Month: November 2012

Balancing the Batteries

peter

I was able to replace the failed battery without trouble, however, due to that misbehaving battery, the rest of the batteries were no longer in sync – meaning some had a lot more charge than others.  I tried bringing the new battery down to the rest of the pack level using a 0.4 ohm resistor (50 feet of 16 gauge wire) – doing this for 5 minutes at a time.  It came close, but when charging, the rear half of the pack reached top charge before the top half.

So, after some consultation with battery experts at work, I realized that if I did a REALLY slow charge (2.3A at 110v), then the resistors on the BMS would be able to discharge as fast as I was charging, thus allowing ALL cells to reach that same point.  Please note that I have 108 cells, so I had to do some math.  2.3A * 110v = 253W.  108*3.4 = 367v.  253W / 367v = 0.69A.  So I’m pushing 0.69A through the pack…ok, resistors can handle that…  V = I*R, or I = V / R.  Resistor on the BMS is 4.7 ohm.  3.4 / 4.7 = 0.723A.  Easy.  🙂

Bad Battery

peter

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