Home-brew battery measuring station

[BrianG]

With all the overinflated battery ratings floating around, it would be nice to have an impartial, third party, and repeatable test procedure performed on various cells. This pipe dream has been discussed a number of times, but never goes anywhere and just fizzles away into oblivion. Getting a company to perform this service is probably not gonna happen, so it's up to the R/C community to get the ball rolling. Unfortunately, the proper test equipment is rather expensive for most of us, so, I thought I'd come up with a home-brew solution made from fairly inexpensive parts.

Warning: No offense, but some of this will make no sense to many people, so don't feel bad. Some of the techy stuff is for electrically-minded people to contribute.

So, here is what I have come up with:



The heart of this is a MMM ESC configured in high power brushed mode. This will be used to effectively vary the load to compensate for various pack voltages and rated capacity. In high-power brushed mode, the MMM could theoretically handle 360A of current. And the LVC can be used to prevent damage from "overzealous" testing.

The control circuit (labeled 556) is a basic timer circuit utilizing a 556 IC (dual 555's in one package) that will produce the throttle signal. Half of it will be configured in astable mode to get a 50Hz frequency. The other half of the chip is configured in monostable mode to get the 1ms to 2ms pulses to emulate a radio signal. This circuit could be (and has been) done with a single 555, but ESCs tend to be a little more finniky than servos, and this would provide better and more consistent control. The pot is a multi-turn version for fine tuning. The control circuit is powered via the MMM BEC, so it's pretty convenient.

The load (labeled 0.1 ohm) could probably be lower, but is currently sized to draw up to 74A @ 2s and 222A @ 6s (). Due to using the ESC, 2s would be the minimum value voltage-wise. To dissipate the power, many resistors would be placed in parallel, heatsinked, and fan cooled (yes, you heard right... a fan!).

Plottable measurements will be a bit tough. An eagletree with the high power module would be ideal, but the circuit above shows a voltmeter and ammeter (actually just a voltmeter across a very low value shunt) as an alternative. Obviously, using meters would be more difficult to measure pulses and true values, but it's an option.

This is something I'm seriously planning on putting together in the near future. Obviously, the MMM would be the most expensive part, as well as the batteries to test. I guess we'll just have to see. Thoughts, ideas, comments, and suggestions are welcome...