Battery Capacity Meter

[Peter Seddon]

I am surprised that this subject has not come up before - at least I can't find any mention of it.

What I would like is a simple electronic device fitted in my model which tells me the capacity of the battery I have used up whilst flying (in AmpHours).

In this way I can tell how much is left!

There are variuos devices that do this but they tend to be heavy and expensive. Is it practical (a microcontroller, display and current shunt) and what are the drawbacks?

regards Peter

[AndyOne]

Peter,

There are specialised commercially made devices for this function, check out...

http://focus.ti.com/docs/prod/folders/print/bq2060.html

This may have more features than you need and it also requires an interface PCB to allow you to program it. It does, however, take into account self discharge aging and temperature effects. The BQ2060 was designed for fuel-gauging batteries in lap-top computers. You will still have to arrange a display if you want better than a 5 led display showing 20% steps.

Of course if you dodn't want to take into account all the long term effects on battery capacity, all you really need is to calibrate the battery with a capacity checker and count out the number of coulombs during discharge and recheck every so often. Watch out though, low temperature can have an alarming effect on the capacity delivered so you may want to take this into account. You could do this with a simple micro controller and a shunt resistor.

Using the coulomb counting method as in the 2060 only guesses at the capacity in the battery based on it's history, it doesn't tell you about the health of the battery and if it is about to let you down. There is a device that measures the state of charge by measuring its impedance so it is also sensitive to battery aging...

http://focus.ti.com/docs/prod/folders/print/bq20z80.html

The 20z80 only works with Lithium rechargeable batteries but the 2060 works with lithium and nickel.

Andy.

[Mr.RC-CAM]

Completely documented project with all the source code (for easy customization): Batmon (http://www.digitalproductsco.com/batmon/)

[Peter Seddon]

Many thanks for that - plenty of food for thought and ideas to produce a simpler updated version.

Regards Peter