Battery cables for battery back-up

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electro7

Senior Member
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Northern CA, US
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Electrician, Solar and Electrical Contractor
Hi,
I was wondering what is recommended for battery cable in an interactive with battery back up system? Anybody have experience with a certain type that works good?
 
I can help. I assume with battery backup you are talking low voltage like 12, 24, 48, 96 volts?

NEC is not going to help a lot as it does not take operating voltage and voltage drop into account. With lower voltage you have to take voltage drop into account and NEC is not much help. Thus you must design on maximum voltage drop calculated on voltages, maximum current loads, and distance. With that in mind you have to throw tables 310 out the window as it is of no use other than sanity check for short distances.

You design for maximum voltage drop usually 2 to 5% at worse case. That could mean using 1/0 for a 20 amp circuit.

To help you we have to be specific with voltage, current, % of VD, and distances. It is simple Ohm's Law, and a sanity check with NEC.

For batteries cable types dual listing RHW-2, DLO or like works well. You want something with fine strands and flexible in larger cable greater than say #4 AWG and use long barrel terminals with shrink wrap.

 
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The communication industry uses Cobra Wire X-FLEX? POWER SUPPLY SYSTEMS CABLE because it is flexible (low stress on battery terminals) and UL Listed with NEC recognized designations (THW, TEW).

http://cobrawire.com/x-flex/x-flex.php

However, you need to use crimp on connectors that are listed for the specific stranding. Burndy, T&B and others have such connectors.

Available from major wire distributors such as Anixter.

Welding and diesel (DLO) cable are not listed or suitable unless they have dual markings of suitable cable type.
 
Okay great. The SMA Sunny Island I will be connecting to allows up to 3/0, so I will make it 3/0 to be safe. It is going to be a 48volt battery bank with 8-12volt batteries, 2-strings of 4, 105ah batteries paralleled after the disconnects to make it a 210ah battery bank. I am going to keep the cables as short as possible. What size would you recommend between batteries? And when I parallel them after each disconnect how should I do that? Polaris taps? Double lug block?

I saw the cobra wire and was wondering if it was code compliant. I think I will go with that.

Thanks!
 
Have you considered using a single string of eight 6-volt batteries? Whenever you have more than one string, you have the possibility of current imbalance.
 
Actually just thought of that but 4- 12v at 200ah. Is there any advantage of going with the 6v instead?
 
electro7 said:
It is going to be a 48volt battery bank with 8-12volt batteries, 2-strings of 4, 105ah batteries paralleled after the disconnects to make it a 210ah battery bank.
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Do not do that. You are just asking to kill the batteries prematurely. If you need 210 AH battery, buy 210 AH batteries. Those are going to be 6 or 8 volt batteries. There are some 12 volt 200 AH batteries out there but they are heavy and difficult to work with.

electro7 said:
I saw the cobra wire and was wondering if it was code compliant. I think I will go with that.
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Cobra is just one of many of the cables used on battery plants. It is very expensive and likely to be over kill. Its intended use is in data centers under raised floors. It is a Zero Halogen aka low acidic smoke. The insulation type depends on which of the cables you use.
  • Telco Flex is RHH/LS
  • Battery cable is UL3311 & 3279. No NEC rating
  • Copp Flex 2000 is RHH/RHW
 
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electro7 said:
Actually just thought of that but 4- 12v at 200ah. Is there any advantage of going with the 6v instead?
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Big advantages. Cable and connectors is obvious. The real issue is batter life. You cannot get two or more parallel strings resistances equal. What that will do is cause one string to take on the Lion's Share of the load which will weaken it much faster than the other strings.

You only use parallel battery strings when necessary like to achieve Amp Hour Capacity. The deal is unless you need more than 4000 AH it is never necessary. They make batteries from 10 AH up to 4000 AH. For large batteries come in 2 volt single cells. 6 volt batteries typically run from 200 AH up to about 600 AH
 
PWDickerson said:
Have you considered using a single string of eight 6-volt batteries? Whenever you have more than one string, you have the possibility of current imbalance.
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Especially since any imbalance on such small batteries is a bigger problem than with larger batteries. A Group 27 battery is almost never a good choice. In the 200Ah range, the 8D is a better choice.

