8 Pylontech US5000 & 8 Long Cables -- Wire Them in Two Banks of Four, or Four Banks of Two?

[AlaskanNoob]

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.

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 would seem to favor 4 banks of 2 for the balancing issues...

[Arandoza]

Check the US5000 manual

• Recommended     Charge/Discharge Current (A) 80*
• Max. continuous    Charge/Discharge Current (A) 100*
• Peak Charge/Discharge Current (A) 101-120@15min   121~200@15sec

 

• 2) For External cable kits:
• NOTE: Power and communication cables connect to inverter belongs to an
• External Cable Kit, NOT include in battery carton box . They are in another
• extra small cable box. If there is anything missed, please contact dealer.
• - 2 * 2000mm power cables (4 AWG, peak current capacity 120A, constant 100A) and communication cable for each energy storage system

The 2 banks of 4 wiring digram you have is incorrect. on the 1st of 4 batteries positive connected to Busbar and on 4th battery negative to Busbar, not both positive and negative to busbars on battery 1 and 4.

However besides for the connections being incorrect 4 x batteries at 100amp each 400amps per set of 4 or Peak 15s 800amp discharge will melt your cables instantly :0 

Also the US5000 Max continuous charge is 100amp per battery, the Cables from the battery to the busbar are "rated" as per the manual at 100amp continous or 120amp peak.

So stricktly speaking you should have  set of cables per battery. When pairing the batteries in sets of 2 and using one set of cables you are potentially sitting with a Cable rating of 100amps continous and the batteries (2 of)  being able to supply 200amps continous.  Its a good recipe for a potential fire  Also seen comments that even thought the pylontech cable sets are rated at 100amps folks have noted from 60 amps upwards they start getting warm, and people seemed to have concerns that the 100amp continous rating is a bit optimistic??

 

AFAIK regarding balancing of LifePO4, this happens automatically, the BMS on each unit will control this. So its not like LEAD ACID / AGM ETC. Cable lengths and resistance will still apply in terms of voltage readings etc. However with the BMS connected to your inverter, this will be reported by the BMS to the inverter so not really applicable? Also all your + and - cables from the batteries to the Busbars from each pack will be 2000mm? (Ext Cab Kit)  so this should not be an issue?

[SYC]

On 2023/03/10 at 8:23 PM, AlaskanNoob said:

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.

 

[AlaskanNoob]

2 hours ago, Arandoza said:

 

However besides for the connections being incorrect 4 x batteries at 100amp each 400amps per set of 4 or Peak 15s 800amp discharge will melt your cables instantly :0 

Also the US5000 Max continuous charge is 100amp per battery, the Cables from the battery to the busbar are "rated" as per the manual at 100amp continous or 120amp peak.

So stricktly speaking you should have  set of cables per battery. When pairing the batteries in sets of 2 and using one set of cables you are potentially sitting with a Cable rating of 100amps continous and the batteries (2 of)  being able to supply 200amps continous.  Its a good recipe for a potential fire  Also seen comments that even thought the pylontech cable sets are rated at 100amps folks have noted from 60 amps upwards they start getting warm, and people seemed to have concerns that the 100amp continous rating is a bit optimistic??

 

AFAIK regarding balancing of LifePO4, this happens automatically, the BMS on each unit will control this. So its not like LEAD ACID / AGM ETC. Cable lengths and resistance will still apply in terms of voltage readings etc. However with the BMS connected to your inverter, this will be reported by the BMS to the inverter so not really applicable? Also all your + and - cables from the batteries to the Busbars from each pack will be 2000mm? (Ext Cab Kit)  so this should not be an issue?

"The 2 banks of 4 wiring digram you have is incorrect. on the 1st of 4 batteries positive connected to Busbar and on 4th battery negative to Busbar, not both positive and negative to busbars on battery 1 and 4."

I don't see why that is incorrect.  The positives are all wired to the positive bus bar, and the negatives all to the negative bus bar.  But if you can explain why that is incorrect, I will pass that on to the Pylontech dealer who came up with it for him to look at.  Can you explain this more?

"4 x batteries at 100amp each 400amps per set of 4 or Peak 15s 800amp discharge will melt your cables instantly :0 "

Good point, and one that applies to both of these options I asked about.  Yes, 400A per set of 4 batteries going through two sets of wires which means 200A could go through the wires and that would be an issue.  The MPPT and the Inverter would need to be adjusted to limit power due to cable rating.  I agree what would make the most sense would be each battery wired to the bus separately, which is a very very very poor design by Pylontech for this battery system.  But it would be handled by limiting the power going to the bank, or being discharged from it.  The Peak discharge of 800A (or 200A per battery) if it happens is a design flaw by Pylontech since even with one battery you would have that issue.  Unless, even more strictly speaking, you used two sets of cables (4 total) for every single battery.  Which shows how ridiculous Pylontech's cables are.

