(ET) 30 year old tool finds battery screwup.

[Christopher Zach <cz bunsen crystel com>]

So about 30 years ago when I first got an Elec-trak I wanted to see if 
the lead acid batteries were roughly in sync, or tell when a 6v battery 
or cell was hitting zero volts so I could ease off on the power and 
bring the tractor in for a charge.

I built a little circuit that used 2 color LEDs (red/green) with a pair 
of zener diodes per 12 volt circuit to tell what the battery voltage 
was. The code is:

Green: Over 14 volts. Fully charged.
Off:    Between 13.8 volts and 10.x volts. Running voltage
Red: Dropping to 10 volts and below.

As a bonus the LEDs would light up brighter as the voltage passed the 
setpoints. Thus if a battery was below 10 volts that LED would be bright 
while one at 10.4 volts would be dim. Go above 15 volts and the green 
LED would turn yellow (bad).

Just dug it out of a box I was cleaning out in the shed and figured I'd 
try it on the Elec-trak with the NICD cells. Now the big tractor has 4 
sets of cells in parallel so that won't work well but the smaller E15's 
each run with a single string in the front.

Started hooking it up every 10 cells. Dark LED, Bright Red LED, Dark 
LED....

Hm. The second set of cells had a voltage imbalance. Got my meter went 
from cell to cell then spotted the problem.

Years ago when I replaced this pack I put one of the cells in backwards. 
In other words it's been running backwards for at least 5 and maybe 10 
years. That resulted in the second set being 1.2 volts low and thus the 
red LED.

Pulled the cell and put in a spare that was at 1.0 volts. Light is out, 
however when I moved the tractor the middle LED came on (showing that 
the second set of 10 batteries has a low cell). Now it's on charge and 
in theory all three LEDs should come to a nice green at the same time.

Kind of amazing this 30 year old thing helped me troubleshoot the pack 
like this. I have the cell on my bench, it was empty but not shorted so 
I'm charging it at 3.2a rate (at 1.4 volts) and will see if it's ok. To 
be honest, it might be fine.....

Chris