Moral of the story (ignoring the problems with longer strings of cells, which doesn't apply here since they are both 24 cell strings) is to use the smallest number of paralleled strings with the batteries sized to the requirements. Not that I think 210Ah is close to "sized to requirements", but just saying.
 
dereckbc said:
You only use parallel battery strings when necessary like to achieve Amp Hour Capacity. The deal is unless you need more than 4000 AH it is never necessary. They make batteries from 10 AH up to 4000 AH. For large batteries come in 2 volt single cells. 6 volt batteries typically run from 200 AH up to about 600 AH
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Or to achieve serviceability and surviveability. I'd never design an off-grid system with a single string -- if that string fails, no electricity until the string is fixed. For the reasons you gave, I'd also never design an off-grid system with more than 2 strings.
 
tallgirl said:
Or to achieve serviceability and surviveability. I'd never design an off-grid system with a single string -- if that string fails, no electricity until the string is fixed. For the reasons you gave, I'd also never design an off-grid system with more than 2 strings.
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If you have a single string of 2V batteries and one fails you can usually cut it out of the string, adjust the CC voltages and continue.
 
tallgirl said:
Or to achieve serviceability and surviveability. I'd never design an off-grid system with a single string -- if that string fails, no electricity until the string is fixed.
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Which requires new batteries. In addition you have a 50/50 chance of the failed string being either an open cell or shorted cell. Open cell OK. Shorted cell and you just doomed the parallel string.

Batteries do not just go bad all the sudden unless abused and/or not cared for. Anyone living off-grid with experience knows weekly PM is required by taking battery temp, hydrometer and voltage readings like professionals do in industry. If that care is given you are going to see a cell or string failing long before it dies. Batteries have to be all replaced together if you expect maximum performance and service life.
 
dereckbc said:
Which requires new batteries. In addition you have a 50/50 chance of the failed string being either an open cell or shorted cell. Open cell OK. Shorted cell and you just doomed the parallel string.

Batteries do not just go bad all the sudden unless abused and/or not cared for. Anyone living off-grid with experience knows weekly PM is required by taking battery temp, hydrometer and voltage readings like professionals do in industry. If that care is given you are going to see a cell or string failing long before it dies. Batteries have to be all replaced together if you expect maximum performance and service life.
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No disagreement that a failed string means they all get replaced at once, unless you're comfortable with premature failure and weird performance. Just disagreeing that the best design for an off-grid system is a single battery string. That and that batteries don't "just fail". You can't stick a battery or take it's temperature and tell whether or not a plate is warping or a cell-to-cell internal bar is failing.
 
Because I have two Sunny Islands (battery based inverters) I was advised to parallel the battery bank to two disconnects, one for each inverter, as both Sunny Islands need to be connected to the same battery bank. So series 4 or 8 batteries together then take 2 positive leads off one end and 2 negative leads off the other. Will this mess with the battery life at all?
 
electro7 said:
Because I have two Sunny Islands (battery based inverters) I was advised to parallel the battery bank to two disconnects, one for each inverter, as both Sunny Islands need to be connected to the same battery bank. So series 4 or 8 batteries together then take 2 positive leads off one end and 2 negative leads off the other. Will this mess with the battery life at all?
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So long as all the cabling (and I mean ALL THE CABLING) is of identical length to where the pair of strings are paralleled you're fine.
 
tallgirl said:
So long as all the cabling (and I mean ALL THE CABLING) is of identical length to where the pair of strings are paralleled you're fine.
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I do not think that it is nearly as critical to keep the length of the cable to both Sunny Islands identical with one battery string as it would be if you were paralleling two battery strings. What is your concern?
 
GoldDigger said:
I do not think that it is nearly as critical to keep the length of the cable to both Sunny Islands identical with one battery string as it would be if you were paralleling two battery strings. What is your concern?
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You're fine having different lengths after (SunnyIsland side) the point of connection for the parallel strings. You absolutely do not want a different cable configuration before (parallelled batteries side) that point.
 
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