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

Maniac_Powerwall

Maniac_Powerwall

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Dec 20, 2018
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Can we combine two cheap 7S BMS instead of one. Two like these in series:


image_hsvofi.jpg
 
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
 
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?
 
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.
 
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 :)
 
Once again no. You dont get any protection. You only get the balance part doing above and then its not a BMS.....
 
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.
 
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
 
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
 
 
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
 
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?
 
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
 
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?
 
of course, you can parallel them and going that way you can control and manage strings separately...
 
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