(ET) Battery monitor circuit

[Chris Zach <czach computer org>]

Ok, as I probably mentioned, I have built a three LED battery monitoring 
circuit for the Elec-trak. Each LED monitors two batteries, and will 
light as follows:

11.0 volts and less: Light LED red (low voltage)
14.7 volts and more: Light LED green (high voltage)

This will then allow you to drive the tractor without worrying about the 
batteries being damaged. If you're driving and you see a red light come 
on, slow down and head for a charger. If one of the LEDs come on early, 
then you know something is wrong with one of the batteries.

Now, here is the interesting part: At first I hooked the monitors up to 
the batteries by using two wires to each battery pair (12 volts) 
directly to the posts. Sure enough, the LEDs would be out, except when 
you turn on the mower deck. Then all three would flash red for a second 
(start up surge drags the batteries down) then go off.

For my second set of tests, I wired the sensors in so that they rode on 
the tractor's power wires. Using the headlight fuse as a common pack 
negative, I used the head light hot wire that comes off the back pack as 
the first 12 volt pair, the headlight wire to the front battery as the 
second pair, and the other side of the front battery and the back side 
of the front battery as the third pair. Wired it all up.

Now however I can see that the back two batteries being sensed via the 
tractor wires are way out of sync with the front battery. Moral: You 
have to use dedicated sensing wires. However it also shows that there is 
significant voltage drop (like a volt or more) in the tractor's wiring. 
Interesting.

With dedicated wires, this circuit is going to be great. No more 
wondering about the status of batteries or whatever. When one starts to 
go bad or if there is a problem you will know. Also I won't reverse any 
batteries by accident.

Chris


Chris