[ausev] Homemade battery meter?

[Brian Lasseter]

On Sat, Jan 10, 2009 at 11:33 PM, Roy Hughes <crashlode at hotmail.com> wrote:
> As im moving along in my car conversion i have been contemplating a battery
> meter gauge. the curtis one that people use is what i'd like, but how to
> make a homemade version? any suggestions?, leds, resistors etc it doesn't
> seem like it should be difficult to make a measurment of the battery pack
> and then tune an LED to come on at each point that i want to measure.
>
> I have a voltmeter to install which is going to tell me what i want to know
> but for someone who is going to just drive the vehicle (like my wife) i dont
> think she would relish an hour instructional on how to read whether theres
> enough 'fuel' to get her home,


I'm not sure what you are trying to accomplish.... whether just a volt
meter or an actual state of charge meter.


Measuring voltage to estimate state-of-charge is easy and can be done
with just a large resistor and a galvanometer.  The LED bar graph just
introduces more parts and complexity.
http://en.wikipedia.org/wiki/Voltage_meter#Analog_voltmeter

You could make something like that yourself, or buy a pre-calibrated
galvanometer for $60 that is appropriate for a car:
http://www.evparts.com/prod-IN2548.htm


However a real state-of-charge meter is a very useful thing to have in
an electric car, and is not very easy to build since you have to
record the instantaneous voltage and current, and integrate those
readings over time to figure out how many Amp-Hours you've burned
versus how many Amp-Hours you think you have.  Peukert's Law and the
temperature of the battery also factor in.  (
http://en.wikipedia.org/wiki/Peukert%27s_law )  Building a state of
charge meter would be a fun romp in learning how to program a micro
controller, but the time you would spend doing that yourself would
likely be better spent on getting the rest of the electric car
working.

A nice capable Xantrex Link 10 state-of-charge meter can be had for a
little over $200 now.  And that includes the current shunt, and I
think a voltage pre-scaler:
http://www.affordable-solar.com/xantrex.link.10.standard.meter.emeter.htm

If I had my car to do over again... I'd probably install one of these
instead of the Xantrex Link 10 that I have:
http://www.paktrakr.com/index.html

Both of those state-of-charge meters give you a very nice Empty to
Full bar graph, and the Xantrex gives you a 0% to 100% display letting
you track battery depletion to the single percentage point, or to the
nearest 0.01 kwh.  (They make the wife very happy.)


-- 
TTFN,
Brian "Lasso" Lasseter

"No Sane man will dance."   -Cicero (106-43 B.C.)