Instrumentation

[geekygrilli]

Hi people

Another week, another question!

How do I make an energy meter for my car? I have read all of your notes on this forum about what %age of DOD you have gone down to, but how do I measure this?

I can/will measure and display total battery voltage and instantaneous current drawn, not a problem. But how do I display the energy remaining in the batteries?

If I measure the voltage it will give some indication of the range I have left, but I don't get how else to do it...

Many thanks

Christopher

[EVguru]

The best all round solution is an E-meter.

Originally the Cruising Equipment E-meter, it's now the Xantrex Link-10.

The one instrument will measure voltage and current, track amphours in and out of the pack, etc.

http://www.energyoutfitters.com/default ... ink10.shtm

You can find them on ebay and with the dollar so low, it's probably a good time to buy from the US. Shop around the price varies a lot. Don't buy the pre-scaler it's overpriced (VERY overpriced) junk. I can tell you the component values to make your own.

[geekygrilli]

Wow - thats exactly what I want, and more. They sell for $200 buy-it-now on ebay at the moment - is that a good price?

From whay I understand, I'll need to buy the unit, the lead and the Pre-scaler and the heavy duty shunt (up to 1000 Amps). But as you said - the prescaler is easy to make.

[EVguru]

You probably don't need the 1000 amp model. Although your controller is rated at over 500 amps, that's mainly in the motor loop where you get current multiplication. You should avoid drawing that much in the battery loop to get a reasonable battery life. The 1000 amp version will be a little less accurate.

US$200 sounds about right. Get it with the shunt, they're much cheaper in the US for some reason. The pre-scaler doesn't work in quite the way you might think. Half the potential divider is already in the meter (118K as I recall). All that's in the pre-scaler are the upper side of the divider (split across several resistors to stay within voltage ratings) some ceramic capacitors for decoulping and a zenner to stop the output going above ~48 volt if the pre-scaler is unconnected. All this costs about $100 and you can't even trim the divider to give an accurate voltage reading.

I'd take a different approach. Set the divider ratio so that the meter reads average battery voltage (i.e. divide by eight for a 96 volt system). If you later change the system voltage you just change the divider ratio and still get the same readings on the meter.