cable size for LiFePO4

Ponderosa Pete

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Feb 1, 2021
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I have two 280ah batteries in series and a 3,000 watt inverter. How do I determine the cable size I need to connect to my batteries?
 

Steve_S

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It depends on the system voltage, 12,24,48 which immediately impacts the Amperage that has to be handled.
The wire length required from Battery to Inverter also adds to the factoring, Resistance of the wire over distance has to be compensated for.
3000W @ 12V=250A, @ 24V=125A, @ 48V=62.5A (not corrected)

Wire is not cheap, especially good proper fine strand copper wires. Generally not something to be relacing if wrong choice made or in the future if/when upgrading. Don't knock it, most folks do upgrade within 2 years of initial install because they realize they need a "bit more" than what they anticipated OR their situation changes (ie have kids or something). I had to upgrade all of my 2/0 Wire to 4/0 when I upped from my 3KW Inverter to 4Kw and it cost hundreds of extra dollars I could have avoided. Bigger wire also reduces wire resistance which is a good thing, in the DC Power world.

If these batteries are DIY'd and you are running 24V or 48V why on earth would you set up Series Batteries ? Series only ups voltage not storage capacity and is risky for a few different reasons. Batteries in Parallel add capacity not voltage and share load/charge evenly.
 

Ponderosa Pete

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Feb 1, 2021
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I'm using a 12v system. DIY. 4 cells = 12.8v and 280A. So I assume I'd put the 2 batteries in parallel? The Inverter is within 2' of the batteries and I was planning to use 4/0 wire. Does this all sound right?
Thanks much.
 

Steve_S

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I would build 2X 12V/280AH battery packs, each with a BMS & Fuse. Set then in Parallel to get 12V/560AH and to have them share the Load/Charge which reduces "pack stress" and also provides a bit of fault tolerance.

To do this I always suggest that you build each battery pack as if it is the only battery in the battery bank, so it can handle the full Charge/Discharge capacity of the system because One Pack may disconnect for any number of reasons leaving only one pack standing.

The problem is getting a 4S BMS with enough capacity to handle that kind of amperage "safely". Rating Labels leave much to be desired, especially with FET based BMS'. Sadly the Overkill BMS only goes to 120A. I cannot recommend a 12V/300A BMS, I'm a 24V guy using Contactor Based BMS' which are more costly.

The wire gauge at 4.0 is GOOD especially for that distance. Do remember that it CANNOT be tight, it needs slack, also to bind the wires together to prevent/reduce EMF/RFI noise, which DC makes when separated. When it comes to this kind of Key Wiring I can only suggest high end stuff like Royal Excelene Welding Cable by Southwire. It is more costly but one of the best wires that can be used for solar systems.

Remember: Battery Pack to Fuse, to Common DC Bus Bar, to Breaker, to Inverter/Charger.
 

Sanwizard

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I would build 2X 12V/280AH battery packs, each with a BMS & Fuse. Set then in Parallel to get 12V/560AH and to have them share the Load/Charge which reduces "pack stress" and also provides a bit of fault tolerance.

To do this I always suggest that you build each battery pack as if it is the only battery in the battery bank, so it can handle the full Charge/Discharge capacity of the system because One Pack may disconnect for any number of reasons leaving only one pack standing.

The problem is getting a 4S BMS with enough capacity to handle that kind of amperage "safely". Rating Labels leave much to be desired, especially with FET based BMS'. Sadly the Overkill BMS only goes to 120A. I cannot recommend a 12V/300A BMS, I'm a 24V guy using Contactor Based BMS' which are more costly.

The wire gauge at 4.0 is GOOD especially for that distance. Do remember that it CANNOT be tight, it needs slack, also to bind the wires together to prevent/reduce EMF/RFI noise, which DC makes when separated. When it comes to this kind of Key Wiring I can only suggest high end stuff like Royal Excelene Welding Cable by Southwire. It is more costly but one of the best wires that can be used for solar systems.

Remember: Battery Pack to Fuse, to Common DC Bus Bar, to Breaker, to Inverter/Charger.
Can you explain why you would put the battery disconnect switch AFTER the common bus bar? I would think if using two battery banks, the off switch of each battery should be before the common bus? That way, you can turn off each battery individually for maint.

