No-Worries Parallel Battery Charging Station

A safe (and better) alternative to parallel battery charging .

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All of the commercially available parallel battery charging fixtures use fuses to protect against inadvertently connecting batteries with different charge states across each other. Some fixtures use a replaceable 15A automotive fuse in the main battery connection, and that is sometimes supplemented with poly fuses between the balance leads, which are hazardous as well.

This approach should allow safe parallel battery charging without having to worry about different states of charge on the batteries.

[Edit 2021-03-19: Rev.3 -- Added comparators to prevent problems with swapped balance leads. Added battery detection to prevent LEDs from lighting up for an unpopulated battery.]

[Edit 2021-05-26: Small changes to schematic to work with different charger.]

I have accumulated quite a few LiPo and Li-Ion battery packs in the last few years. I have a pretty good 200W battery charger that can charge four batteries simultaneously, but sometimes I would like to charge a few more in parallel to speed up the process. There are parallel battery charging fixtures available from several manufacturers that can do the job. All of them require that the state of charge of the batteries be pretty close before connecting them in parallel to avoid potential catastrophic failures: smoke, fire, explosions. These fixtures include fuses between the batteries which open if too much current flows between the batteries, preventing the catastrophe. One fixture provides a battery checker that displays the battery voltage so that you can ensure that the batteries won't be too far apart in voltage before inserting them into the fixture. All of this relies on the user to know what he is doing...and what's the point? If there is a battery that is discharged deeply, you will have to charge that battery individually anyway since it cannot be connected to the others that are only moderately discharged.

I think there is a better way.

First, a bit of background.

Way back in the last millennium, I was designing battery charger ICs for Linear Technology Corp. I believe that I designed the first commercial Ideal Diode implementation into the LTC1960 and LTC1760. (I may be wrong about that, but if you don't toot your own horn nobody will toot it for you.) It wasn't called an Ideal Diode. There were 5 of these "low forward voltage diodes" on these two chips. It worked beautifully!

Later, a very capable IC designer working for me turned the concept into the LTC4412 "near" Ideal Diode Power Path Controller as a stand-alone part. The LTC4412 is now listed as an Ideal Diode, and it has been copied by others. The Ideal Diode power path controller is integral to this approach.

Problems to Solve:

  1. Mismatched batteries should not charge or discharge each other with high currents.
  2. The charger performs a simple "sanity check' on the connected battery before committing to perform the charging operation. I believe this is just checking voltages at terminals to see if there is a battery connected. If the voltages and impedances aren't within acceptable limits, then the sanity check fails and the charger aborts.
  3. After the charging begins there is some additional sanity checking. If the impedance on the balance leads is not correct then the charger aborts soon after beginning to charge the battery.
  4. The charging station must survive a careless user: there must be no requirement to connect batteries or balance leads in any order, swapped balance leads should not destroy components. Dead cells or bad cells should be handled safely.

This parallel charging station must fool the charger into thinking that all is well.

The Schematic to date:

There are now enough components to bump up against the limit of the free 500pin Diptrace license. I removed the ability to expand the charging station by stacking them in series -- not giving up much capability with that feature. Much of the schematic "chaos" is caused by PCB layout issues. I was able to reduce bypass capacitors on comparators by locating the comparators close to each other so that one bypass cap would serve two comparators.

This fixture will charge 1S-6S LiPo, LiFeSO4, Li-Ion, or LiPoHV packs, as long as the cell counts are the same. I'm not sure that it would be a good idea to charge any Nickel-based batteries in parallel. 

Theory of Operation:

There are four LTC4412 ideal diode circuits to connect the charger to the four batteries to be charged. The ideal diodes prevent batteries with higher charge...

