Battery Testing

I got the Big Red Van back from the shop last week and it seems to be running fine. So... with that I'm putting it back into daily use and have started pulling batteries out of the Saturn.  My plan is to cycle every cell to determine their actual amp-hour (ah) capacity and internal resistance (ir).  I bought a little charger that is really made for remote control car/airplane type batteries, designed to quick charge them, but it will work very well for what I need as well.  It's called the Powerlab 6 from Revolectrix, and it will discharge and charge the cells at up to 40a, and create cool looking graphs.  Fairly unique is it's ability to calculate the cells internal resistance, which is a strong indicator of health. Each cell will take 7-8 hours to cycle.

Yes, this will take me a few weeks to do since there are 48 cells, I figure I'll be lucky if I can get 2-3 done per day. But I am also going to pull the motor/trans out and re-do some things in the car so it's not just lost time. 

As of now, I've got 14 cells done.  It's not stellar news in that quite a few cells are below spec, but it's about what I expected based on the voltage sag I've recorded while driving.  What is interesting is that there are some cells that are excellent, some that are normal, and some that are weak, but there is no pattern to where they are in the series string of batteries.  A good cell will be right next to a weak cell, quite randomly.  It makes it hard to believe that any kind of mistreatment caused the cells to degrade, perhaps they were just mismatched from the factory?  Typically not the case, but I'm not sure how to explain such a wide variance.  Some cells are testing over 135ah, others are below 125ah.  Internal resistance numbers are varying from 0.5mohm to 1.2mohm.  

Below is a picture of the setup, there is a 12v deep cycle battery connected to the Powerlab 6, connected to the battery cell. I've got a power supply on standby in case it needs it on the charging side, and the laptop is connected to the PL6 via USB and logs all the data. The fan is there to keep everything cool.

It's really cool to do these tests, I've seen all the battery charts before, but it's a whole different understanding of the cells to actually create the charts.  Here are two volts vs. time files for cells.  The top one is a good cell (139ah, 0.5mohm) and the bottom one is a weak cell (124ah, 1.2mohm). Notice the differences:

1. sharpness of curve at end of discharge/charge cycles. The weak cell is much more gradual.

2. Difference in voltage between the two shelves. The good cell holds a higher voltage on discharge, and charges at a lower voltage on the charge side.

3. Length of time at CV. The good cell gets full in about 20 minutes after hitting 3.60v, the weak cell sits there for over 45 minutes.