Parallel battery cables

[MikeyG]

Getting ready to do a major refit/relocation of my battery bank and I guess I'm seeking confirmation of my thought process. 8 100amp lithium batteries feeding a 3000w victron charger/inverter. For cable size between batteries I believe I just need to size for maximum possible amperage draw which would theoretically be 250 amps, is it that simple? I understand the voltage drop/length map but the base size has me a little confused.
Thanks!

 

[DavidM]

Well, you could just use 1/0 cable which is rated to carry 285 amps and would be perfectly safe. But would it cause too much voltage drop? If your batteries are 5' (one way) away from your inverter 1/0 will result in a .25 volt drop, probably tolerable.

But if the batteries are twice as far away then the voltage drop is .5 volts and that may be too high, then use 2/0 or 4/0.

Sizing cable should always consider current carrying capacity (ampacity) at a minimum, but also voltage drop in the cable.


David

 

[Pete Meisinger]

Simple. Go big or go up in smoke.

pete

 

[High Wire]

Without digging too hard and simplified math, 3000W output at 120VAC will draw at least 250 amps input at 12VDC. Now account efficiency of the inverter, say 80%, you have to plan for even more amps. Your estimate is light. Bottom line: Follow the tables in the inverter manual for cable size.

 

[MikeyG]

I was thinking size to the inverter but it seemed too simple. 4/0 it shall be, inverter will be within 18 inches of positive lead but goal is get all the potential out of my power.

 

[Comodave]

For that short distance 4/0 will not be that expensive. That is what I would do.

 

[foggysail]

I am not sure about voltage drop concerns other than the heat generated when charging. Most chargers are current sources not voltage sources. Sure, the heated cables will detract from the energy that should be going to the batteries.

Now as far as the batteries powering the inverter.....yes, that cable loss should be minimized. The other consideration is does the OP plan on operating that 3KW inverter at full load continuously? Is the inverter rated at 3KW continuous usage?

Personally I would make life easier for myself and use no greater than 1/0 copper. If you connect the + cable onto one end of the bank and the - cable onto the opposite end of the bank you will essentially force current to balance between the batteries minimizing the bus bar voltage loss.

I have a 3KW true sine wave inverter on my boat that is energized by two banks of golf cart batteries with each bank paralleled ....ie. 8 golf carts forming 2 twelve volt banks for a maximum ampere capacity of 430A each for a total capacity at 12V of 860 amperes. And I am only using #2 cu with a total length of 20’ round trip of wire. Never had any troubles. But to each his own.

 

[psneeld]

No expert here but my understanding is there is a "dead short" rating for batteries...well at least lead acid. I am not familiar with Lithium but they may be the same.



If the cable is rated for that, which is what any fuse protection must be rated for (not sized mind you), then you I would think you should be OK.


The other method many go by is the heaviest amp draw and either use a chart for minimal voltage drop or just ampacity rating....and there are varying opinions on whether that is too small.


I am ONLY talking battery jumper wires, not runs to the inverter.

 

[foggysail]

No expert here but my understanding is there is a "dead short" rating for batteries...well at least lead acid. I am not familiar with Lithium but they may be the same.



If the cable is rated for that, which is what any fuse protection must be rated for (not sized mind you), then you I would think you should be OK.


The other method many go by is the heaviest amp draw and either use a chart for minimal voltage drop or just ampacity rating....and there are varying opinions on whether that is too small.

Are you suggesting the wire rating should equal the fuses ampacity? The overcurrent condition should be hastily removed with the fuse.

 

[psneeld]

I said "rated not sized"...if discussing the new ABYC fuse specs for battery protection discusses battery overprotection. Interruption rating.


A big argument by some "experts" ensued and said the over-current protection provided by the Blue Seas battery terminal fuses didn't meet the new requirements...however I showed them where the fuses were OK as stated in the new standards if their "specs" met the battery short circuit rating.


I have a guess that no wire size will probably meet that suggestion practically.


So going with system max amp draw is probably the safest, especially if overcurrent protected.



Other than that I am not sure what you are asking.


My first suggestion is that the jumper wires be capable of handling at least the short circuit rating of the batteries if they are rated....unless it is so high that would be impractical.

 

[MikeyG]

Sneed, I think we're on the same page but then what would be the short circuit rating of 8 100 amp lithiums and is this not where the fuse becomes important? Overthinking myself into a rabbit hole...

 

[psneeld]

The individual battery has a short circuit max interruptionrating that overcurrent protection fuses/CBs have to be rated for ( where they just melt together and no longer provide protection). It is not their amp rating.

I thought along those lines in my first post, but I think it is way higher than needed. The safe wire size would be the same as the wires that go to the inverter, but can probably reduced till each one would carry the max load with similar resistance based on their length.

Again the easiest answer is just match the runs to the max load draw or upsize if additional draws are possible.

 

[syjos]

The Victron 12/3000/120, if that is what the OP has, require a 400 amp DC fuse per installation manual. I size the battery cable for that. Usually 4/0.

 

[DavidM]

Can someone tell us about the various classes of high current interrupting fuses that are used with large house battery banks that have a very high short circuit sourcing capability.

For example Class T is one such type but I don't know when to use one.

David

 

[syjos]

Go to Blue Seas website and there are circuit design guides and other info.

Here's a sizing guide.