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BOM Spreadsheet, with sources. Everything can be purchased from LCSC.

ms-excel - 13.50 kB - 04/09/2021 at 22:16



Top paste mask for Rev.3 PCB.

gbr - 16.14 kB - 04/09/2021 at 17:53


Gerber PCB layout files for the Rev.3 pass. Does not include top paste mask for stencil.

x-zip-compressed - 130.88 kB - 04/09/2021 at 17:51


  • Changes for New Charger

    Bud Bennett05/25/2021 at 16:48 0 comments

    I just purchased a ToolkitRC M6D charger, and a 600W 24V power supply to go with it. With this setup I can charge four 4S Lipos at 10.8A (2.7A/battery) in each slot and use channel 2 for another four batteries. This new charger doesn't behave like my older SkyRC chargers. Changes had to be made to the parallel charger circuitry to get it to work with the new charger.

    When I received the new charger it refused to charge anything. The calibration was so far off that it could not reconcile the voltage at the main plug with what it was measuring on the balance leads. I kept getting a balance voltage error. This was the case whether I was using the parallel charger board or directly plugging the battery into the charger.  I thought that it might be a software problem, so I installed new firmware -- no dice. 

    Time to download the manual and read it...ugh. I found out that if you hold down the enter key while powering up the charger that it will enter a calibration mode. When I did this it was obvious that the charger calibration was way off. I think the main battery voltage measurement was in error by nearly 2V. It is amazing how many different voltages and parameters can be adjusted...but were not adjusted by the factory. After calibrating all of the voltages against my trusty DVM, the unit came alive and decided it was OK to charge a battery!

    But it was not OK to charge using the parallel charger. The diode, D1, was causing the charger to exit with an error. I shorted D1's leads but still could not get the charger to pass its sanity check phase. The next step was to reduce the resistors R1, R4, R6, R8, R10 and R12 from 1k to 75R. After that the charger was happy and started the charge cycle. I did a quick back-of-the-napkin analysis and concluded that the 75R resistors would not cause any real problems, such as heat dissipation or significant battery discharge.

    The only negative to making the above changes is that it is possible for an LED to light up as a battery is connected. This is a nuisance, but not a real problem since the user will probably start the charging cycle soon after installing the batteries. After the charge cycle has ended there are no LEDs that remain lit.

    I charged a few batteries in parallel with mixed results. The matching of the cell voltages at end of charge was not very good. I dug out my 10x loupe and examined the connections on the PCB very carefully, and found several open connections in the balance lead muxes. After fixing the soldering errors on the two boards I intended to use with this new charger things proceeded as expected. It now regularly charges all of the cells to within +/-10mV across all of the batteries.

    At this point in time I can't foresee making any more changes to the parallel station. It is working well for me. This project will be marked "Completed" sometime soon.

  • Third Pass Testing

    Bud Bennett04/08/2021 at 19:10 0 comments

    I finally got the Rev.3 PCBs and the extra components to populate them. I decided to transfer the more expensive components from the Rev.1 and Rev.2 boards to save some money. This took some extra time, but worth it.

    There are a lot of components, so it took some time to populate the board. I wish that I had ordered a paste mask stencil along with the PCBs as it would have save a lot of time with the solder paste syringe. I usually don't buy a stencil because it usually gets tossed when I have to do a revision.

    The first function that I tested was that the battery LEDs did not light when a battery was not populated in the charging station. That works as designed.

    Next, I tested the overall charging function (which should not have changed) and verified that the batteries in all four slots were being properly charged and the balance leads connected.

    Finally, I tested the situation where three (or more) batteries are connected to the charger, but two of the batteries have swapped balance leads:

    • I connected a 2.2Ah 3S LiPo into BAT1 and the balance lead connector for BAT1. This battery had a voltage of 11.7V.
    • I connected a 2.2Ah 3S LiPo into BAT3, with a voltage of 11.6V, but connected its balance leads into the  BAT4 connector.
    • I connected a 2.2Ah 3S LiPo into BAT4, with a voltage of 11.93V, but connected its balance leads into the  BAT3 connector.

