Help with Battery setup

[Rebotco]

I'm re-jigging and re-siting my leisure battery bank - 3 x 110Ahr 12v sealed wet cell batteries.

I am a leisure boater with a low electric demand, about 60Ahr/24hrs max when cruising. (most of that is the 12v fridge!)

Everything is pretty conventional, but how do I calculate the optimal size of the large cables :- 

• 1. To interconnect the individual batteries?
• 2. To the starter motor, about 1 metre away?
• 3. The 1200 watt inverter, about 3 metres away?
• 4. The isolator switch, about 1.5 metres away?
• 5. The fridge, about 8 metres away?

Thanks in advance to the wizards of the black arts of electrickery (which I ain't).

[Robbo]

I would use 50mm2 for everything (size based on your inverter) apart from the fridge where I would use around 8-10mm2 to avoid voltage drop.

Edited October 1, 2017 by Robbo

[Tony Brooks]

Fridge - easy, 1sq mm CCSA for each meter between source and fridge so 8 sq mm conductor cross sectional area for both pos and neg.

The BSS demands (I think) a minimum of 25 sq mm CCSA for battery & startser cables. That has a NOMINAL rating of 170 amps so that shoudl be fine for the interlinks, starter and battery cables.

I would probably use the same for the inverter as we can estimate it demands about 120 amps at full output.

[Rebotco]

Thanks gents.

Its good to get such clear advice so quickly.  

[Tony Brooks]

Remember too thick a cable can do no harm apart from to your wallet and possibly by making it more difficult to run. Its better to over specify cables than under specify them and never rely on any current ratings on the drum or in a catalogue for runs of more than a meter or two.

[OldGoat]

13 hours ago, Rebotco said:

I'm re-jigging and re-siting my leisure battery bank - 3 x 110Ahr 12v sealed wet cell batteries.

I am a leisure boater with a low electric demand, about 60Ahr/24hrs max when cruising. (most of that is the 12v fridge!)

Everything is pretty conventional, but how do I calculate the optimal size of the large cables :- 

• 1. To interconnect the individual batteries?
• 2. To the starter motor, about 1 metre away?
• 3. The 1200 watt inverter, about 3 metres away?
• 4. The isolator switch, about 1.5 metres away?
• 5. The fridge, about 8 metres away?

Thanks in advance to the wizards of the black arts of electrickery (which I ain't).

Umm - you didn't ask, but

• Does the engine / starter have its own dedicated battery? If NO then that's not a good idea, flat or well discharged main batteries (and you will get that situation, means you can't start the engine to charge them.
• If yes to the above then 2 x 110 Ah is not really adequate for your load.
• Your question was about the leisure bank, but you included the starter motor in the question, which I probably have misinterpreted....

[Rebotco]

7 minutes ago, OldGoat said:

Umm - you didn't ask, but

• Does the engine / starter have its own dedicated battery? If NO then that's not a good idea, flat or well discharged main batteries (and you will get that situation, means you can't start the engine to charge them.
• If yes to the above then 2 x 110 Ah is not really adequate for your load.
• Your question was about the leisure bank, but you included the starter motor in the question, which I probably have misinterpreted....

Ah yes - well spotted!  My mistake there, I do have a separate starter battery, so should not have included the starter motor cable.

Does this have any effect on the advice already given?

Many thanks.

[OldGoat]

9 minutes ago, Rebotco said:

Ah yes - well spotted!  My mistake there, I do have a separate starter battery, so should not have included the starter motor cable.

Does this have any effect on the advice already given?

Many thanks.

Oh, no - it was a "just in case" comment. Folks tend to be mean with batteries, which is strange because they are one of the lesser expensive items in boating. FWIW if you can bear to search through recent threads, you'll notice that anything from four 110 Ah batteries upwards is suggested as a minimum number, especially if you don't want to have to charge them every day. At least when designing the bank space - see if you could fit more later if my doom-and-gloom comments holds true.  

[Tony Brooks]

56 minutes ago, Rebotco said:

Ah yes - well spotted!  My mistake there, I do have a separate starter battery, so should not have included the starter motor cable.

Does this have any effect on the advice already given?

Many thanks.

And the alternator output to be sure but with just three domestic batteries I assume it was fairly modest.

It depends on engine size, type of starter, to a degree and definitely length of cable run. Going up a cable size or two will never hurt.