Measure battery capacity

[j123456]

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]

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]

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]

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]

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]

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]

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]

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.