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.