E-Bike battery capacity testing
Last year I bought an e-Bike battery from eBay, and it's worked fine.
It's rated at 36v/10Ah (amp hours) which basically means it can provide 10 amps for an hour at 36v.
(Watt hours is a better measure of power stored in a battery, because the voltage of the battery changes between fully charged and empty but it seems Ah is more common.)
I decided to measure how much power the battery would provide on a full charge, but this meant I would need a few things:
I powered the battery off and checked the wiring. I believe one of the bulbs wasn't seated very well in it's holder and once that was reset it seemed to work fine again.
I restarted to system but would have to add 0.318 to the final result as the meter had reset it's count to zero.
Many hours later the system turned off again, but this time due to the battery running out of power.
That's 9.379 Amp hours. Adding the previous 0.318 from the first run and we have a total of 9.697 Amp hours.
That's a lot closer to 10Ah than I was expecting from the cheapest 10Ah battery I could find on eBay. Cool.
It's rated at 36v/10Ah (amp hours) which basically means it can provide 10 amps for an hour at 36v.
(Watt hours is a better measure of power stored in a battery, because the voltage of the battery changes between fully charged and empty but it seems Ah is more common.)
I decided to measure how much power the battery would provide on a full charge, but this meant I would need a few things:
- a connector to match the one on the battery
- a power meter
- a way to use between 30 and 42v.
The connector was fairly straightforward and after a bit of googling I discovered it was an M16 16mm 4 pin aviation plug so I grabbed one of those.
Similarly a cheap power meter was easily procured from eBay.
Similarly a cheap power meter was easily procured from eBay.
For the load I decided to use 3 12v car bulbs in series. These would have the ability to work at 10-14v each giving a combined draw of 30-42v, exactly the working range of my battery. Bulbs and bulb holders were acquired and installed into a makeshift housing and I was good to go.
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| The power drain apparatus is go |
Unfortunately there was an issue. While the watt meter would count the amp hours used it would reset this count if power was lost. This would mean when the battery cut off at 30v I would need to be watching the meter to see it's last displayed value before it was lost.
The solution to that was to have a go-pro watching the screen taking time-lapse photos.
Things started off okay but after a short while the lights went out. The meter still had power and was displaying the amount of amp hours used, but no current was being drawn.
 |
| 0.318 Amp hours used when the bulb failed |
I powered the battery off and checked the wiring. I believe one of the bulbs wasn't seated very well in it's holder and once that was reset it seemed to work fine again.
I restarted to system but would have to add 0.318 to the final result as the meter had reset it's count to zero.
Many hours later the system turned off again, but this time due to the battery running out of power.
The final meter reading was this:
That's 9.379 Amp hours. Adding the previous 0.318 from the first run and we have a total of 9.697 Amp hours.
That's a lot closer to 10Ah than I was expecting from the cheapest 10Ah battery I could find on eBay. Cool.
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