Could we use two 7S BMS instead of 14S BMS

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Can we combine two cheap 7S BMS instead of one. Two like these in series:


image_hsvofi.jpg
 

Korishan

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No. they will not work equally. I mean, you *could*, but you won't get balanced results. You could still end up with 1/2 of the string out of balance with the other 1/2
 

daromer

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No.

As Korishan said. basically you can end up with 7 of the cells at 4.1V and the others balancing at 4.25V....

beware of that BMS is a protection and management system. its the last defence to protect your gear and your self. Do you really want to cheap out on that?
 

Cherry67

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No AS Well.
The real Problem ist Not the balancing.
Nightmare Happens when one switches Off due to over or undervoltage.
Maximum ratings of the Mosfets ist exceeded.
 
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No i just got one of these for testing right now. And i wanted to know if i could use them that way. Just for interest. Right now thinking about building a little system for testing cells via solar with a backup battery made of the 18650s i have right now. This should be very cheap or made from things i could use later on. So im thinking of using what i got or wich things i would need for sure :)
 

Walde

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Use 1x7s + 1x 8sfor 14S
and the last and the first double

or 2x7s for 13s

:D
 

daromer

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Once again no. You dont get any protection. You only get the balance part doing above and then its not a BMS.....
 

Korishan

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Agree with daromer and mike. The bms is there to do a job. If you piece together two units, you've effectively took part of that protection away.

As mentioned earlier, if the bms units are not exactly the same values, 1 of them could shut off before the other one. This would cause for sudden issues. Spikes in voltage could be an issue, hammer effect could happen (where the units kick on/off rapidly), or the smoke could be released.

Most bms units are designed to work alone. They handle a full string (the system voltage) of the battery, not portions of it.
 

Korishan

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You could check out Stuart Pittaway's diyBMS setup. Adam Welch on YT did several videos on the project.

I think this is the first video in the series.
https://github.com/stuartpittaway/diyBMS And this is Stuarts software
 

Charly144

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I am going to use this one. Ive got it for eur 33,- on Black Friday...theres a lot to befound about it in several forums under the header china bms

Regards
Karl

14S 40A/60A active bms 2017 new Li-ion smart bms pcm with android Bluetooth app UART bms wi software (APP) monitor
https://s.click.aliexpress.com/e/cpMzWuSc
 

daromer

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Charly144

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I do have 14s80p and will have 5 strings (each 130-150Ah)in parallel. Every stringe managed by one single bms of this type...

Its the one shown in daromers video.

A friend is writing an integration on a raspi which consolidates information and functionality of all single bmses

Best
Karl
 
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But we could use bmsses like these. Either mine or the other in series without a problem? I think i wouldn't use these small bms for a big powerwall like yours. Can it balance it good?
 

daromer

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Maniac you cannot serie or parallel bms systems. They Will not work as you expect. Buy a proper bms instead.it doesnt cost much more to get 14s instead of 2*7s
 
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Maybe my fault. I meant could we parallel packs equiped with these? Like some people paralleling packs from hoverboards. I think they got very similar BMS system?
 

Charly144

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of course, you can parallel them and going that way you can control and manage strings separately...
 
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