The next part of the puzzle will be to figure out how all that stuff (IR Homing Module, laser and ultra-sonic distance sensors, motor controllers and the main controller) is going to fit on the new chassis.

Stay tuned!

Frank

 

 

 

Wall-E2 Battery Charger PCB Part I

Posted 18 October 2017

Wall-E2’s on-board battery system has gone through some major changes over the last three years or so. Back in October 2015 I implemented an on-board battery pack & charger using two Sparkfun boost chargers and two 2000 mAh LiPo batteries as shown in the following image.

 

Battery pack showing charging modules and switching relay. The tan capacitor-looking component is actually a re-settable fuse

This was a technical success, but a practical failure; the LiPo cells just didn’t have the oomph to handle the high motor currents, and the single relay design had some problems as well.  So, in December 2015 I replaced the entire on-board battery system with a much higher discharge capacity RC battery, the GForce 2200 battery pack, as shown below

Current Wall-E2 battery pack

However, using this battery required that I physically disconnect the battery and connect it to an off-board charger for recharges, which meant I could never implement my idea of an autonomous robot.

So, about a year ago, I re-implemented the on-board charging system, but with two significant changes; I used two 2-cell stacks of 3700 mAh 18650-style 3.7V li-ion batteries (specifically the NCR18650B cell) instead of the 2000 mA LiPo flatpacks, and two Adafruit PowerBoost 1000C chargers instead of the Sparkfun units.  I also added a second Axicom DPDT relay to solve the previous overvoltage issue.  The result is shown in the photograph below

Finished charging module connected to two 2-cell 3.7V battery packs

This system worked very well, and with the modifications discussed in this post from last January (January 2017), I had a complete system for autonomous on-board charging.

This system has worked extremely well, but now I wanted to duplicate it for a planned upgrade to a larger robot platform, and I really wasn’t looking forward to hand-wiring another board. What I needed was a PCB, so I (or anyone else) could fabricate any number of charging modules without the PITA factor of a hand-wired perf-board implementation, so I decided to see If I could make a PCB using the free version of DipTrace.

After fumbling around for a while in DipTrace, I soon realized that in order to do a good job with a PCB design, I needed a component and associated PCB pattern for the Adafruit PowerBoost 1000C, and AFACT, none existed – at least not in a form compatible with DipTrace.  So, after some more fumbling around, I came up with the following model for the PB1K

DipTrace component model for the Adafruit PowerBoost 1000C

Creation of this component was complicated by the fact that I needed ‘flying lead’ connections to the ‘charging’ and ‘finished’ LED drivers on the PB1K module – signals that aren’t exposed to the outside world.  Eventually I hit upon the idea of adding a couple of ‘off-board’ pads (shown above at the lower left corner of the pattern), and assigning them to the ‘Finish’ and ‘Charging’ functions of the component layout. The ‘Chg Pwr’ signal already existed on the breakout header as the ‘Power’ signal, so I didn’t need anything for that.  These modifications allowed me to integrate the PB1K module into the charger schematic and the PCB layout using the normal DipTrace project work flow.

After getting what I thought was a successful layout, I clicked the ‘Order PCB’ button in the PCB editor, and ordered their minimum of two PCBs for $30 + shipping (I actually received 4ea PCBs so it was an even greater deal than normal!).

PCBs as received from Bay Area Circuits – nice packaging!

Four PCBs for the price of two – cool!

Test run to populate PCB with actual components

Size comparison between PCB and hand-wired versions. First iteration of PCB is slightly larger

After inspecting the PCBs, I realized I had screwed up on a couple of items.  The pad pattern for the two-wire terminal blocks wasn’t correct (too small), and at least one of them was reversed – oops!  In addition, a couple of PCB traces weren’t correct. So, it was back to the drawing board (literally) for a ‘version 2’ PCB.  On this iteration I learned a good bit more about DipTrace’s schematic/component/pattern relationship, so I felt like I was getting at least some value out of my screwups.  After removing a superfluous 2-wire terminal block and moving a couple of parts around, I was able to make the V2 PCB smaller than the original hand-wired model.  This time I remembered to use DipTrace’s verification routines, and this allowed me to catch some additional problems that had made it through into V2.  In addition, I used DipTrace’s print facility to print out a 1:1 PCB layout on paper, so I could double-check parts placement with real components, as shown in the following photo

Second iteration PCB printed at 1:1 scale on paper for component fit testing

So, many hours and a couple of iterations later, I have what I believe is a successful PCB implementation, and I sent off a new order to BAC.   As usual, I wound up spending far more time designing the PCB than I ever would have in making a second hand-wired unit for the new robot, but where’s the fun in that?

Stay tuned

Frank