72v li-ion battery problems

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Halifaxe   1 mW

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72v li-ion battery problems

Post by Halifaxe » Apr 29 2020 8:27am

Hello, I suspect there might be something wrong with my battery because of the cell diffrence under load as shown in the pictures below. ImageImageImage

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goatman   10 MW

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Re: 72v li-ion battery problems

Post by goatman » Apr 29 2020 9:13am

I find it interesting that the odd numbered pgroups voltages are different than the even numbered pgroups or column 1 is higher than column 2. maybe its the way the bms operates and not the battery.

Halifaxe   1 mW

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Re: 72v li-ion battery problems

Post by Halifaxe » Apr 29 2020 9:39am

I also find that very strange.

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bww129   100 mW

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Re: 72v li-ion battery problems

Post by bww129 » Apr 29 2020 9:51am

What battery is it? Are the BMS cell voltage taps wired in exactly the same position for each cell? It doesn't take much extra added resistance to mess up a measurement at high currents: 20A through 1mOhm is 20mV.

Halifaxe   1 mW

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Re: 72v li-ion battery problems

Post by Halifaxe » Apr 29 2020 10:05am

Its a 72v li-ion battery with 140 LG INR18650MHI 3200mah - 10A cells. As far as i can recall the taps are not wired in exactly the same position.

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bww129   100 mW

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Re: 72v li-ion battery problems

Post by bww129 » Apr 29 2020 10:33am

Do you know the voltage accuracy of the BMS? Some of the higher quality ICs used in BMSs are accurate down to 1-2mV, but that depends on implementation as well.

You probably shouldn't worry too much about small difference in cell voltage difference under load, especially if the pack is new and uses quality cells. Since you have the ability to monitor cell voltages you can track them over time which will be a better indicator of cell health.

Halifaxe   1 mW

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Re: 72v li-ion battery problems

Post by Halifaxe » Apr 29 2020 10:49am

Okay, that is reassuring, the voltage accuracy is 3mV. Thanks for your fast and informative responses.

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bww129   100 mW

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Re: 72v li-ion battery problems

Post by bww129 » Apr 29 2020 11:16am

goatman wrote:
Apr 29 2020 9:13am
I find it interesting that the odd numbered pgroups voltages are different than the even numbered pgroups or column 1 is higher than column 2. maybe its the way the bms operates and not the battery.
Some of the ICs used in BMSs have dual ADCs and can sample two voltages at the same time. It is possible that the cells in column 1 were sampled by ADC1 and cells in column 2 were sampled by ADC2, hence what appears to be two different groupings, but it isn't possible to know for sure without any knowledge of the BMS, what ICs it uses, and exactly how it is wired.

john61ct   100 GW

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Re: 72v li-ion battery problems

Post by john61ct » Apr 29 2020 2:01pm

did not read all

A balanced pack is only "balanced" **at a particular voltage**

Cells vary, both in capacity and in resistance / performance, the relation between SoC and voltage level.

That is why balancing is needed.

Doing so does not make the above variations go away, just "lines them up" at the top (or bottom or middle)

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thundercamel   1 kW

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Re: 72v li-ion battery problems

Post by thundercamel » Apr 30 2020 10:37am

What John said. From what I've seen with my batteries, it's normal for them to all be balanced uniformly at the top of charge. They get out of line a little as the state of charge decreases to empty, but then they all line up again next time they are charged.
My Ebike builds - Existing bikes, affordable motor kits, self built 14s6p batteries - Now with more recumbent!

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