wiring multiple batteries - advice please

[PaulR]

Our starlight 35 has 3 batteries - one we use primarily for engine starting , the other 2 are wired in parallel and used for domestic services with selection of either the engine or domestic via usual off/1/2/both rotary switch.

We have been advised to ensure that supply and service connections for the parallel pair are best wired diagonally one from one of the pairs negative terminal and the other from the OTHER battery in the pair's positive terminal and NOT to connect the parallel linked batteries to the boat only via one battery with the second battery only hooked up to the first battery and not itself directly connected to the boat wiring.

Not a problem - we can redirect the wiring from the positive connection (which goes to the 1/2/both switch) to the second one of the parallel pair rather than the first one but our query is this.

All 3 batteries have a shared connection to the negative boat system - effectively the negative cable connects first to the negative of the engine battery - then from that connection links to the negative of the first of the 2 parallel wired domestic batteries and from there to the negative of the other parallel wired domestic battery. Our understanding (maybe flawed) is that by moving the positive connection to the second of the domestic batteries rather than as currently it first connecting to the positive terminal of the first domestic battery will create this recommended diagonal connection across both of the domestic battery pair rather than the first connection for both neg and pos connections being to the first dom battery only.

It has now been suggested to us that best practise is not to have the negative cable daisy chained along the 3 batteries but to connect the engine battery and the parallel wired domestic batteries separately to the boat - meaning we would need to run a second negative lead from the battery box to the boat - is this right /usual?

Welcome guidance please

 

[lpdsn]

You could calculate it yourself. The effect occurs because the batteries further away see lower charging voltages (and load voltages) because of voltage drop in the cables, so the multiple batteries in the bank end up with different histories.

I did see a website with calculations for a four-battery domestic bank a few years ago. The difference between battery A and battery D was noticeable. It didn't seem to make a whole lot of difference for a two-battery bank.

 

[pvb]

All 3 batteries have a shared connection to the negative boat system - effectively the negative cable connects first to the negative of the engine battery - then from that connection links to the negative of the first of the 2 parallel wired domestic batteries and from there to the negative of the other parallel wired domestic battery. Our understanding (maybe flawed) is that by moving the positive connection to the second of the domestic batteries rather than as currently it first connecting to the positive terminal of the first domestic battery will create this recommended diagonal connection across both of the domestic battery pair rather than the first connection for both neg and pos connections being to the first dom battery only.

As I understand it, your understanding is correct. You'd have been better posting a diagram though!

It has now been suggested to us that best practise is not to have the negative cable daisy chained along the 3 batteries but to connect the engine battery and the parallel wired domestic batteries separately to the boat - meaning we would need to run a second negative lead from the battery box to the boat - is this right /usual?

I don't know who's suggesting all this stuff, but your negative arrangement is fine as it is, always assuming adequate cable sizes.

 

[macd]

I did see a website with calculations for a four-battery domestic bank a few years ago. The difference between battery A and battery D was noticeable. It didn't seem to make a whole lot of difference for a two-battery bank.

Perhaps this is the one, lpdsn. It's been referenced repeatedly on here: http://www.smartgauge.co.uk/batt_con.html

 

[lpdsn]

Perhaps this is the one, lpdsn. It's been referenced repeatedly on here: http://www.smartgauge.co.uk/batt_con.html

That's the one. Thanks for posting it.

 

[pvb]

Perhaps this is the one, lpdsn. It's been referenced repeatedly on here: http://www.smartgauge.co.uk/batt_con.html

I'm always sceptical of people who profess to have carried out incredibly complicated calculations, so complicated that they can't be revealed, but we just have to trust their word. In the Smartgauge example you've quoted, one of the parameters assumed is "Interconnecting lead resistance = 0.0015 Ohms per link" - that's about the same as 3 metres of 35mm2 cable. They sound like rather long links to me.

 

[William_H]

I don't think OP should be worrying about wiring arrangements. Cables should all be quite large for starter current on either battery so for normal charge and domestic discharge volt drops will be quite low. So very little unbalance of batteries in parallel. Perhaps OP should be thinking of redesigning to remove the rotary selector switch and go for engine start and charge always to the small battery with VSR (voltage sensing relay) to connect the 2 large batteries in parallel to the engine battery when engine running with domestic load only on the 2 large batteries. So 2 simple on off switches for the 2 batteries plus another for emergency engine jump start from the domestic batteries. olewill