Poor Man's BMS

BMS: Battery Management System. Usually a computer controlled gizmo that automatically controls upper cell voltages during charging (and regen braking) and lower cell voltages when they are almost dead.

I needed BMS to diagnose bad battery pairs in my pack made up of batteries of varying types and condition. The only way to truly diagnose battery issues is to monitor the voltage of each pair while I am driving. In other words, while I am putting the batteries through hell.

I already had six small voltmeters sourced from China ready to go - I just hadn't gotten around to executing it. Until yesterday. Now I have a removable BDU. Battery Diagnostic Unit. I just made that up. There's nothing like a good TLA (Three Letter Acronym) to make an idea seem credible. I'll stop now and move on to pictures.

Here is the BDU:

Crude, but awesome. And yes folks, that is a plug from a Caterpillar engine harness.

Here is the BDU in action:

It was very quick to find one pair of batteries that was 2 volts lower under heavy torture than the rest as I was driving home from work. So I split the bad pair to load test them and found another one had bit the dust. Another visit to my secret stash of batteries solved that problem.

The bad news is my car keeps murdering seemingly good batteries. I see a fancy lithium pack in the future. Just not yet.