Help wire my giant 20kWh pack in 3s

jestronix

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Nov 7, 2017
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54
I'm building a 12v system, long story no I can't go higher voltages and yes there are big advantages all round for 48v.

Max current will be 1400w continuous and 2800w spikes.

Ok so I'm building 80p packs 4x20 cells. 33 packs ! So how to wire them?

I've had thoughts of big lugs all in parallel for all 11 packs per serial. Then a lug that will take 1400w into each inverter. Running 2 inverters all up.

What are the advantages of a big pack like this ? I know the disadvantages:)

Disadvantages

Big amps !
Discovering a bad pack is harder! Just so much parallel goodness 800p !
Efficiency loss.
Insane time to balance - can I still use longmons ?
Loose some range in voltage , eg I'll only go down to 10.5v and 3.5v a cell.

Advantages

Only 3 cells to monitor ! Take a long long time to go out of balance.
Can use my current inverters and solar chargers and all my other 12v stuff.
Cells are treated lightly and cycle volt range is conservative
Just the I'm different and mad factor.
 

Korishan

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Jan 7, 2017
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Ok so I'm building 80p packs 4x20 cells. 33 packs ! So how to wire them?
Really big bussbar connections! No, seriously, you need large ones. For cost reasons, it might be better to use copper pipe and use that as the bussbar interconnect. Because that much wire is going to be expensive since you are going to want to go with at least 2/0 wire because you'll have large runs of it.

Can use my current inverters and solar chargers and all my other 12v stuff.
Personally, I'd go with a 48V setup, then convert to AC to step it down to 12V (this is so you can truly use the full amp range), then rectify it back to DC. In the end, I think it'll be cheaper, less headaches, and a whole lot easier to manage and trouble shoot.
Then, as time/money permits, migrate your 12V inverters to 48V ones.

But, that's IMHO.
 

daromer

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1400W is alot on 12V but its doable of course. 48 Is better in that sense but we all use what we got. Just make sure to have enough bus bars. Especially in the connection point if you run many smaller packs. I would though do bigger packs instead of all those small ones.

No problem with Batrium and longmons. You can have up to 250 Longmons on 1 system so I dont see the problem.
In general 2-400Ah per longmon depending on setup. So if you got more Ah per pack in total just add more longmons. You can run 1 or 2 or even 10 longmons on same pack. Though in that case it might be better to split the packs up so you get pack specific monitoring and can find the packs that might have issues if thats the case.


Running 3s is NOT easy. if your low limit is 10.5V you wont be able to use alot at all from your pack... If we say that 10.5V is lov cutoff thats 3.5V per pack
But you will atleast have 0.3-0.4V voltage drop. Thats atleast 3.6 or in worst case 3.7V. Thats not much use of the cells since 3.7V is way above 50% left in resting voltage. Thats the BIG issue going 12V systems on LiIon with nominal of 3.7V.

In that sense 24V or even 48V is alot easier to work with. And going higher voltage on the battery bank just to DC/DC down for the inverter is no sollution if you ask me. In that case its better to sell the gear and buy new.

So if you still going to do 3s and 12V make sure you have as little voltage drop as you can ! BIG busbars and all.
 

electrotech

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Dec 7, 2017
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16
Could you change the battery bank to 24V, get a 24V inverter & use a 24 > 12V step down for any other 12V gear?

Any cables/bussbars between banks need to be equal lengths to keep pack aging equal.
 

DarkRaven

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Sep 2, 2017
Messages
1,235
126A continuous and 252A peaks, you are looking a 100mm solid copper busbars (5x20mm) or 70mm stranded wire. Preferably even a bit higher to minimize voltage drop. And make sure you have connections as short as possible or you have to use even bigger stuff. You have virtually no tolerances to play with as you will hit the low voltage cutoff early anyway so every bit of voltage drop will make that happen even sooner.
 
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