Great point on the BMS.  I think the Wiring Unlimited guide was meant for lead acid and you're right the BMS should make that not an issue.  So I'm thinking the 4 banks of 2 is perhaps better because it won't depend on the BMS but I agree it shouldn't matter.

Thanks for the info, and I hope you'll explain further why you think the 2 x 4 version is wired incorrectly.

 

[Arandoza]

Have a look at the PylotechUS5000 manual page 18 onwards, none of the battery diagrams are wired like the proposed 2 by 4.

 

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwip_MyNlNf9AhVOY8AKHaI1A6AQFnoECAsQAQ&url=https%3A%2F%2Fwww.teci.co.za%2Fwp%2Fwp-content%2Fuploads%2F2021%2F02%2FUP5000-V1.0-Product-Manual.pdf&usg=AOvVaw3jG5P_j-9SjJZRRyrnIREo

 

in terms of the power cables. It's always + or - on the first battery to the Busbar, and also one + or - from the last battery to the busbar. So drawing power across the packs + on first pack to - on last battery. And the BMS worries about the Balancing of the packs, and the cells within each pack.  

Your diagram for the 2 by 4, shows both positive and negative on the first and last packs being connected.  Also the 2 x + and 2 x - terminals on each battery pack have a common busbar which they are connected to behind the faceplate on the battery.

In the diagram for the 2 by 4, it would appear one is pushing and or pulling power from both ends of the combined pack? makes no sense?

I cant see a benefit, two positive cables on the first battery and two negative cables on the last battery will have double the current capacity? 200 amps?

However the interconnecting cables, (4 and 10) between the packs,  can still only carry 100amp, so why?  It just makes no sense?

Check with your pylontech dealer what their reasoning is??

Also bear in mind all batteries data cables would be interconected so they can be monitored and would be aware of each other. So the 2 x 4 packs or 4 x 2 packs is not really relevant, the only thing that the BMS cannot communicate is how the batteries power cables are physically connected to the inverter and or busbar, so it cannot protect overload conditions on the power cables.

How one connects the Power cables is really about max load carrying capacity, and protecting your power cables from Heat (increases resistance in cables) , and overloading. Ensure all packs have the same length cable connecting them to the busbar, and try to keep only as long as needed, increased length means increased resistance, and can create voltage drop if long enough.

Ideally you could use one set of cables from each battery pack, back to the busbar, thus ensuring the cables 100amp limit connot be overloaded, one can then safely use all available 8 x 100 amps (38.4 KW charge / discharge : 48v x 800amp ) all the batterys would still be connected via the data cables from 1 to 8 and would communicate as normal. And your serial/console or CAN / RS485 cable back to inverter for reporting from the batterys to the inverter of the SOC / Charge / discharge rate / and any other parameters it may supply.

Going back to the recommeded details from the manual :

• Recommend Charge/Discharge Current (A) 50                    - Two packs interconnected and then back to busbar (should not not exceed to 100amp continous limit) 
• Max. Charge/Discharge Current (A) 74~89@60sec             - Two packs interconnected and then back to busbar (would exceed the 100amp cable rating, by 78amps after 60 seconds the BMS should shutdown the pack(s) / drop the load? or other protection (fuse) before / after the bus bar, what happens if dead short? )
   
• Peak Charge/Discharge Current (A) 90~200A@15sec         - Two packs interconnected and then back to busbar (would exceed the cable rating, by 80 to 300amps  after 15 seconds the BMS should shutdown the pack(s) / drop the load? or other protection (fuse) before / after the bus bar, what happens if dead short? )

You would have to look at your inverters max charging capabilites to consider if you would be able to exceed the max Charge conditions, which I think in many cases is not so likely? However the discharge scenario is more likely, if there are any dead shorts and there is no protection to cut out supply from the batterys one could potentially,  reach the max discharge rates, melt cables etc?

 

One last thought is that if you ever want to add more battery units, the newest battery unit should be the master, as it's BMS should be backward compatiable with all existing batteries, so consider building your rack from the ground up, so there is space to add at the top of the rack later, without having to repack the rack   Also something I have noticed is that the packs closest to the floor run cooler, especially in summer.

All The best  

[Micdemas]

I would say 4 banks of 2 is better because of the peak current in your cables(so it is 100 amp per stack 50 amp per battery). I think the problem with the first arrangement is you can't guarantee they will share the current equally (it could be 200amp in the one set and 0 amp in the other), everyone is saying it is wrong, I thought so too, but I just saw it recommended in the LV-HUB manual(https://portal.segensolar.co.za/reseller/docs/LV Hub Application Note.pdf) which you might need if you are using CAN-BUS....

I hope you find the right solution and good luck.

[Sc00bs]

@AlaskanNoob There is another option, run another set of cables from the open port on the packs fig 6 & 7 in the diagram. 

They will need to be separately fused but you will double the amperage to 200A that you can draw from the packs without having to go to busbars. 

I was educated about this option a few months ago on the forum