Bad Battery

The BMS was complaining about a battery for some time now, but when it got to the point where the complaint was EVERY morning, it was time to find out which one was bad.  This is difficult when there are 108 of them. However, there is a method to finding the bad one:

  1. Drive a few miles to drop all the batteries to normal operational voltage (3.33 for LiFePO4)
  2. Expose all the batteries so you can see the BMS chips
  3. Measure the voltage at all batteries (mark the ones that are low)
  4. Start charging.
  5. Mark the batteries that go into shunt mode first
  6. When charging is done unplug the charger and measure all the batteries.
  7. Wait 1 hour for batteries to settle and again measure voltage.

Quite likely you will have identified the bad battery in step 3, but steps 5 and 6 will clearly confirm this.

What makes a battery go bad?  If the battery is new, then it will fail within a month or less.  If the battery has been in the pack for a while, then it has received some form of trauma (such as a connector flopping on and off or been hit or something like that).

In my case, the battery had a loose connector, which caused one of the bars to bounce on and off the battery.  This is a Bad Thing ™.  To see why, check out this photo:

Bad Battery

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