Batteries in parallel - shore power charger question

[Cardo]

New to us boat came with two domestic batteries. Each one hooked up to its own on/off switch that linked the batteries in parallel when both switches were on. The shore power charger lead then went through a battery isolator before going to the two batteries. The manual for the charger suggests running things this way to stop one battery from discharging the other.

I've made some changes and replaced the two batteries with four batteries all directly wired up in parallel. I then have all four hooked up to a single on/off switch. I was intending to simply connect the battery charger direct to the batteries and skip the now redundant isolator.

Is what I'm doing ok? All four batteries are brand new and are of the same kind. Surely I shouldn't need to worry about them discharging each other? The alternative would be to have all the batteries running to their own individual on/off switches, which seems overkill and the SmartGauge would not be happy.

Thoughts?

 

[William_H]

Batteries in Parallel

Yes essentially you are right in that all 4 batteries should be OK in parallel hard wired permanent. However when the batteries get old and specifically when and if one dies with a faulty cell then you may have a low voltage for the whole bank. Dragged down by one bad battery. This then will be time when you would want isolation switches on all batteries. However if of course it is not too much trouble to disconnect each battery you will still be able to isolate the bad one. good luck olewill

 

[VicS]

Common practice to simply parallel all four I should think.

The thing to avoid is paralleling the house battery with the starter or reserve battery.

On the subject of interconnecting batteries to form a large bank there is some relevant advice on the Smartgauge website http://www.smartgauge.co.uk/batt_con.html

 

[bonny]

One Spare?

HI,
Yes, Connecting all in parallel is not a problem and simply gives a single battery with an effective AH rating of the sum of each.

It would be wise to ensure that at least 1 battery is kept switched separately, this would of course require split charging to each of the banks. Normally this one battery (as VicS has stated) would be for engine starting.

You originally stated that the boat came with 2 domestic batteries.
Do you mean "only 2 domestic batteries" or "2 domestic batteries and a separate engine battery"?

An engine battery - car battery is designed for very short periods of high discharge current - eg for engine starting. These are not designed to be discharged to any great depth and do not like being repeatedly discharged beyond about 25% (ie not less than 75% charge). ie a 100AH battery can only be used to about 25AH.

Domestic - or Leisure batteries are designed to be repeatedly discharged more deeply, Typically utilising around 75% of their capacity down to about 25% of full charge. A similar 100AH Leisure battery can be repeatedly used to about 75AH. Unfortunately Leisure batteries do not like heavy discharge currents involved in starting engines, which can buckle their plates.

Having said that, starting an engine from a bank of 4 parallel leisure batteries is not likely to be a problem because the starting current would be shared bay all 4 (if as you state are all new and identical). However, if these are your only power, how would you start the engine if you accidentally run them flat.
This is where a single separate battery for starting comes in, As a guide a similar sized car battery for a vehicle with the same engine size/type as your boat would do with a minimum of around 50AH.
Of course, if you dont need a battery to start your engine (small outboard) then don't worry.

 

[Cardo]

Thanks for the replies.

I have hooked them up as per the SmartGauge website, which I found very useful.

In relation to the number of batteries, I have 4 domestic batteries in parallel plus a dedicated starter battery. The shore power charger has a dedicated charging lead for the starter battery.

Next plan is to use the redundant on/off switch as a method of paralleling the domestic bank with the starter battery in case the starter battery goes belly up. Judging from the labels on the switches, I think this is what it was originally put in for.

 

[VicS]

Thanks for the replies.

I have hooked them up as per the SmartGauge website, which I found very useful.

In relation to the number of batteries, I have 4 domestic batteries in parallel plus a dedicated starter battery. The shore power charger has a dedicated charging lead for the starter battery.

Next plan is to use the redundant on/off switch as a method of paralleling the domestic bank with the starter battery in case the starter battery goes belly up. Judging from the labels on the switches, I think this is what it was originally put in for.

Just a comment on the last paragraph. Consider using the switch to interconnect the domestic circuits with the starter circuit rather than paralleling the batteries themselves. You don't want to parallel a good battery with a duff battery. You want to be able to leave the duff one isolated, but maybe that's what you meant anyway.

 

[Cardo]

Just a comment on the last paragraph. Consider using the switch to interconnect the domestic circuits with the starter circuit rather than paralleling the batteries themselves. You don't want to parallel a good battery with a duff battery. You want to be able to leave the duff one isolated, but maybe that's what you meant anyway.

I had been contemplating simply creating one large parallel circuit that I would use for a short period of time simply to get the engine going. However, your suggestion seems a lot more sensible. Thanks.

 

[William_H]

Jump starting a dead battery

VicS theory is fine although much depends on the type of failure of the engine battery. If it has a shorted cell then yes it is best to have it disconnected however in many other types of failure it does not hurt to leave the starter battery in circuit although dead.
If like me you have jump started cars over the years you do not disconnect the dead battery and while you usually have the donor battery on charge (engine running) I think that for most cases the simple jump in parallel is OK. Of course in your case the simple parallel switch will be simpler and may be adequate.
Just don't tarry after paralleling the engine battery before starting the engine. If you leave them parallel for a long time you could indeed flatten the good batteries. good luck olewill