(ET) Pack is unhappy, weird!

[Christopher Zach <cz bunsen crystel com>]

So I've been vacuuming leaves with the E20 for the last week or so and 
it's been a busy little tractor. The 90 BB600 cells are arranged in 
three strings, one in the front, and two in the rear. All paralleled, no 
big issues.

Till today. Using my Home Assistant telemetry to monitor charge I see 
the last two charges on Nov 1 and 2 were normal. A spike up to 1100 
watts that tapers down over 2-3 hours to 200 watts, then a small 
increase to 300 then stops as the timer runs out. Same for November 2.

However today I went out to check it and the batteries were flat. Odd, I 
did use it on the third and parked it with low cells but nothing too 
bad. Put it on the charger, went inside, and when I came out an hour 
later the tractor was whistling.

That is not good. Came from the front of the pack, so I immediately 
unplugged it, went to the shed to get my 8mm socket wrench, and removed 
the busbar on the front pack. One of the cells was boiling and when it 
stopped charging and was disconnected it went down to quiet but the cell 
was *hot*. Several others were warm, so after checking it I charged up 
the rear pack for 15 minutes, enough to drive it to the water hose and 
hose down the front pack.

I'll check it tomorrow, but the battery that was boiling had plenty of 
water in it (I haven't watered since last fall) and a couple of other 
cells looked ok as well. Tomorrow I'll check the pack, I may have just 
had a couple of cells short which discharged the rest of the pack into 
the cells. However since it was running fine a few days ago whatever 
happened happened fast.

Never dull. I may need to sit down and put 2 contactors in the back 
strings so I can have them turned off when the tractor is "off". This 
will screw up charging, but I'll have to figure that out.....

C