8 x Pylontech batteries, best to wire 2 banks of 4, or 4 banks of 2?

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alaskannoob asked Mar 10, '23

8 x Pylontech batteries, best to wire 2 banks of 4, or 4 banks of 2?

I've got 8 x US5000, 4 long positive cables, and 4 long negative cables (same lengths in either scenario). The question is where to put those cables in the battery bank. The network cables will talk to the system and treat it as one big battery bank of 8 regardless, from what I understand, but I have some questions on differences in electricity flow between the two options.

My MPPTs can only charge at 400A, and I will set the inverter to not take more than 400A. So the cable ratings shouldn't be an issue.

The 2 banks of 4 is an option I've seen some do, also referred to as "double stacking." The questions I have on this option, what about the two batteries in the middle of each stack, will they get unbalanced since the current goes to the top and bottom batteries first? Also, does this drive a bigger circuit breaker/fuse for the cables because there are 4 batteries that can deliver short circuit current rather than the other option which would be limited to only 2 batteries providing current per set of cables? The manual says each battery has a short current/duration time of <2000A/1ms.

The Victron Wiring Unlimited Guide leads me to believe 4 banks of 2 might be better?

Because it suggests wiring batteries in parallel with one of these methods:

Thoughts?

Pylontech

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4-of-2.png (815.5 KiB)

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Answer 1 · 2023-03-10T19:55:41Z

patricknl answered · Mar 10, 2023 at 07:55 PM

4x2 would be my preference. Pylontech mentioned one pair of cables per two batteries somewhere in the manual I believe. Using two for two pairs is the same but as you said: there are units in the middle that aren't directly connected. The BMS should monitor this and compensate the powerusage of the outer two but still.

In case 40kWh isn't enough you can add one extra per pair without too much hassle, only needing an extra set of cables per battery added. With 2x4 you would need to build an completely new stack.

the ICU rating is 10kA upto four units so that wouldn't change I believe... With eight you need a 20kA icu rating.

Just my two cents :)

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Answer 2 · 2023-03-10T20:06:20Z

alaskannoob answered · Mar 10, 2023 at 08:06 PM

I tend to agree. I asked the battery dealer about this and he said we should go with the 2x4 bank as he says it's more "conventional." He pointed me to this image from the manual, but it's from Section 5.10 which is about "multi-group mode" so I'm not sure if it's applicable or not.

So far the best reasoning I've seen is for 4x2 though. But there are people wiring them in both ways online.

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Answer 3 · 2023-04-20T05:33:46Z

wushoubo answered · Apr 20, 2023 at 05:33 AM

it is based on the size of you inverter. For example, 10KVA, the max. current is about 220A, then two pairs is enough. While you have a 15KVA system, 4 pairs can meet the demand.

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Answer 4 · 2023-04-20T06:51:20Z

kevgermany answered · Apr 20, 2023 at 06:51 AM

I'd go for four pairs. Less impact if you need to remove a battery. You lose two, not four. Ignoring BMS control.

You're not showing the data cables, need to take them into account.

I believe @Alexandra has set up big banks, hope she chimes in.

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alaskannoob commented · Apr 20, 2023 at 08:06 AM

We actually decided to go with each battery connected to the bus individually. Just for an abundance of caution. Less current through the wires that way and if we do have a short circuit, it's only one battery rather that multiple. Otherwise we'd be frequently putting 100A through the cables rated for 125A max, so by going with 8 sets of cables, our cables should last longer and not get warm.

8 sets of cables, 8 t-class fuses close to the positive terminal of each battery.

Probably overkill, but we want to do everything we can do to make this system as safe as possible.

Data cables will have one BMS-CAN cable from one battery to the Cerbo GX, and then a small data cable from that battery down to the next battery, so on and so forth daisy chaining data cables until the last battery.

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Alexandra ♦ alaskannoob commented · Apr 20, 2023 at 06:04 PM

Two thumbs up....

It is not cautious, it is most correct.

I am a definitely a fan of the individual bus bar set up.

People always look at the initial cost, but what is a $35 cable kit and fusing on a $1500 battery? And you are investing in and installing a 10 year system....

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Answer 5 · 2023-04-20T11:36:09Z

ponzoa answered · Apr 20, 2023 at 11:36 AM

In the case of the US5000, they are rated at 75A so 2 x batteries would be able to provide 150A. The Pylontechy cables are rated at some 120A so if your inverter can demand more than this, or your solar array could provide more than this, you should definately run a set of cables from each battery individually to the Bus bars or lynx distributor ensuring that the current rating of the bus bars are sufficient for your max. load plus some margen for peaks which the inverters can work with.

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alaskannoob commented · Apr 20, 2023 at 03:38 PM

The US5000 should be able to provide 100A max continuous charge and discharge per battery. Of course, we'll see what their BMS actually allows beyond their advertising claims...

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Answer 6 · 2023-04-20T13:12:05Z

maa answered · Apr 20, 2023 at 01:12 PM

I vote for 4 stacks

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