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2p7s80p strange setup - need help with BMS
#1
Hi guys,

I need a bit of help. for last 6 months I had a fully working system - 7s80p. It worked like a dream. I've been using it to heat the water in my LazySPa jacuzzi Smile it was great!

Recently I have decided to expand with another line of 7s80p connected to first bank parallely. Don't want to change to 48V at least not for now.

Initially i've used cheap Chinese 7S system and it did the trick, so this time I  got the same brand 14S kit. Since it isn't 14s but 2p7s I have no bloody clue how to connect the sensing wires. Have you got any ideas how to do it? Please? Smile

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#2
if you are staying with 7s you need 2 7s BMS's , if you are moving to 48v your 14s will work. the seller of the bms should have instructions on their web site.
later floyd
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#3
You can either use 2x 7s BMS's, or your cold run balance leads for each of the parallel packs to it's sister string and stay with 1 BMS. However, this puts a heavier load on the current BMS as it will have a harder time keeping twice as many cells in balance. Depending on which BMS you are using, it could be as low as 50mA balance, maybe even 35mA. There are some that are 100, 150 or even 200mA balancing.
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#4
Thank you guys. In the meantime I have ordered active balancer which works in range of 3S to 17S i think that would be some solution, but again 2p7s cause me brain freeze how to connect. It looks like 2x BMS (7s) is only sensible solution Sad.

Are there any other systems like Batrium which can monitor bank/cell individually and not require a mortgage?
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#5
it'll always be 2p7s, no matter which option you go with.

To make them balanced across both strings you'd just run a small wire, about 24awg or so, from string-A pack-1 to string-B pack-1, string-A pack-2 to string-B pack-2. Easy Smile
You could even use those alligator clips to make the connection.

btw, Active balancer won't do you much good on that scale. Also if you "need" that much balancing, you have other issues to deal with first before building the packs. Find the cell(s) that are causing the issue and replace them. Don't keep putting a bandage on them.
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#6
And an active balancer does not change that you need a bms to monitor all packs Wink
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#7
(10-03-2020, 11:57 AM)Korishan Wrote: To make them balanced across both strings you'd just run a small wire, about 24awg or so, from string-A pack-1 to string-B pack-1, string-A pack-2 to string-B pack-2. Easy  Smile
You could even use those alligator clips to make the connection.

I thought about doing that but wasn't sure if that would work - sounds like a solution Smile

I ordered active balancer to work as a BMS since it has flexible range 3s-17s. The balancing wasn't main reason as the packs/banks are fairy leveled. All have been re-worked after 6months of heavy usage. In the meantime I have been testing new 7 banks and re work where needed. The system now is really stable. 

If I pick up your brain again please. Can I hypothetically connect my system as 7s(2s80p)? Would that work?
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#8
As daromer says, an Active balancer is not a bms. You need a bms in the conjunction with the balancer.

You "can" tie several 2s bms units together in series, but why would you? Unless I'm not understanding your question.
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#9
7s80p in series with another 7s80p is fine that makes 14s80p.
You would need 1x 14s BMS + 1x 14s balancer (active or not, maybe with built in BMS).

Trick is the ground 0V connections, that's why using 2x 7s BMS in series is a bad idea, if you connect eg monitoring to the "upper" BMS you're starting at ~+24V & may get shorts to 0V easily. Very likely to get magic smoke...
Similarly you can't use most 14s BMS (except batrium would be OK) to monitor 2x 7s80p in parallel. A 14s BMS will expect cells to "ladder" from 0V to 48V not "step down" to 0V in the middle. Another almost certain magic smoke event....
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