72 Volt battery problem.

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Teeman   1 mW

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72 Volt battery problem.

Post by Teeman » Jul 31 2020 2:29pm

Hello i just received a 72 volt 21 amp hour battery but it will not charge to 84 volts the charger works but shuts off at 72.5 volts I tried a other charger and it only chargers to 72.5 What could be wrong? I have 3 other 72 volt battery's and they always charge to 84.

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MadRhino   100 GW

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Re: 72 Volt battery problem.

Post by MadRhino » Jul 31 2020 3:13pm

The BMS is responsible, probably set to 72.5v overcharge cutoff.
Make it fool-proof, and I will make a better fool.
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Trek Session 10 mod. Variable geometry. 70mph
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Santa Cruz V10. 50mph

john61ct   100 GW

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Re: 72 Volt battery problem.

Post by john61ct » Aug 01 2020 4:03pm

The voltage it is nominally labeled may not correspond to the usual 20S layout

markz   100 GW

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Re: 72 Volt battery problem.

Post by markz » Aug 01 2020 8:31pm

What chemistry is the battery, LiFepo4 has different voltages

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amberwolf   100 GW

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Re: 72 Volt battery problem.

Post by amberwolf » Aug 01 2020 9:13pm

Teeman wrote:
Jul 31 2020 2:29pm
Hello i just received a 72 volt 21 amp hour battery but it will not charge to 84 volts the charger works but shuts off at 72.5 volts I tried a other charger and it only chargers to 72.5 What could be wrong? I have 3 other 72 volt battery's and they always charge to 84.
LInk to webpage it was purchased from, so we can get the specifications to be able to help you?

How many series cells are in it?

What chemistry are they?


If the cells are badly imbalanced, the BMS will stop the charger from continuing, until it can drain down the high cell(s) enough for it to continue, assuming it is a balancing type (some aren't, and thus can't fix this kind of problem). If this is the case, and it is a balancing type of BMS, then it could take weeks of being left on the charger continously to fix that bad of an imbalance.

But if it is that bad, there's probably something seriously wrong with it that rebalancing wont' fix.

Teeman   1 mW

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Re: 72 Volt battery problem.

Post by Teeman » Aug 02 2020 6:05pm

Thanks Guys for all the help, I will try to run the battery down to cut off and leave it on the charger for a week to see if it will rebalance the cells.

markz   100 GW

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Re: 72 Volt battery problem.

Post by markz » Aug 02 2020 8:09pm

Teeman wrote:
Aug 02 2020 6:05pm
Thanks Guys for all the help, I will try to run the battery down to cut off and leave it on the charger for a week to see if it will rebalance the cells.
Open it up and take measurements of each group of cells. Buy a $5 digital multi meter and start measuring and writing it down. If you bulk charge then you over charge some cells for sure if its unbalanced. Balance charging will take forever, I would buy a few cell phone chargers from the thrift store and charge up each group pair, just dont over charge them, keep an eye on them, those chargers could be 5V and you want 4.10-4.20V for each pairing. Thats just what I'd do, for what I have. But if you can balance charge then do it for a week but keep an eye on things.

john61ct   100 GW

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Re: 72 Volt battery problem.

Post by john61ct » Aug 02 2020 9:49pm

Nothing useful is served by just "running it down" other than faster wear, unless in a controlled CC load test.

Make a list of the questions we've asked so far, and track down the answers

in order for us to be able to further hep / educate you

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amberwolf   100 GW

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Re: 72 Volt battery problem.

Post by amberwolf » Aug 02 2020 10:53pm

Teeman wrote:
Aug 02 2020 6:05pm
Thanks Guys for all the help, I will try to run the battery down to cut off and leave it on the charger for a week to see if it will rebalance the cells.
Don't do that.

Answer the questions you've been asked so that we can help you.

If you won't help us help you, it's going to be hard to fix your problem, and you may just make it worse (or fail completely).

Teeman   1 mW

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Re: 72 Volt battery problem.

Post by Teeman » Aug 03 2020 12:47pm

Again, Thanks for all your help. So far as to taking this battery apart, That is not going to happen Its just not in my dna.I did levee the battery on charge out side and its up to 74.9 volts so it is balancing, I will let go to see what happens, If it works great if not i will be looking for a repair shop.

markz   100 GW

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Re: 72 Volt battery problem.

Post by markz » Aug 03 2020 1:37pm

Teeman wrote:
Aug 03 2020 12:47pm
Again, Thanks for all your help. So far as to taking this battery apart, That is not going to happen Its just not in my dna.I did levee the battery on charge out side and its up to 74.9 volts so it is balancing, I will let go to see what happens, If it works great if not i will be looking for a repair shop.
I hope you have the money to buy a new battery from a reputable builder such as Grintech www.ebikes.ca or any other reputable builder, like www.em3ev.com or any of the other battery suppliers that build good batteries like unitpackpower on aliexpress/alibaba or OSN Power, easy information to gleen from the search function.

If you can't take simple measurements from a $5 digital multi meter then its going to be very hard to know whats going on with the battery, it could be very out of balance. Do not drain it, you could damage it because you do not know whats wrong. You could very well bring a group of cells to 1V or 2V which is not good at all, 3V is about the lowest you want to go with certain types of chemistries, 3.20V is healthier. You need to figure out the voltages with a meter, write it down in sequence then get back to us. Charging could do the same damage, your balance charger could be out of calibration, if its the cheap RC kind it could very well be.

If you can't take off the shrink wrap or hard case to inspect the connections, inspect for physical damage - Which is all very simple stuff, that can be dangerous yes, if your wearing loose jewellery, have metal stuff on your desk like scissors, screwdrivers, paperclips loose. I like what AvE does on his channel, he tapes his marriage ring up with tape when taking apart a battery.

Then just buy a new battery, not some random buy on ebay because its cheaper.

Teeman   1 mW

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Re: 72 Volt battery problem.

Post by Teeman » Aug 03 2020 4:34pm

Again So much thanks, The battery is up and running, The problem the hole time was the charger. I have 3 other 72 volt battery so i made a connector for the new battery, and it worked.Right now the battery is at 84.4 volts. I am going for a ride.

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