Electrical question: should one leave the motor battery ...

[rays]

switched on when sailing? A friend told me he had been advised to switch off the dedicated motor battery whilst sailing (and using the normal electical stuff) to avoid running it down. I thought the whole point about having separate batteries for "domestic" and "motor" was to stop this from happening. I must confess that we often leave the fridge on whilst sailing. Hasn't caused a problem yet.

CD

 

[Vulcan]

If it is only used to start the motor, then switching the battery off is fail-safe to avoid any current drain. Providing it is quick to switch on again in case motor is needed in a hurry.

 

[VicS]

It wont run it down provided there are no faults and the engine auxilliary circuits are switched off.

And of course provided it is truly a dedicated starter battery.


Your car battery is not switched off when you are not motoring and provided you dont leave something switched on then it does not run down. At least not in a few hours or even days.

 

[michael_w]

I would always leave it switched on. Never know when you might want the engine in a tearing hurry, MOB and similar emergencies.

 

[LadyInBed]

If you use a 1-2-Both switch the question should not arise.
You switch to domestic battery when sailing, so by default the engine battery is open circuit.

 

[fireball]

We always leave the dedicated Engine battery turn on when sailing - or when on the boat for that matter.

 

[DAVIDO]

Which makes it even worse if youve flattened it /forums/images/graemlins/grin.gif

 

[Csail]

Ours is never switched off even leaving the boat for sometimes months and always fine.

 

[KREW2]

I must be doing it all wrong, when sailing I turn off the dedicated starting battery and run domestics off the 2x 110ah leisure batteries, switch to position no 2, when motoring I have all 3 on charge, switch to position all.

 

[VicS]

[ QUOTE ]
I must be doing it all wrong

[/ QUOTE ] Not really. If you have the domestic bank and the starter battery connected to a 1, 2, off, both switch you do not have a truly dedicated starter battery in as much that you could run your domestics off the battery you normally reserve for engine starting and you could start the engine from the domestic bank.

A truly dedicated starter battery would not be wired in using such a switch. It would be connected to the engine only and have just an isolating switch. Some form of split charging via a diode splitter or a vsr would be employed to charge the domestic bank.

In your case you would start with just the starter battery in circuit and then when that has been recharged switch via the "both" position to the domestic battery bank to charge that, leaving it selected until the next time you needed to start the engine when you would switch, via the "off" position preferably, to the starter battery again

 

[Guest]

If the battery banks are truly isolated from each other ... then as long as you switch of alternator excite (ignition switch) then leaving on should be ok ...

Only thing I can think of in an isolated battery set-up is if you have stray currents - which is not unusual in a boat ... then it may be better to switch of completely.

In the set-up where batterys go through a selector switch ... then selecting engine battery for starting ... charge for a while ... then switch to domestic effectively answers your question.

Most here have assumed you have completely isolated engine battery - not that common ... IMHO most n=boats have provision to use batterys on other services ... so is your engine battery isolated or not ?

 

[Geordie]

Assuming you have a 1,2,both switch and 1 is starter battery while 2 is domestics battery, the procedure is:
1. Before starting, switch to 1.
2. As soon as the engine is running switch to both.
3. After you get out of the marina/harbour and your sails are set and you switch off the engine, switch to 2.

The rationale is:
At 1. Only the starter battery is being used to start the engine.
At 2. Both batteries are being charged.
At 3. The starter battery is isolated and therefore retains its charge, while the domestics battery is being used to power the navigational equipment etc.

Easier said than done. It requires a strong memory and discipline which is why many people opt for a split diode system or a relay system which looks after the above process automatically.

In an emergency, if the starter battery fails, switch to 2 and try staring on the domestics battery or, switch to both and try starting on both.