Electrics Question

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lukewhite

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Before I start, I hope that this is a sufficiently different question to those already posted about batteries etc....
On my Moody 36 I currently have 2 110Ah 'leisure' batteries. I wish to add a dedicated engine starting battery and am wondering what size I would need to start the Thorneycroft T90 (36hp).
 
I think your engine is 4 cylinder, 1.5 litre. I would get a car battery and would size it on one suitable for a Peugeot 1.9 litre diesel engine, (as fitted in a 306), to give more than enough starting power.
 
We have the T95 and i used to go for the biggest that would fit in the box but apparently , and after research it is best to do what the post above indicates.
Smallest within reason.
 
If space is tight, you might consider a dedicated starter battery such as a Red Flash or an Oddyssey. More expensive, but half the volume typically and can be mounted in any orientation. Speak to Merlin or Barden for advice. Assume you have split charging already.
 
"I wish to add a dedicated engine starting battery".

Why?
I've owned numerous boats with identical twin batteries and adequate switching arrangement, never any problems.
 
Thanks for your answers so far. Space isn't really a problem, I could lie down in the spare floor space in the engine compartment. Malaprop, I currently live on the boat, albeit in a marina and so the boat gets more use than usual. I have quite often managed to empty the first battery (Yes I know it needs replacing as it should last way longer) in the space of 12 hours while out sailing. I thought that by adding a dedicated engine battery and then later, when funds allow, replacing the useless battery I would be able to stay away from shore power for longer.
 
I fitted a Red Flash 1100 a couple of years ao. No problems with it, but in hindsight a small cheap car battery from Halfords would do just as well as a dedicated start battery, if you have the space.
 
To put a figure on it, go for a 60Ah battery for dedicated starting. Would increase that to 70Ah if you have a Windlass dependant on it while running.

This is sufficient for the average 1.6 litre diesel. Mine is 1.1 litre Yanmar 30hp.

This allows you to put in more Ah for the domestics.
 
If you are on a budget that's a reasonable plan. Leisure or deep cycle batteries don't like putting out high currents to start engines and a better battery for that would be an automotive "cranking" battery. You will need to add a switching or split charging circuit of some kind but that's very simple and you only really need one of about 75 AH as a dedicated engine start battery if your engine starts reasonably easily. Bigger only if it doesn't.
 
Agree that if space is not a problem then an Automotive cranking battery 70/75 is OK. More important in my view is to have a split charging system using a VSR that can parallel the batteries so that you can use the domestics in an emergency to start the engines. Also ensures that your engine battery is topped up first. You can supplement your domestic charging by solar panels or wind power to reduce your dependence on engine charging.
 
I just use a 80AH halfords battery as a dedicated cranking battery for a T90. Separating the cranker from the domestic is a good way of making sure you have enough amps to get the motor going. I also carry a gel Red Flash as back-up but so far that's been a case of overkill.
 
It didn't work like that!

"Both" was for starting and charging.
"One" for domestic on odd days of the month.
"Two" for domestic on even days of the month.

It did take a while to learn . . .
 
[ QUOTE ]
are halfords all that cheap

[/ QUOTE ] No I bought a 60Ah leisure battery there last year. Just convenient to pop in there and get one. It is possible to pay higher prices though!

No carriage to add on and no petrol used to collect!
 
you say your 110Ah battery may need replacing because it should last way longer than 12 hrs. Not necessarily so. Just running a fridge and some electronics can easily use 7 or 8 amps, which would pretty much drain your battery. Never mind that you shouldn' go below 50% capacity too often etc.. On a channel crossing running instruments, plotter, fridge, autopilot and some radar you'd be hard pressed to stay below 10amps of consumption. (We use 11 or 12 but have a 580Ah battery bank).

For a starter battery, any car style batter in the 60-80 AH range should suffice.
 
The appropriate criterion to look at when sizing a starter battery is its CCA, Cold Cranking Amps. For your engine, this should be around 600a. The AH capacity is almost irrelevant, since very little is used in starting.

Most car batteries are high CCA and relatively low AH - ie, they have lots of thin plates in each cell so they can deliver high currents (and accept high re-charge rates).

However, this type of battery is not suitable for long slow discharges typically needed for yacht live aboard domestic use or caravans, where fridges or lights will be run for longish periods without any charging. To minimise battery deterioration over these deeper discharges, fewer, thicker plates are most suitable (deep cycle batteries). But these give low CCA, so they're not the bees knees for starting . . .

'Leisure batteries' are a compromise, suitable for '1,2, Both' installations when you may alternate batteries between starting and domestic use. They'll be bigger to achieve the appropriate CCA, but capable of withstanding a reasonable number of discharges to 50% capacity.

I hope that confuses the situation suitably for you . . .
 
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