Connect lots of small Packs in parallel.


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Bobbych

Member
Joined
Feb 18, 2020
Messages
30
So in this 18650 power wall/pack community it seems the norm is to build large packs of cells in parallel then connect them in series to get the required Voltage.

But if 1 cell went down the voltage of the whole pack would drop significantly and would probably be disconnected by the BMS or to low for the inverter to use.

I recently read somewhere, maybe on here but cant find it now, that maybe best to use lots of 24v Packs (say 7s1p) and connect them all in parallel. So if one pack went down the voltage would not be affected at all but capacity would be reduced by a small amount and it would be far simpler to replace/repair a small 7s1P pack.

I Guess that each small pack would need its own BMS but given the size these would be small BMS's

Or is this a bad idea?
 

OffGridInTheCity

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Joined
Dec 15, 2018
Messages
2,228
One problem with 7s1p and doing 1,000(s) of them for a powerwall is that you would need 1000(s) of BMSs - which is not practical and the expense would be out of this world as DIY typically means you are doing it to save $.

Also, once a 'powerwall' is built properly, it doesn't need individual cell replacement - its more about monitoring the system and perhaps replacing an entire pack that is not compatible - I had that happen trying to mix Sony G5/G7 cells with the others.

Even with small batteries, like a 3s4p that powers my RV toilet - a single BMS is fine. If something goes wrong I'm more likely to buy/find 12 cells and just build a new battery.
 

Bobbych

Member
Joined
Feb 18, 2020
Messages
30
One problem with 7s1p and doing 1,000(s) of them for a powerwall is that you would need 1000(s) of BMSs - which is not practical and the expense would be out of this world as DIY typically means you are doing it to save $.

Also, once a 'powerwall' is built properly, it doesn't need individual cell replacement - its more about monitoring the system and perhaps replacing an entire pack that is not compatible - I had that happen trying to mix Sony G5/G7 cells with the others.

Even with small batteries, like a 3s4p that powers my RV toilet - a single BMS is fine. If something goes wrong I'm more likely to buy/find 12 cells and just build a new battery.
Makes sense to me now. Not a good idea on a large scale. Thanks
 

Korishan

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Jan 7, 2017
Messages
7,075
Generally speaking, no matter what battery design you have, if there are bad cells, then they will all be effected. Doesn't matter if they are smaller packs, or larger packs, big or small strings, etc.
As mentioned, Monitoring is the key here, and with proper monitoring then the problem cell can usually be identified long before it starts causing a major issue.

As far as connecting multiple strings in parallel, that's quite fine and doable. And, it's generally recommended. This way you won't have to bring the whole system down to work on a particular string.

So basically you would have your 7sXp configuration as normal. But, instead of something like 1000p, you'd make them 100p, and then have 10 strings of 7s100p.

Depending on the BMS system you go with, you would either need to have a separate BMS for each string, or a single BMS that has multiple monitoring sensors. Batrium uses a central BMS with multiple monitors called longmons. Another one is Stuart Pittaway's bms, the diyBMS, if I remember the name correctly.

The other option that's possible is that once you connect a string into the master group, all parallel segments are connected with the neighboring sections. So you could have 10x 10p packs that when they are seated in their connections becomes a single 1000p configuration. This does take a lot more planning and prep to do, though. And you have 'always' have to make sure the packs are the same voltage before connecting a pack to the setup. Otherwise sparks can fly, and possibly too high of current load happen.
 
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