How do I measure the ah of a battery?

[tsakis]

It might be a really silly question but I simply don't know the answer...

I got a battery from BMS battery that does not write anything anywhere for the ah of the unit.

I possess a cycle analyst as well and I do want to figure how many ah do I have left to not deeply discharge the battery..

The problem is that I asked for 10ah and due to lack of stck the offered to change it for a 12 ah one.. We had some mails but I don't really know what they ment so now I am not sure which is it....

Sould I dischage until close to Undervoltage protection or this will damage my pack?

 

[neptronix]

The BMS in your pack is designed with a low voltage cutoff to prevent you from overdischarging it.
That is in fact half of the reason it exists! ( the other half is to make sure none of the cells get overcharged )

SO.. drain that sucker until it cuts the power. But watch out, when it cuts the power it will also cut the cycle analyst's power

 

[tsakis]

Thanks! Potentially this is the battery: http://www.bmsbattery.com/48v/129-48v-10ah-lithium-ion-alloy-shell-ebike-battery-pack.html

Or the 12 ah version....

So if it passes successfully the 11ah margin then I will cut it off and figure that this is the 12 ah one...

PS: I am impressed with the weight though and that is why I believe it is the 10ah version. 3,9 kg!!!!

 

[neptronix]

Good luck, hope you got the 12ah.

That chemistry, which is rather new, has low discharge power, kinda like a ping. But it's soo much lighter. It has high density per weight and volume compared to lifepo4, in fact i believe it is as light as the RC lipo that many of us like for it's weight and size.

Keep us updated on how that pack performs, am interested in seeing how they pan out.

 

[tsakis]

I will..

 

[Pure]

Usually the BMS will cut you off at or about 2 Ah short of capacity. So if you get to 10 Ah then you know you have the 12 Ah pack. I mentoin this because you comented on getting to 11 Ah and I don't want you to freak when it cuts you off at 10 and have you possibly thinkiny you might have discharged a 10Ah pack to 0.

When it does trip your CA should have the data saved for you to make note of after reseting the bms.

 

[ColinB]

Capacity is measured by running the battery at low current levels until the voltage drops to a set level. The questions are: what current, and what low voltage? Personally, I would not do this. The cells could be damaged by over discharging and could shorten the life span of your new pack. True, they have a BMS, but then you are relying on the BMS settings to save you. Was it set conservatively, with the life of the batteries in mind, or severely, to deliver maximum capacity at the expense of cycles? Since you have a cycle analyst, I would always aim to discharge the battery to a maximum of 80% capacity to be nice to the battery, and consider the BMS a last resort.

That said - perhaps you can compare your battery by weight? An extra 2 amp hours should be easy to measure.

Colin

 

[tsakis]

This might be great but BMS weight specs are not accurate at all as many of us stated... :-(

 

[dnmun]

do you have picture of your battery? no need to destroy it by hoping the BMS will shut it off if you over discharge it. just show us a picture of the cells and we can tell you how many amp hours it will have.

 

[dumbass]

Capacity is measured by running the battery at low current levels until the voltage drops to a set level. The questions are: what current, and what low voltage? Personally, I would not do this. The cells could be damaged by over discharging and could shorten the life span of your new pack. True, they have a BMS, but then you are relying on the BMS settings to save you. Was it set conservatively, with the life of the batteries in mind, or severely, to deliver maximum capacity at the expense of cycles? Since you have a cycle analyst, I would always aim to discharge the battery to a maximum of 80% capacity to be nice to the battery, and consider the BMS a last resort.

That said - perhaps you can compare your battery by weight? An extra 2 amp hours should be easy to measure.

Colin

Cilin....Well said but lets add one more thing. Most BMSs only monitor the individual cell while charging and monitor the pack while discharging. Therefore, if there is a runt cell the packs LVC may not trip in time to save the poor little fella. Bob