A SECOND HOUSE/DOMESTIC BATTERY

[Bubble32]

Bubble has its original factory-fitted electrics with two batteries - starting and domestic - under the starboard saloon berth. This isn't ideal as it gives the boat a slight list to starboard and, also, the only thing I have which uses a lot of power continuously is my recently-replaced fridge compressor which is on the port side!

The batteries are lead-acid; a standard car battery for engine start and a 140Ah lorry battery (an exact replacement of the original) for domestic.

Recently retired, I am planning some long passages for next year and am considering adding a second domestic battery, chiefly to power the fridge. This would leave the existing domestic battery free for the critical things such as the autopilot, instruments, lighting, windlass etc.

My question is this:

Is it possible to install a second domestic battery which charges from the ship's main charging system but is NOT paralleled to the existing house battery with thick cables? My tentative thoughts are these:

Install the second domestic battery on the port side. Use to power the fridge ONLY, via a relay, so very little change to the existing wiring for the fridge. Given that the fridge is not safety-critical, but the things left to the existing house battery are, I am wondering if it is possible and safe to somehow connect the second, port-side domestic battery to the existing starboard-side one via a VSR or some other device so that it will charge only when the primary domestic battery is fully charged. In that case could the charging connection be wired using thin cables (such as for a car battery charger) rather than the usual heavy ones?

I have looked everywhere and can't see evidence that anyone in our forums or elsewhere has an arrangement such as I am proposing. I guess that means it isn't possible. Most grateful for any advice.

Thanks.

[symphony2]

That is overly complicated. Why have a dedicated battery just for the fridge? I know it is potentially a big consumer, but doubling your capacity would more than cope with a fridge. In temperate climates, fridges usually take 35-45amps a day which is less than 15% of the usable capacity (never less than 50%) of a 280Ah bank. I have that size bank and have never worried about fridge consumption in the UK. If you are intending to spend time at anchor or in warmer climates then the issue is replacing the power used, so alternative means, typically solar are the first thing to explore.

There is no reason why you cannot parallel batteries that are not adjacent to each other. However perhaps you could explore using a high power AGM start battery such as a Red Flash elsewhere and put the two new house batteries in the current location. Also if you are expecting greater overall usage then consider similar capacity AGMs which have advantages over the type of battery you have in terms of charge acceptance and longer life.

[tiger79]

I have looked everywhere and can't see evidence that anyone in our forums or elsewhere has an arrangement such as I am proposing. I guess that means it isn't possible. Most grateful for any advice.

It's certainly possible, but it isn't sensible.  You'd be better doubling the size of your house bank and running everything from it.  Bigger banks accept charge more readily.  And they last longer.

[Keweetoo]

Also consider getting some solar panels - I have a 100W folding panel which tucks into the wet locker when not in use, but once at destination I unfold and place on deck - it runs the fridge quite happily via a controller of course and tops up the service bank.