Battery wire length

Hi again. I now have 2 x 200 ah 24 volt life04 batteries and would like to know if, as I’m wiring them in parallel, positive to positive, negative to negative and then positive from battery A to bus bar and negative from battery B. Do all the wires have to be the same length? Also how would you fuse the batteries to a 3000 watt hybrid inverter? Thanks

If the round trip distance from inverter +ve to inverter -ve is same distance whichever battery the current goes through you are good to go. It doesn't matter if, for example, the +ve cable is longer than the -ve cable.

You need to size the fuse according to the current carrying capacity of the cable, rather than the inverter. Fuses, protect cables from overload.

When installing battery packs in Parallel to a common DC Bus we highly recommend using the exact same length of wires (+) & (-) from Packs to Busbar. If installing a Fuse/breaker between battery pack & busbar, that is extra in-between. Fuses/Breakers are always on the (+) and Shunts are always on the (-).

Each 200AH Battery should be fused according to max output potential. So if the BMS can handle 200A "Continuous" then 200 X 1.25 = 250A Fuse/Breaker should be used. With 24V You can use either MEGA, Class-T or MRBF Fuses without issue. I am 24V and religiously use MRBF because they are a great match. DO NOT SKIMP HERE ! Quality Fuses/Breakers can & will save your gear & life ! Crap knock offs can harm & injure !

SPECIAL NOTE !!!
There are MANY busbars out there and an awful lot of them are JUNK ! Either too thin copper or poorly made or using non-thermal plastics... Boy we have seen the gamut of crap being flogged. Those cheap "pairs" for < $50 USD are the worst offenders that people buy cause it "looks" like a good deal. I Very Highly recommend quality busbars like PIKE Industries or Victron/BlueSea because you really do get what you pay for. I personally use PIKE Industries ones. https://pikeind.com/industries-busbars/



Pike Industries busbars.
Top one offset with 3/4" so that the MRBF's clear over the accompanying (-) wire
(+) & (-) wires from Batt to Busbars are kept tight together to prevent EM/RF noise.










An A-Typical Component system with a parallel battery bank

Steve_S said:

When installing battery packs in Parallel to a common DC Bus we highly recommend using the exact same length of wires (+) & (-) from Packs to Busbar. If installing a Fuse/breaker between battery pack & busbar, that is extra in-between. Fuses/Breakers are always on the (+) and Shunts are always on the (-).

Each 200AH Battery should be fused according to max output potential. So if the BMS can handle 200A "Continuous" then 200 X 1.25 = 250A Fuse/Breaker should be used. With 24V You can use either MEGA, Class-T or MRBF Fuses without issue. I am 24V and religiously use MRBF because they are a great match. DO NOT SKIMP HERE ! Quality Fuses/Breakers can & will save your gear & life ! Crap knock offs can harm & injure !

MRBF Terminal Fuse Block - 30 to 300A - Blue Sea Systems

Easily and economically satisfies ABYC 7 circuit protection rule by mounting on a 3/8 battery post, battery switch or bus bar.

www.bluesea.com

SPECIAL NOTE !!!
There are MANY busbars out there and an awful lot of them are JUNK ! Either too thin copper or poorly made or using non-thermal plastics... Boy we have seen the gamut of crap being flogged. Those cheap "pairs" for < $50 USD are the worst offenders that people buy cause it "looks" like a good deal. I Very Highly recommend quality busbars like PIKE Industries or Victron/BlueSea because you really do get what you pay for. I personally use PIKE Industries ones. https://pikeind.com/industries-busbars/

View attachment 219339

Pike Industries busbars.
Top one offset with 3/4" so that the MRBF's clear over the accompanying (-) wire
(+) & (-) wires from Batt to Busbars are kept tight together to prevent EM/RF noise.










An A-Typical Component system with a parallel battery bank
View attachment 219340

Click to expand...

Thank you very much. That real helps

I have a quick question about this. Do you add the class T fuse length together with the positive cable to get your total length ? Or is the length defined by the section of positive cable on each side of the class T added together?

Sorry for jumping in but it’s a part of solar systems that always confused me.

Steve_S said:

When installing battery packs in Parallel to a common DC Bus we highly recommend using the exact same length of wires (+) & (-) from Packs to Busbar.

Click to expand...

Technically it works if all the positive wires are one length and all the negative wires are another length, though there’s nothing wrong with both of those lengths being equal.

SeaGal said:

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You need to size the fuse according to the current carrying capacity of the cable, rather than the inverter. Fuses, protect cables from overload.

Click to expand...

It depends on which is the lowest current carrying item you intend to protect. Often the current carrying capacity of the inverter is less than the cable servicing it. In this case I size breakers and fuses to protect it. Consequently the cable is also protected. It is perfectly OK to go down in size so long as loading does not create nuisance breaker trips or blown fuses.

Mattb4 said:

Often the current carrying capacity of the inverter is less than the cable servicing it. In this case I size breakers and fuses to protect it

Click to expand...

thanks for clarifying that... My rule-of-thumb is fuse to be 20% greater than inverter charge / discharge capacity and cable to be at least 20% greater capacity than fuse.

Obviously if the installation requires longer runs it is very likely that the cable will have been chosen to minimise voltage drop, hence is often significantly greater capacity than the inverter / fuse.

SeaGal said:

thanks for clarifying that... My rule-of-thumb is fuse to be 20% greater than inverter charge / discharge capacity and cable to be at least 20% greater capacity than fuse.

Obviously if the installation requires longer runs it is very likely that the cable will have been chosen to minimise voltage drop, hence is often significantly greater capacity than the inverter / fuse.

Click to expand...

Reasonable way to set things at the maximum fuse or breaker size. I think sometimes the maximum size all too often becomes the minimum size in peoples mind. Sizing wire (or cable) is the exact opposite where there is a minimum current carrying capacity it must meet or exceed for safe design.

Mattb4 said:

Reasonable way to set things at the maximum fuse or breaker size. I think sometimes the maximum size all too often becomes the minimum size in peoples mind. Sizing wire (or cable) is the exact opposite where there is a minimum current carrying capacity it must meet or exceed for safe design.

Click to expand...

I have always started from the basis that everything should be overspecced (for cabling / capacity / rating) for what you want to do, and then underfused.

That way, worst case, you pop a few fuses before you adjust. Or your system has a particularly heavy day and it pops and you say "Oh, well, I'll just change that out".

To the point that I have started on a 12V system knowing that I'll end up on a 48V system eventually... but that means that ALL my cabling is safe for 12V, so by the time I change things out for 24/48V, it'll be MORE THAN safe (and all my connectors, charger, etc. are designed to accept both 24 and 48V already - in a way that's for future proofing, but it's also a safety device if it should get higher voltage by mistake when I rewire!).

A few months back, I plugged something into a PDU and it popped and I panicked. Until I remembered that I'd deliberately put a 3A fuse into the PDU so that it was far safer than necessary (at the time I didn't imagine having to put 3A devices on it any time soon).

I had gone to the effort of downgrading that fuse DELIBERATELY before I even started. And I'd rather change 100 fuses than fry 1 piece of equipment or my house.

Overspec. Underfuse. It's the only way to live cautiously.

Thicker cables than strictly necessary. Appliances with wider inputs than strictly necessary. Busbars, fuseholders, etc. that can take a voltage more than strictly necessary.

And then put in lower-rated fuses that are between what you expect to pull, and what your equipment/cabling can safely take.