    I applied 100mA of charging current -- BAT3 LED lit, as expected. As the voltage increased on BAT3 the BAT1 LED turned on dimly. I put the scope on the BAT1 LED and found that it was oscillating with low duty cycle at about 200Hz. As I increased the charger current from 100mA to about 400mA the frequency increased and the duty cycle changed until the LED was no longer oscillating. This behavior was expected.

    I increased the charging current to 1A, which is a more normal situation, and noticed that the BAT4 LED came on at full brightness when BAT4 began to charge. I backed off on the charger current and the LED turned off. When the voltage on the two charging batteries increased further the BAT4 LED lit. I was not able to get the BAT4 LED to oscillate. It appears that the Rev.3 design is functioning as intended and saving the balance lead PFETs from overdissipating when balance leads are connected improperly. I put my calibrated finger on the PFETs while they were experiencing current drain from the misconnected batteries -- I could not feel much increase in temperature.

    At this point I believe that the design is pretty solid. I will publish the Gerbers and a BOM to the files section.

  • Third Pass Design Issues

    Bud Bennett03/18/2021 at 18:16 0 comments

    I was nearly set to accept the second pass design as the final product until I started thinking about other stuff that could go wrong. I revisited the issue of when the user accidentally misconnects the balance leads between two batteries. I was completely wrong in my previous thinking that the balance lead PFET switches would survive -- they won't without some help. The problem doesn't become clear until you analyze what happens when three, or more, batteries are connected to the station and two of the batteries have misconnected balance leads. (The details section of this project must be corrected.)

    Even before the charger is enabled, there is a problem. The lowest  voltage battery will draw current from BAT1 and connect its balance leads to the charger balance leads. If two batteries have misconnected balance leads then it is possible for the battery with the higher voltage to have its balance leads connected to the lower voltage battery -- very bad.

    What happens then is that the higher voltage battery dumps current into the low voltage battery without regard to any current limit. Fortunately, the internal battery resistance comes into play and the two batteries connect and disconnect at a rapid frequency until the voltages equalize, or the PFETs overheat and fail. 

    Things get worse when the charger begins charging the connected batteries. The added charger current tends to inhibit the internal battery resistance from disconnecting the balance leads and the PFETs fail quickly. 

    I verified the above scenario with simulation and experiments on the bench with controlled battery voltages. It's not pretty.


    I think there is a solution. I added a comparator to sense if the first cell of a battery is above the voltage on the first cell balance lead that is connected to the charger and prevent the PFET switches from closing. This is a schematic of one of the channels with the comparator in place:

    The MCP6546 is relatively cheap, open-drain, comes in a SOT23-5 package, and has built-in 3mV hysteresis. R61 and R62 create a voltage drop of between 70mV-100mV. If the batteries first cell voltage is higher than what is on the balance lead bus then the comparator pulls down on M56 and prevents the PFET switches from connecting. It's not foolproof. The battery voltages will eventually equalize and the PFETs will get connected, but there will be much less voltage and current to deal with so the PFETs should survive.

    I seems to function correctly. I simulated it in LTspice with the schematic below:

    A full blown implementation is too simulation intensive, so I simplified it a bit. The comparators are implemented with voltage controlled switches, S1 and S2.. I also had to use generic NMOS devices and diodes because the more complicated models were generating artifacts in the simulation. The charger is located in the upper-left -- just a current source and voltage clamp. I modeled the 2-cell batteries with large values of capacitance with 25m-50m Ohm series resistance tagged with an initial voltage condition.

    The result of the simulation is shown in the plots below:

    The full charging cycle takes place in 50ms. When the charger is engaged at 4A, it begins to charge the battery with the lowest voltage, BAT3, at full current. When BAT3 rises to 3.5V/cell the current begins to share with BAT1, but BAT1's STAT pin is held low by the comparator. This is normal charging operation because the STAT pin doesn't affect the behavior of the LTC4412. Note that BAT1's balance leads are not connected to the balance lead bus. Neither are BAT2's balance leads connected due to the mis-wiring.

