Determining actual capacity of battery?

[Haku]

How can I find out the actual capacity of an ebike battery?

Could it be done with simply using a couple of 12v car bulbs and continually measuring the volts+amps until the battery turns itself off, then adding up the resulting numbers?

 

[piotrmacheta]

Battery capacity varies with current drawn so a simple test using bulbs will only give an approximate answer. Usually battery capacity is quoted at a discharge rate of about 0.1C but an ebike uses peaks of 2 or 3C so the capacity will be much less than the quoted spec but for LiPo's it doesn't make too much difference.

The best way is to use a wattmeter and run it in the bike till the controller trips out. If the current drawn by the motor is high due to you, say, going up a hill then the battery voltage can sag and trip the cut-out earlier resulting in a lower battery capacity than you expected but as the battery is being used under real conditions then this is the true capacity of your battery for your application.

 

[Haku]

You make a good point regarding actual use versus a lab test, I could probably make my own watts-up type device but buying the device would be much easier, something to put on my "to get one day" list because it'd mean fiddling with the wiring on the ebike to accomodate the device.

I'm curious because I now have 3 identical ebike batteries, all over 2 years old, and want to find out the capacity of the not so older two.
I got a 3rd ebike last week for a steal, a 2 year old Urban Mover UM36X in black, looks and runs really well, better than my ageing 4.5 year old UM36 that has seen much better days.

I think I'm ok for battery capacity usage (even carrying a 2nd) until next spring but after that I'm going to seriously look into re-celling the oldest battery, especially as the price of a set of cells is 1/7 to 1/4 the price of a whole new battery pack.

 

[piotrmacheta]

If all you want to do is compare your batteries against each other then your idea of putting the same load on each one and seeing how long it lasts is good enough but you must be very careful not to discharge the batteries below about 3.1V per cell. I use an alarm to warn me of this (a Lithimon). I connected my Wattmeter between battery and controller.