Batteries in series and parallel; wire gauge

[jambouk]

1. If the wire to a pair of 12v DC batteries in series is 10AWG (5.3mm2) and is say 2m in length, what gauge and length wire should join the two batteries, or as long as it is at least 10AWG and less than 2m it is unimportant? Are there issues with using thicker wire to join the two batteries together.

2. And if these two batteries were in parallel, what are the requirements of the wire joining these two batteries together? Again, is using thicker wire than the live and positive to the battery pair problematic?

3. And if you can tolerate answering another question, where would you fuse these two set ups?

Thanks, James

 

[Juicy6]

10AWG will result in a minimal voltage drop. For 2 meters and estimated consumption of 5 Ampere you will see only 0.07 volts drop. Use same wire between batteries. A 10-15 Ampere fuse near batteries will suffice for protecting cables.

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=12&phase=dc&noofconductor=1&distance=2&distanceunit=meters&amperes=5&x=47&y=18

Christer, Sweden

[DaveL59]

you could put a fuse between the batteries on the +/- connected pair, UPS's like APC tend to have a 100A fuse linking the 2 batteries in series to protect in case the other 2 terminals short. But then they are usually arranged with the terminal edges facing each other because of space constraints. 

For batteries in parallel the wire requirement isn't any different really, using thicker interconnect would be fine, but you face other issues in terms of balancing the batteries so they are at the same voltage when connecting them together and in use and during charging. One weak battery in the parallel set will bleed charge away from the others during charge and use as it'll have a different internal resistance.

[jambouk]

Thanks both. I need to get a bit more 10AWG wire then.

For the batteries in parallel, how quickly would you notice a problem with one battery bleeding the other (time frame or charge cycles), and if say both batteries originally had 80 amp hour capacity, but now through aging one effectively had 70 amp hour capacity and one 65 amp hour capacity though both have a resting voltage of 12.95V, is this considered to be a big difference, or would a big difference need to be in the order of 25%?

Regards

James

 

[Kev M]

I would personally go down the bigger battery option rather than use batteries in parallel.

The only exceptions to this would be if you needed to maintain power at all times ( you could change one out without losing power) or if there was a physical space constraint.

Using batteries in parallel can be fraught with difficulties.

[DaveL59]

1 hour ago, jambouk said:

Thanks both. I need to get a bit more 10AWG wire then.

For the batteries in parallel, how quickly would you notice a problem with one battery bleeding the other (time frame or charge cycles), and if say both batteries originally had 80 amp hour capacity, but now through aging one effectively had 70 amp hour capacity and one 65 amp hour capacity though both have a resting voltage of 12.95V, is this considered to be a big difference, or would a big difference need to be in the order of 25%?

Regards

James

 

I'd say so long as they've able to give the same voltage but then one will be flat before the other and that's where the damage to the weaker cell will be worsened. It'll have a faster volts drop which will then be sucking charge from the stronger one under load and then when they get to low capacity if you keep the load running one will go very flat. it's very not good to run a lead battery totally flat unless they're designed for deep discharge.

If you really want to go that route, take a look at solar forums and how they set up the battery banks, of course a much larger scale from necessity but will give you an idea of the issues. Batteries in parallel as Kev says, best avoided but I guess it depends what you're trying to achieve and are working with.

[pete_l]

1 hour ago, jambouk said:

For the batteries in parallel, how quickly would you notice a problem with one battery bleeding the other (time frame or charge cycles), and if say both batteries originally had 80 amp hour capacity, but now through aging one effectively had 70 amp hour capacity and one 65 amp hour capacity though both have a resting voltage of 12.95V, is this considered to be a big difference, or would a big difference need to be in the order of 25%?

http://www.smartgauge.co.uk/batt_con.html

HTH

[jambouk]

Thanks again; I’d read that link earlier today.

The aim is to not spend any money. The existing batteries are there and are in a reasonable shape so seems a waste to get rid of them and then spend money on new ones. I appreciate these will tire and die over time, and then they can be replaced with bigger newer ones, but as long as it’s largely safe, I’ll stick with this for now and fuse it all. 

Appreciate the help.

James