    When the voltage of the two batteries gets within 200mV of BAT2 (at about 34ms) the comparator allows the balance leads to connect and STAT1 is asserted. This causes current to flow from BAT2 into the other two batteries through the balance leads. Note also that the charger is not charging BAT2 at this...

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  • Second Pass

    Bud Bennett03/10/2021 at 03:21 0 comments

    Not much to report about with these new PCBs, but I did learn something.

    I added a LED to report the charging status of each battery. Unfortunately, I did not account for how dominant the BAT1 connection is. Here is the problem:

    1. If BAT1 is not the highest voltage of all the connected batteries, then when the charger begins to charge the unpopulated battery LEDs light up. I expected this.
    2. If Bat1 is the highest voltage of all connected batteries, then when the charger begins to charge, the unpopulated battery LEDs are not lit. I did not expect this, but it is not really a problem since the user knows which batteries are populated and can discount any activity from the LEDs of unpopulated batteries.

    It is still better to know which batteries are charging and which are not. 

    The addition of resistors into the GND connection of the balance leads did not cure the problem of the charger claiming that the battery was reverse polarity when the last battery was removed from the parallel charging station.

  • Results From First Prototypes

    Bud Bennett02/09/2021 at 18:07 0 comments

    This project was delayed by other priorities for more than 3 weeks. I decided that the most prudent way to proceed was to just populate the BAT1 channel and the 5V regulator to see if the charger complained about anything when I tried to charge a battery. Before connecting a battery,  I measured some voltages at the gates of the balance lead FETs, the charger input, and the voltage drop across M1 (the ideal diode PFET). All the voltages appeared to be correct, and the drop across M1 was only 25mV with a 2.5A load current (that's 10mOhm.) I also applied >25V at the battery and balance leads to make sure they were capable of 6S LiPo voltages.

    The charger (a SkyRC Q200) did not complain and proceeded to charge the battery (a 1.6Ah/3S LiPo) without issue. 

    Then I populated all of the PCB and began testing the parallel charging function. I was not very rigorous about this testing. Basically, I just attached BAT1 and BAT2 and started the charger. When the green light lit I checked the cell voltages to see if there were any outliers. I did the same thing when adding BAT3 and BAT4. After all of the batteries were in place and the MATCH LED was lit, I stopped the charger and reset the charger current to 4X the battery Ah, which in this case was 6.4A. This channel of the charger was limited to 50W, so I could only get 4.2A from the charger at 12V output.

    I swapped out the 3S batteries for 4 850mAh 4S LiPos. I charged these batteries from about 3.8V/cell to completion -- it took 35 minutes. I immediately checked all of the batteries using the battery meter function of the charger:

    BAT1 - 16.78V (4.19, 4.20, 4.18, 4.20)

    BAT2 - 16.77V (4.19, 4.20, 4.18, 4.20)

    BAT3 - 16.76V (4.19, 4.19, 4.18, 4.20)

    BAT4 - 16.77V (4.19, 4.19, 4.18, 4.20)

    The batteries are within +/- 10mV, which is a pretty good result (though I'm not sure that the charger or parallel station had anything to do with that result.)

    The only "glitch" that I noticed was that the charger would complain about a reverse battery insertion when I unplugged the last battery from the parallel station. That may be a result of the balance leads not grounded on the parallel station board -- I did not want any ground loops.

    I also tested it with an old SkyRC B6 charger (50W single battery charger) and it worked just fine.


    As far as I can tell, after some pretty basic testing, it works as intended. After playing with it for a couple of days I have a list of small changes:

    1. Add LEDs on each battery channel to indicate how many batteries are matching so the user can increase the charging current to speed up the process, if desired.
    2. Add resistors to the balance lead grounds to possibly prevent the complaint from the charger about reverse voltage.
    3. Both right-angle XT-60 connectors had to be placed on the bottom of the board because I got the connectivity wrong. If there is a revision, this should be corrected.
    4. Make some right-angle XT60 to XT30 adapters to allow for some more room when charging LiPos with short leads intended for quad copters.

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