(ET) That battery monitor thing

[Chris Zach <czach computer org>]

Ok, I have pretty much finished thrashing out the battery monitor 
thingie. In fact I expect to make an order for a 50 light version of 
this for my electric car next week.

The schematic is below. Thanks to Lee Hart for making such cool ascii art.

      _________________________
      |    D1   |               |
      |  6.2v  _|_/   green     > R2
      | zener //_\   __|\|__    > 1k
   +__|__       |   |  |/|  |   >
 12v ___      A |___|       |___| B
   -  |         |   |  red  |   |
      |         >   |__|/|__|  _|_/ D2
      |   R3    > R1   |\|    //_\  6.2v
      |   47    > 1k            |   zener
      |__/\/\___|_______________|

The key here is to use a red/green LED that comes in one package. At 
first we were fiddling with simple resistor ladders, but they were too 
boring and sensitive.

At 12.4 volts, the voltages across the diodes are a perfect zero since 
the current between R1-D1 matches R2-D2. Now when the voltage goes above 
12.4, current starts to flow thru the diode on the green side. When the 
voltage hits 14.7, the green LED is lit and you know you're overcharging.

Likewise, when the voltage drops below 12.4, current starts to flow 
"backwards" thru the red LED. When the voltage hits 10.9 volts, the red 
LED lights on, and you know you're running low.

It's really a simple circuit, but very clever. Normally all lights are 
off, and you know things are good. When you see a flicker of red, your 
eye picks up on it fast and you know something's wrong (ie: batteries 
are going low). See one RED led without the two others, and you have a 
battery imbalance issue to address with a hard overcharge.

Likewise if you see two greens and one dark while charging you know 
something's up there as well. This will be key for me, since I need to 
make sure my Prizm doesn't overcharge and cook it's batteries.

Hope this helps. Maybe I'll build a meter with three LEDs in it and 
start selling "super meters" or something :-)

Chris