Measure battery capacity

j123456

Finding my (electric) wheels
Sep 3, 2018
12
1
70
Hi,
This summer I built a very inexpensive ebike using a Raleigh bike and a conversion kit. I bought an inexpensive bottle battery. Pretty pleased with my new bike but I'd like to know what I've really bought. In particular how good, or bad, is my battery. It's meant to be 36v 10Ah and it maybe is.

I'm sure I read somewhere an article using a volt meter and a power meter to access the capacity. I have a voltmeter and a plug-in Lidl power meter which shows Watts used, although I am unsure how accurately. Does anyone have a lead on this? There must be some loss in terms of charge versus discharge for example.

Thanks
 

anotherkiwi

Esteemed Pedelecer
Jan 26, 2015
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Most Watt meters have a mAh reading, the easy way is to put it inline between the charger and the battery to see how many mAh you put in when you charge it.

You are correct there will be some difference between what you put in and what you get back out but it will give a decent idea of capacity.
 

j123456

Finding my (electric) wheels
Sep 3, 2018
12
1
70
Most Watt meters have a mAh reading, the easy way is to put it inline between the charger and the battery to see how many mAh you put in when you charge it.

You are correct there will be some difference between what you put in and what you get back out but it will give a decent idea of capacity.
Thanks. I was worried the efficiency would be much worse than that. My power meter gives kWh and I know my battery should be 36v x 10amps 360Wh. Hopefully the meter resolution will be good enough.
 

anotherkiwi

Esteemed Pedelecer
Jan 26, 2015
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I have one similar to this



Shows:
- Watts Peak
- Amps peak
- mAh used
- Wh used

As well as battery voltage, running time and real time Amps and Watts
 

Laser Man

Pedelecer
Jul 1, 2018
196
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Michelmersh SO51
I assume that the Lidl power meter plugs into the mains?
The battery charger will be working at roughly 85% efficiency (15% lost as heat) - that gives you an idea of how much power is actually going into the battery.
Anotherkiwi's type of meter is much more accurate, but probably over the top unless you are trying to fine tune the system.
 

j123456

Finding my (electric) wheels
Sep 3, 2018
12
1
70
Correct. Thanks for the 85% suggestion. Not going to buy a better meter at the mo although I understand the limitations of the Lidl one. Just want a rough idea.
 

harrys

Esteemed Pedelecer
Dec 1, 2016
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Chicago, USA
I like theTenergy meters that anotherkiwi posted. Not really over the top, as they only cost $10USD and a 5 week wait on ebay.

Since my ebikes have connectors on the battery cable, I used it to assess my battery usage. I've also put it between my charger and the battery. I've always found that the power used during riding is about the same as the power needed to recharge.I'd estimate the accuracy no worse than 5%. I've put two meters in series and the amp readings are identical although volts an be 0.5 volts apart at 42V..
 

j123456

Finding my (electric) wheels
Sep 3, 2018
12
1
70
I can only get those meters imported from the states which trebles the price. I can get something that looks the same for about 10 bucks though. Probably same factory. I'll use the one I have and maybe upgrade later. Thanks

Voltages 41.9 full 36.4 part empty 270Wh.
So 7.5Ah.
I'll do a bigger discharge and recharge soon.