What size engine start battery?

[PabloPicasso]

Wow this is all etting very technical. In my original question I wanted to know if a small battery could usefully be used to start the engine. It seems from discussion here that this would be impractical or usafe on a boat.

So I coiuld use my 85ah existing batt to start engine and get a greater capacity leisure battery say 120ah to run domestic?

I would need to find a space for this new battery though, perhaps in the gap aft of the gearbox, but access may be difficult there

 

[Plevier]

Well I wanted to answer VicS's question!

I suggest your economical solution with a reasonable safety margin is a normal starter battery of around 35Ah if you have space for it. If space is more problem than cost, then a Red Flash - but a bigger one than a 900 to have a bit of spare capacity in the bag.

But it's your judgement, your choice!

PS You must reserve it just for engine start - wire it so it can't be connected to the domestic loads.

 

[bluedragon]

Been wondering the same thing (why have a big engine start battery). Our Volvo 2020 starts immediately (1-2 seconds?) even in winter (after 10 secs glowplug). So a small battery would do the job I'm sure. If bleeding the system or similar circumstances, then what's wrong with jump leads to the service battery?

 

[sailorman]

Been wondering the same thing (why have a big engine start battery). Our Volvo 2020 starts immediately (1-2 seconds?) even in winter (after 10 secs glowplug). So a small battery would do the job I'm sure. If bleeding the system or similar circumstances, then what's wrong with jump leads to the service battery?

If bleeding the system or similar circumstances, then what's wrong with jump leads to the service battery?
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a change over switch does that job @ no messing about

 

[VicS]

what's wrong with jump leads to the service battery?

There is always a small risk of a spark igniting any hydrogen when connecting or disconnecting jump leads .

It's not very likely but it can happen and every now and then it does happen. The risk of having a battery explode in your face is something to be avoided.

It is why the last connection made and the first broken when using jump leads on vehicles should always be to a point, such as a bolt on the engine, well away from the battery itself. Not always so easy to do on a boat.