Extending battery bank - size of cable required?

[dulcibella]

At the moment I have a domestic battery bank consisting of two 110 AH 12V batteries (due for replacement) connected in parallel. I wish to increase the size of this bank to 4 X 110 AH. The only practicable site for the two extra batteries will involve a cable run of about 2 metres to the existing site. I guess that it is important to minimise resistance in these connections so that one pair of batteries is not preferentially depleted (or the other pair starved of charge). I am thinking in terms of 25 sq mm cable for these connections - will that be enough to make the resistance insignificant for practical purpose?

 

[David2452]

25mm2 is pretty small but it depends on what you are drawing (and charging at) rather than the overall capacity of the bank, if you have a decent invertor for instance 25mm2 is woefully inadequate.

 

[dovekie]

Agree with David 2452, you will probably need much bigger. Hopefully a real electrician can say if this is right, but you should consider a) the current draw if you ever have to use the domestic bank for engine starting, and b) the long term effects of even tiny voltage differences when repeated each time when charging.

 

[Alpha22]

If you have the height, consider replacing each 12V battery with 2x6V "proper" deep cycle batteries...... like the Trojan T105 @ 200Ah for a pair, and not much more space needed than your existing 100Ah 12V battery. If you have the height then a pair Trojan L16 will give you 400Ah in only slightly more foot print. The price per Ah is not much more that the 110Ah 12V batteries, but if looked after, the Trojans life expectancy will be double!!!!!

http://www.pbase.com/mainecruising/deep_cycle_battery ......... If you need any more information.

 

[William_H]

I think the cable size is not nearly so important as you might imagine. Presumably the existing domestic bank has heavy cables that will carry most of the current in an emergency engine start. Charging regime should not be much affected by lighter cables to some batteries. The volt drop will mean less current at high charge currents but the difference will diminish at the lower currents of the bulk of the charging phase. The batteries getting more charge will develop a higher voltage so deflecting current ot he less charged batteries. Likewise discharge currents will tend to ballance out as the batteries providing most current will drop more in voltage to ballance any volt drop in the cables.
So of course it is nice to have heavy cables being more robust and having less volt drop and an obsessive would go for heavy cable but really it does not matter so much.
The obsessive would also wire the 4 batteries so that cable lenght (volt drop) is the same for each battery. ie take the negative from the remote set and positive from the original set but really it makes very little if any difference. (just theoretical) good luck olewill

 

[VicS]

25mm2 is pretty small but it depends on what you are drawing (and charging at) rather than the overall capacity of the bank, if you have a decent invertor for instance 25mm2 is woefully inadequate.

Agree with David 2452, you will probably need much bigger. Hopefully a real electrician can say if this is right, but you should consider a) the current draw if you ever have to use the domestic bank for engine starting, and b) the long term effects of even tiny voltage differences when repeated each time when charging.

I think the cable size is not nearly so important as you might imagine. Presumably the existing domestic bank has heavy cables that will carry most of the current in an emergency engine start. Charging regime should not be much affected by lighter cables to some batteries. The volt drop will mean less current at high charge currents but the difference will diminish at the lower currents of the bulk of the charging phase. The batteries getting more charge will develop a higher voltage so deflecting current ot he less charged batteries. Likewise discharge currents will tend to ballance out as the batteries providing most current will drop more in voltage to ballance any volt drop in the cables.
So of course it is nice to have heavy cables being more robust and having less volt drop and an obsessive would go for heavy cable but really it does not matter so much.
The obsessive would also wire the 4 batteries so that cable lenght (volt drop) is the same for each battery. ie take the negative from the remote set and positive from the original set but really it makes very little if any difference. (just theoretical) good luck olewill

Id use the same size as the existing as the minimum but preferably a little larger.

I had a quick look at the question yesterday evening and was coming out at AWG 8, which is 8.3mm² as a suggestion for likely loads, and charging currents excluding winches, bow thrusters or other high current bits of equipment.

For supporting emergency starting even heavier AWG 4, or thicker, perhaps.

 

[PCUK]

Buy 70mm welding cable from the welding suppliers. Cheap and highly flexible!