Wire size

[Lin]

What would be the minimum cable size to connect two group 31's in parallel?

[bevans6]

Totally depends on what amperage you are going to pull out of them, and if they are in series or parallel.  What are you doing with them?


Brian

[luvrbus]

2 gauge seems to be norm on the newer buses in parallel

[Lin]

I am replacing the house bank with three group 31 agm's.  We do not generally have any really heavy loads, but it would not hurt of be prepared for the worst.  The present cables are only #4. and seemed a bit small to me although a tech at Interstate said that would be the minimum.  I have got enough #2 around, so I think I will go with that suggestion.  Thanks

[Oonrahnjay]

  I am replacing the house bank with three group 31 agm's.  We do not generally have any really heavy loads, but it would not hurt of be prepared for the worst.  The present cables are only #4. and seemed a bit small to me although a tech at Interstate said that would be the minimum.  I have got enough #2 around, so I think I will go with that suggestion.  Thanks 

      Plain "2 gauge" seems awfully small to me (I get scared thinking about 4 gauge) but I know that designers are trying to get low cost, low weight, easy to bend and install - so  maybe.  Parallel means 12V = higher amps for the same wattage (but I guess that parallel means more wires to carry the load).  If the I'state tech guy says OK, I'd probably go for it but I might wonder about it.

[Seangie]

Lin - if you have a smartphone, I-phone, Android or Windows, blue-sea systems has a free nifty app that gives you reccomended wire size based on a number of calculations including, load current, length of conductor, allowable voltage drop, temperature (in engine room?) In or out of conduit...works pretty good.  They might have a computer based app from their website as well.

-Sean

www.herdofturtles.org
1984 Eagle Model 10S

[luvrbus]

2 gauge copper wire is rated for 208 amps no way will he exceed that with 2 group 31 in parallel you cannot take out more amps than the battery is rated for

[bevans6]

Of course you can take more out of the battery than the 20hour amp/hour rating.  You could take a thousand amps out of those two batteries easy, but not for long and you might not even damage them.  Think of surge ratings with a starter motor on a cold morning. There is nothing inherent in a battery that limits current in any way, except it's melting point.   Not that that would happen with a house bank unless an inverter was asked (or decided) to surge.  Anyway, I used 1/0 wire in that same application, based on the highest load I would connect to the bank.

Brian

[Len Silva]

As long as you never expect to use an inverter, the 2ga. should be fine.  The paralleling straps should be rated for the expected load.  If you are using screw top batteries, you can always double up on the straps, now or later.

[Lin]

Len, we do use an inverter, but the microwave is generally the biggest load and only for a few minutes at most.  I found a source that had Alliance group 31 AGM blemished batteries for $100. each.  They said they are made by Deka.  They want $200 for AGM 4d's and $250 for 8d's, so if these group 31's are really good, I will probably consider the 4d's for my start batteries.

I have mentioned in the past that my Trace 2500 watt inverter would not start my compressor or an AC unit.  I am curious to see if the new batteries will change that.  I will give it a try tomorrow.  Although it is not something that I would do regularly, having an option to run the compressor without the generator would be nice.

[bevans6]

Please let us know if your test works, I'd like to know what a 2500 watt inverter can start motor-wise.  Still choosing my next inverter...

Brian

[Lin]

It started and ran the compressor.  Even though the batteries had had a reasonable resting voltage until recently, I knew that they were weak just by how much they would drop overnight with very little load, but I guess I did not know how bad they were.  I had always blamed the inverter; now I owe it an apology. 

[Don4107]

Size matters, but length is important too.     How far from the batts to the inverter?  Remember to feed the positive lead from one and the negative from the other.

[Lin]

Don, the inverter is right next to the batteries and those cables were sized equal to or greater than Trace's specs.  Since the batteries I bought were termed as blemished and discounted, it is possible that they have been sitting around for a while.  They were not fully charged when I got them home.  I should have brought a volt meter with me when I picked them up to try to convince them to let me take the ones that tested best.  They are charging now slowly and are at 12.6v, which per Deka is 80%.  They say that 12.8 or above is 100%.  In the hope of getting the best use out of them, I have ordered a Ctek charger that was highly recommended.  Although the built in charger on the inverter is adjustable, I suspect it is not really the best for these batteries.  The Ctek will have a battery temp sensor to help keep the charge tuned correctly.