Batteries to 'Both'?

[Guest]

Batteries to \'Both\'?

This may be a silly question but here goes...When starting the engine is it ok to have the batteries switched to both i.e. 1 &2 or should you switch to 1 (on my yacht this is the engine battery) and then switch to both once the engine is running?
Over the years I have received conflicting advice on this subject from apparently experienced skippers.At the moment I start on 1&2 and then switch to 2 when under sail/moored up.Comments please.

 

[lilianroyle]

Re: Batteries to \'Both\'?

I think that its best to use just one, the dedicated engine start
battery. If your number two battery (domestic?) isnt as fully charged as the number one it will reduce the overall voltage of the two batteries linked together.

 

[salamicollie]

Re: Batteries to \'Both\'?

I would always start in Position 1 as this means that if your Domestic bank is flat the Starter battery will not empty its charge into the domestic bank, probably stopping you starting the engine and possibly frying the cables...

There are wiring schemes using blocking/splitting diodes that avoid this but they can be rather complicated - one way is to feed your charging input to a spplitting diode and then to the batteries and then dedicate the batteries but then unless you have a smart charger you can have charging problems due to the lower voltage arriving at the terminals - see http://www.adverc.co.uk/technical/technical-boat.htm

 

[WayneS]

Re: Batteries to \'Both\'?

Select your engine starting battery only, then switch over to both to charge.

Only start on both if your starting battery will not start u. Sometimes though it would be better to use the domestic to start, and only use both if that still does not work.

Wayne

 

[Chris_Stannard]

Re: Batteries to \'Both\'?

The answer depends on your boat's wiring. The staart battery and the domestics should be different types of battery, the start an ordinary car battery and the domestics semi-traction batteries. You should start on the engine battery. However, once running you should not move the switch as this can produce spikes which will ruin you electronics and instruments.
You have the additional problem that, if you just switch to the start battery are all the batteries being charged. You also run the risk of all of your electronics resetting as the start current will drag the voltage down to such a level that the electronics switch off.

The real answer is ask someone who is competent in electrics to go through your system with you and decide which is the right way to do things as not all boats are wired the same way. It might cost a few quid but could save you embarrasment when everything fails, or loads of money if you get it wrong and something is damaged. Alternatively buy PBOs excellent book on boat electrics and see if you can figure it out for yourself. (no connection with the PBO)

Chris Stannard

 

[gtmoore]

Re: Batteries to \'Both\'?

After receiving similar advice from Chris I bought the PBO book (Practical Boat Owners Electrics Afloat) and it really is excellent - definitely recommended if you don't want to get into the subject too deeply but still need to know whats going on.

 

[VMALLOWS]

Re: Batteries to \'Both\'?

Disagree with all the posts in reply to your query. Any mutual (dis)charge between parallel batteries will be minimal because the voltage differential is so small. Far better, in my view, to spread the starting load over the available batteries. Then leave them to recharge which will be quicker than if you take it all out of one. (just remember to isolate them again .... or you'll probably find the fridge/tele/whatever has exhausted both by the morning!)

 

[LadyInBed]

Re: Batteries to \'Both\'?

Using your rational of not switching batteries once the engine has started, how do you ever charge the domestic battery?
The main reasons that instruments reset when starting an engine is that the battery isn't of sufficient capacity for the size of engine or the battery is knackered.
Re Instruments and spikes, modern instruments can either handle spikes or have spike suppression circuits. One/Two/Both switches are 'make before break', so switching from one to two (via both, never via off) will not cause spikes unless the switch is faulty.

 

[alex_rogers]

Re: Batteries to \'Both\'?

The output from the alternator often goes to both batteries through charging diodes, regardless of the position of the battery switches. It all depends on how you boat is wired, hence all the conflicting posts.

 

[jfkal]

Re: Batteries to \'Both\'?

1& 2 for easier starting. Back to house after start if you have splitting diodes for charging. Stay on 1& 2 if not as long as the engine runs. Switch to House as soon as engine is off

 

[LadyInBed]

Good Point - nm