Does that make sense?
 

Steve_S

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Well, it depends on the particular desired setup.
You can put a switch between every pack & Common DC Bus to isolate each pack.
You can put a Switch after the DC Bus to disconnect the entire Bank from the gear (most common set)
You can set a Multi-Switch (like I did) which allows me to flip from the LFP bank to the FLA bank or BOTH (not that common).
Some folks have Heavy Anderson Connectors between Battery Packs & Common DC Bus so they can physically disconnect packs from the BUS.

Every system is a bit different depending on usage (vehicular/mobile to fixed residential or .... and then a myriad of possible connections and design variations to suit the purpose. There is no single "this is the answer" generic setup that everyone, everywhere can use.
 

Martijn

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I just received a 200A BMS, for a 24V battery, the cables attached are 2 AWG, I wonder if this is sufficient? I was thinking of using larger cable for the rest, but are these short cables fine for the BMS?
 

Sanwizard

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I just received a 200A BMS, for a 24V battery, the cables attached are 2 AWG, I wonder if this is sufficient? I was thinking of using larger cable for the rest, but are these short cables fine for the BMS?
How often will you be pushing 200amps? Most folks swap out the cables for something that matches their workload.
 

Martijn

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How often will you be pushing 200amps? Most folks swap out the cables for something that matches their workload.
Not often, I will be using a Victron 24/3000 inverter, with a cont output of 3000W, but peak of 6000W. There will be an induction stove of 3500W, which will hardly ever reach that output, I think.

3000W/24V would be 125A.

I'm just a bit surprised that a 250A (not 200, my mistake) BMS comes with 2 AWG cables.
 

Sanwizard

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Not often, I will be using a Victron 24/3000 inverter, with a cont output of 3000W, but peak of 6000W. There will be an induction stove of 3500W, which will hardly ever reach that output, I think.

3000W/24V would be 125A.

I'm just a bit surprised that a 250A (not 200, my mistake) BMS comes with 2 AWG cables.
Those high thread count silicon cables can take a lot of abuse.
 

Sanwizard

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No. Its a matter of using the correct cables and fuses based on your load. The fact that Chinese BMS makers use under sized cables for their rated loads is just something we need to deal with.
 

Sanwizard

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My 200amp Heltec BMS comes with two 7awg wires per path. That is not a problem for me, as my load will never be over 50 amps at most.
 

Martijn

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I ALWAYS overcable inverters and winches.
Wire is much cheaper than trouble.
Electric winches are evil. 😁 (our truck weighs 4500kg, not including the camper box)

No. Its a matter of using the correct cables and fuses based on your load. The fact that Chinese BMS makers use under sized cables for their rated loads is just something we need to deal with.

So first thing we need to do, is try to remove the existing cable from the BMS, and with that, trying to keep the copper piece that the cable connects to, or creating your own piece for a larger cable.

OK, will do. 😉
 

DJSmiley

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I recently installed a 24V system in a boat, with a 24V 100A BMS (The 8S JDB one, similar to the Overkill one, but the JDB's are more easy to get in NL and cheaper than shipping an Overkill one across the ocean)

That one has only 2 (silicon) wires of 10AWG (which equals to approx 2x 6mm2 here in Europe). But since those are silicon, they can resist more heat, and since they are short, the voltage drop is still not that much.
I've run 50mm2 to the invertor as per Victrons specifications. Customer wasn't going to pull >2400W, so a 100A BMS is fine for this setup, the Victron was already installed in his boat, along with the wiring.

If needed, he can always add a second set with a BMS to make it capable of (approx) 200A in total, but I doubt that's needed. At least not for now based on the customer's setup. (Also has approx 300W of solar, and the boat is engine driven, so when out in the water the alternator also charges)

Basicly I swapped the old 4x 230Ah lead (2P2S) for 1x 300Ah 8S LFP. Victron charger re-configured with MK3 for LFP, replaced the solar charge controller (Old one was broken - It charged the lead acid bank to 35V... end of the bank), and reconfigured the alternator charger (old one, but it had adjustable boost/float so that worked out fine)
 
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