72v Battery showing 80v on multimeter

[matthillster]

Hi everyone,

I'm in the process of putting together a self built high power ebike using a chinese Enduro frame, Sabvoton 7280 controller, 3kw hub motor and a 72v battery I bought second hand off ebay.

It's proving pretty daunting as I've never done anything like this but very gradually it's all coming together.

Something that's confusing me a little though is that although the battery I bought was advertised as being 72v, the controller says it's getting 80v from the battery and my multimeter agrees with it. Is this normal or have I been mis-sold a battery?

It seems that the controller is happy getting 80 volts but I'm not sure the battery percentage meter will be accurate / usable.

Also I can't get regen braking to work and I think it's because the controller automatically disables it if the battery voltage is too high so as not to over charge the battery or melt the controller. Has anyone else experienced this and if so is there any work-around?

Any feedback / information guidance on any of this would be greatly appreciated as I'm starting to tear my hair out a bit with it.

 

[Hwy89]

A 72 volt battery is most likely 20 cells or groups of cells connected in series. These groups of cells will have a working voltage range of 4.2 volts down to around 3 volts. If you divide the 80 volts you measured by 20 cells you will find that each group is at 4 volts. Almost fully charged.

 

[matthillster]

Thanks!

So when I recharge it, should I just charge it to 72v?

 

[john61ct]

**If** it is a 20S pack, 81-84V is the range for charging "termination" or CV setpoint voltage

per cell
4.20V is stressful maximum "vendor spec Full" not good for longevity but maximum range

4.05V nice and gentle, maybe using 85-90% capacity

Also depends how charge terminates

CC only "charge TO a voltage and stop"

vs CC+CV hold Absorb until trailing amps minimum is reached, say 0.03C

latter being closer to Full as per "vendor spec"

 

[amberwolf]

So when I recharge it, should I just charge it to 72v?

That would only be around half full or less.

You should charge it to the full voltage the charger allows, if you want to use the full capacity. For a 20s pack, that is usually called 72v, that would be about 84v. Check the charger label for the actual charger voltage, and measure it without it connnected to the battery (otherwise i will just be at the battery voltage).

If the battery will not charge to the full marked charger voltage, then you could leave it on the charger at least overnight, and possibly for several days or more, so that the pack's BMS can rebalance the cells in it and allow it to fill up completely.

If even after that it still won't charge to the full charger voltage, then one or more cell groups in it has a problem, and would need to be repaired or replaced to allow fully charging the pack.

 

[hemo]

Matt in plain English your battery if 20s/72v nominal will have a min voltage and a max voltage.
A safe use min voltage will be approx. 64/66v and a max charge voltage will be 84v, the 72v is a nominal voltage which is approx. 50%. For better cell life/longevity charging only to about the 90% mark of 82v is recommended.

 

[matthillster]

Thanks for the info everyone! really useful!

 

[T-Sizzle]

Haha I always thought my battery was fully drained and stop riding when it hits 72v.. My battery literally has twice the capacity I've been utilizing smh!!

 

[john61ct]

Haha I always thought my battery was fully drained and stop riding when it hits 72v.. My battery literally has twice the capacity I've been utilizing smh!!

That's how to get (maybe) 5x cycle lifespan

 

[999zip999]

What type of batteries are we talkin about or just assuming this is lithium ion and not lifepo4. How about more information on the battery like a link or a photo of battery and charger. Get used to using your multimeter all the time to check for positive and negative , high and low voltage and the voltage of your charger.

 

[larsb]

What he said.

And if you find it’s a 20s lithium battery that’s supposed to be charged to about 80-84V and you’re not using it yet then discharge it to 72-76V with a light bulb or something like that to stop the aging while you don’t use it. Lithium batteries shouldn’t be stored at full voltage.

 

[999zip999]

They both can be charged to 84 volts. lithium ion can be discharged to about 62volts till empty. Lifepo4 lvc is about 70volts. This is very important to know. Give us a link for the battery where did you get it ?