Tag Archives: battery pack

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

 

 

Wall-E2 Charging Station Design, Part V

Posted 18 Dec 2016

The new charging system for Wall-E2 consists of three major parts:

  • The two 3.7V Li-Ion battery packs and battery chargers (one charger for each battery pack)
  • The contact array that connects the charging platform to the chargers.
  • The charging platform and charging power supply

I have spent the few days or so working on the first two items above, building up the battery pack and charging circuit for Wall-E2, and working out the details of the contact array for connecting Wall-E2 to the charging platform.

Charging Module:

This is actually the third time I have attempted a charging system for a  7.4V Li-Ion battery pack consisting of two 3.7V cells.  The first one was for my original Wall-E, and it is still cooking along.  The second one was for Wall-E2, and it didn’t go as well.  After all the work of building up the module and tucking it into the robot, I discovered that the system just wasn’t robust enough for Wall-E2’s higher power requirements, so I wound up going with a high-current RC battery and an external charger.  This wasn’t really satisfactory either, as the battery pack was just too big and awkward, and having to physically disconnect the pack from the robot to charge it was a real PITA.  Plus, I still harbored the desire to make Wall-E2 more human-independent by giving it the capability of recharging itself.  So I made another run at the dual-pack charging universe, and this time I found an article by the Adafruit guys about a ‘simple balance charger’ using two of their Li-Po chargers and a manual 3PDT switch.  This article very closely matched the charger setup I had used previously, except for one extra pole on the switch.  In the Adafruit circuit, this third pole was used to switch the positive side of the upper 3.7V pack from the load to the upper charger. My previous designs didn’t have this switch, and on closer examination, I realized that without this third pole, the upper charger might see the entire 7.4V on its output port – oops!  The reason for this is that the chargers aren’t truly isolated from each other – they share a common ground, and when the circuit is in series (RUN) mode, the upper charger’s plus output is still tied to the upper battery’s positive terminal, while its negative output is still at ground.  This puts the entire  7.4V across the upper charger – a BAD thing!

So, I needed a third pole, but although small 3PDT manual switches are easy to find, small/compact 3PDT relays are not.  In my previous designs I had used the very nice Axicom V23105 2PDT telecom relay, but 3PDT relays in the same form factor were nowhere to be found – grrr!  So eventually I decided to use two of the Axicom relays and gang the coils to get a 4PDT relay, one pole of which would go unused.  Also, learning from previous mistakes, I made sure I could easily remove/replace the battery packs and the chargers if necessary.  The final charger schematic is shown below, along with some images of the finished charger module.

Dual cell balance charger. Note the two Axicom relays ganged to form the required 3PDT switch

Closeup of the completed charging module. Note the two Axicom relays used to implement a 3PDT switch

bottom wiring layer of charging module

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

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

Signal/Power Contact Array

The next challenge was to figure out how to connect the robot to the charging station.  In addition to supplying +5V to the two chargers, I wanted to bring out the power, charging and charge-completed status signals from both.  This requires a total of 8 contacts ( 6 status signal lines, +5V in, and GND).  I also decided to bring out the power line to the robot, for a total of 9 contacts.  The idea here is to place contact strips on the bottom of the robot, which will make contact with spring-copper sliding contacts on the charging platform.  I played with a number of contact layouts, but ultimately decided on a straight-line array of contacts due to space restrictions in the robot.  In the image below, the contact array layout is shown, with the ‘Pwr LED2’ position partially implemented.

 

Bottom side view of 4WD robot showing contact array layout with the ‘Pwr LED2’ position partially implemented

Some time ago I purchased a length of beryllium-copper finger stock used for fabricating EMI gaskets, with the intention of using the individual fingers as contacts for the charging station.  In order to do this, I needed a way of capturing each finger in the top surface of the charging station.  I went through several iterations as shown below

After playing around a bit with  a single contact finger and the contact arrangement shown above, I realized that a small misalignment between the robot and the charging platform could cause a finger to bridge two contacts or even connect with the adjacent circuit.  So, I went back to Visio and redesigned the contact layout, as shown below

 

two-row contact layout.  Contacts are 15x10mm with 16mm center-center spacing

As shown in the following photos, this is a much more robust arrangement in terms of contact mis-alignment protection, at the cost of taking up more space on the bottom of the robot

Contacts just prior to engaging

Contacts mis-aligned low

Contacts mis-aligned high

Contacts in the fully engaged position

Contacts just before engaging

Contacts well before engaging

With the above arrangement, there is basically no possibility of a contact finger bridging the gap between two contacts, and even drastic mis-registration of the robot onto the platform will result in correct contact engagement.  That’s my story, and I’m sticking to it! ;-).

Here’s a short video clip of a few simulated engagement/disengagement cycles

 

The next step was to fabricate the robot-bottom contacts from copper tape and wire them to the charging module. Here are some photos of the finished product.

Interface contacts fabricated and wired to charging module

When I looked at the completed module, I recognized that I still had two issues remaining.  The first and more important one is that I needed a strip of insulation to having the sliding contacts short to ground or each other as they moved across it on their way to their final destination. The second one was that it would be nice to label the contacts so that I wouldn’t have to trust my very untrustworthy memory.  As I thought about this, it occurred to me that I could kill two birds with one stone by placing a label strip on the chassis, as shown below – cool huh!

Added labelling to contact array. This was a ‘two-fer’ as it also prevents contact shorting to chassis

Frank