Battery questions.

[Uricanejack]

A couple of questions about batteries.

My boat like most has two deep cycle batteries No 1 and No 2.
I have an isolation switch with the usual options
OFF, No 1, Both, No2.
In the past it was always my practice to regard one of my batteries as a house battery and one as a starting battery.
Using one battery only for lighting etcetera while at anchor or moored and keeping the other isolated and charged up so I would always be able to start.

Reading my owners manual it would appear this is a bad practice possibly causing damage. My owners manual recommends keeping on both and not changing position of the switch while engine running as this will cause damage. Why?

I would have thought if I run down 1 battery overnight if I switch to both to start it would cause my good battery to loos charge to the low battery. Damaging batteries. If not why not?

At the moment I believe my no2 battery is pretty much shot. While my No 1 is OK.
Had a problem earlier this year when both went right down. Both reading only about 7 or 8 amps Had to jump start the boat. My No1 battery seamed to recover my No 2 not so much.
It does charge up to about 13 amps but very quickly looses charge. And drops down to read well below 11.
My no 1 appears to hold charge.

It was my hope if I left cycling along with good battery it might recover.
Was this dumb?
It doesn’t appear to have worked.

If I replace my No 2 battery with a new battery and leave a possibly weakened No 1.
Will this deplete or loose efficiency of new battery?

Should I replace both batteries together.?

Is there a way of setting things up so I always have a fully charged battery?

 

[VicS]

With two batteries controlled by a 1 2 both off switch I would do the same as you. Namely use one as starter battery and one as house bttery.

I'd select the starter battery, start the engine. After allowing it to recharge I would switch VIA BOTH to the house battery. This would remain selected until the next engine start.

It is important that the switching is made via BOTH to avoid a situation in which the batteries are completely disconnected from the running engine. If this was done there would likely be a voltage surge which would damge the dodes in the alternator.

When the engine is not running the switching can be via OFF unless it is necessary to maintain a power supply for navigational instrumentation etc.

Generally I would avoid having both batteries in use or charging together, except when necessary to boost the power available in an emergency or when switching as described above.

By operating in this way it does not mattter in the slightest if one battery is much newerthan the other

I'd probably also opt for a small automotive type of battery for starting and a larger, perhaps deep cycle, one as house battery.

To avoid the risk of damaging the alternator nobody except the skipper should be allowed to operate the battery selection switch

Other people will now suggest their personal preferences.

 

[onesea]

No Expert on the field so I have simplistic system. A dedicated starter battery and a house battery, both are of different types. Starter is an Optima came with the boat, the Domestic is an AGM. They are are charged through a split relay, so once the engine battery gets to the correct voltage the domestic battery is charged.

Simple with only isolator switches for each battery to deal with. The only downside is that if the engine battery goes duff you need jump leads to use the domestic battery.

 

[Robert Wilson]

I have just read "Electrics Afloat" by Alastair Garrod (in association with Practical Boat Owner) as I'm about to embark on an extensive overhaul/winter programme on Khamsin.
I have always been "in the dark" as far as electrics are concerned (plz forgive the pun) and although the narrative is rather technical and scientific at times for one such as myself, I found it extremely informative and helpful. Plenty of graphics, diagrams and other useful points.
I believe it answers ALL your questions. And from what I have gathered, you are correct in starting with No1 battery, then changing to No2 (house) VIA BOTH.
I have been guilty of running on BOTH thinking I was doing the correct thing, but it appears that is not so. Batteries, like syphon-linked water vessels "equalise" thereby doing neither any good, and potentially harm.

IMHO I would replace both batteries now, and keep the remaining "half decent" one as a spare for some other use. Then use them/charge them independently.

The book mentioned above is readily available from all "boaty booksellers" and I thoroughly recommend it to you. Please forgive me if I am wrong, but you sound like myself - keen to get things done but with limited expertise and knowledge.

I await the techies wrath if I'm wrong..............