Re: My Battery Monitor

[stevenr@flash.net]

Hello All,

Lee, Darrell and others have made many good points about our battery
monitoring system.  But it seems to me we first have to define what we
want to accomplish.  Do you want to just measure battery voltages or do
you want to measure battery voltages and also have the ability to charge
a low battery.

To measure battery voltages only, I think can be done using the 555 timer.
Lee has good arguments for not using it, but my experiments with the device
show that these arguments can be overcome very simply.  Here is what I did;
I used two opto isolators with the 555 timer chip.  One turns the 555 on or
off (the 555 while oscilating pulls a max of 31ma says my trusty Fluke meter) 
and also isolates the control line.  The second opto isolates the data line.  
The circuit is powered by the battery using a small 200ma (I think) metal can
type To2 5v regulator, input diode to protect from battery reversal, filter
caps, resistor etc.  This little power supply circuit worked fine all the way 
down to approximately 5.4V before the 555 lost control.  We really don't care 
about any battery voltage below 10.5V do we? (assuming a 12v battery)  So for 
each battery you need one conmtrol line to turn on the 555 and one data line. 
Paralleled together with the other 555's since you will only turn on one 555 at 
a time (it also keeps the 555 from draining the battery while sitting idle in 
the garage).  This circuit was hand wired to a cheap Radio Shack circuit board 
approximatly one inch square.  The StampII output pin was buffered with a 
transistor to drive the "Power" opto and the "Data" opto pulls down the data 
line which can be any voltage the opto can handle.  At the StampII input pin 
the pulses are detected and counted by the "Pulsin" command.  I potted the 
entire circuit in epoxy from "Epoxies ETC".  The epoxy I used is a heat 
conducting type that disipates the heat (I could barely feel any heat in any 
device before I potted it) and I just brought out 5 wires for all the 
connections.
You can drive over this thing with a car (I did!), the epoxy is chemical 
resistant and water-proof.  Heating and cooling the epoxied unit while running 
had no affect on it's performance.  I did not take it to 200 degrees C, but I 
don't think any EV would see a temp that high anyway.  What remains is the 
steering logic at the uP to select the batteries in sequence.

The other option is a battery monitor with recharge capability.
I think Lee is correct that a relay system would be the best in terms of cost
and simplicity and garanteed isolation.  As mentioned before some sort of 
steering logic is needed to insure only one relay is turned on.  In this case 
only one 555 (or ADC) is needed.  The uP could use the same steering logic to 
select a relay that it uses to select the particular 555 on the battery.

I'm not hung up on using the 555, I'm only reporting my experience with it.
The opto isolators come in just about any configuration you can imagine and
voltage ratings in the 100's of volts.  I used the 4n37 because that's what I 
had.  But I don't think you need a relay rated at 30 amps.  I assume a full 
pack charger would still be used to charge all batteries at once and a smaller, 
mabey 10 amp charger to bolster the low ones.

What do say Lee, Darrell, anyone !

Thanks

Steve Richardson