Sunday, August 15, 2010

Battery testing

I have removed everything from the car to complete that to do list I created a long time ago and have been putting off (more on this later). While I had the batteries out I wanted to do a test to determine how much capacity is remaining now that the cells are about 1.5 years old and have over 15k miles. I've also calculated it takes me roughly 120Ah to get to work and 80Ah to return home (2k foot elevation climb to get to work). Using 15k miles, and the 70 mile commute that gives me about 214 trips I've made to work so far. Each round trip (120Ah + 80Ah) / 160Ah batteries = 1.25 charge cycles. Therefore, 214 x 1.25 is 267 charge cycles I've used.

Below is my battery testing solution.
  • Costco a 2300 watt power inverter. This allows me to test four batteries at a time to get my 12v source.
  • 1500 watt space heater. This gives me just over 130 amp load on the batteries which is close to 1C and will give a decent enough load to test.
  • E-Xpert pro battery monitor from TBS Electronics with a 500 amp shunt. This will track the Ah used for me and give accurate results compared to trying to calculate this myself based off of ever changing voltage/amperage as the batteries drain.
  • CellLog8 allowed me to easily monitor each cells voltage to determine when a cell was too low during the load test or too high during recharge.
  • For charging I used my original battery charger, a 12v charger I had, and my bench power supply. This allowed me to charge up to 45 amps and helped speed up the testing greatly. I only did quick charges and once the amperage needed was between 5-10 amps I stopped charging and begin the test. This means the cells were NOT fully charged (probably 90 - 95% is my guesstimate) which is important when reviewing the results below.

My original testing plan was going to be to drain the batteries until the first cell reached low voltage, then rotate in another cell and continue testing to get results for each cell. After doing this only once I realized that the capacity of the cells was extremely close and it wasn't worth the extra time. I won't post all the results but basically my lowest cell produced 145Ah. I had other groups producing 148, 150, etc. They were all extremely close. Even the two cells that have terrible voltage sag still produced over 145Ah like the others. Now 145Ah is just over 90% of original capacity, however, remember my charging method was manual and I didn't give it the time to do much constant voltage charging. So to my surprise, I think I'm still near original capacity. The only thing is I didn't do this testing when the cells were new and I'm told they will have at least 160 but usually more, so it's possible I've lost more than I know. Either way this is good news and I'm quite happy with the results.

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