How much grunt?

ghostlymoron

ghostlymoron

Well-Known Member
Joined
9 Apr 2005
Messages
9,889
Location
Shropshire
Visit site
I have a battery conundrum. I have wiring in place on my Mirror Offshore Motor Yacht for 2 batteries but am struggling to find space for them without blocking maintenance access to the rear of the engine (fuel filter and water pump). I can fit a leisure type battery of reasonable size in there but not the starter battery as well. I have a small space under the toilet plinth which I think could house a 'compact' battery but would it have sufficient grunt to start my MD1? I've looked at Red Flash batteries but they're out of my price range. Any ideas?
 
You are unlikley to use enough power on a Mirror Offshore to need two batteries in terms of capacity, but having two batteries makes it easier to avoid flattening your engine starting power. I doubt many had two batteries fitted originally. (If you have the original engine with Dynastart suspect the engine charging available from the engine will also be fairly limited anyway.)

If your engine has a hand-starter and you are able to start it with this, then you wouldn't need to worry too much about flattening a single battery from an engine starting point of view.

If you can't hand-start, you could fit just a single battery, together with a battery monitor (e.g. NASA BM1) and keep an eye on it to ensure you don't get below a certain votage (others may be able to suggest a suitable value) before you run the engine to charge or switch off all lights or other power draws. I believe you can also get a relay through which you could operate these other loads which will cut out when the voltage fallls below a certain level (though this would need to be below the 12.8 volts usually used by relays to control charging).
 
Yes LS, I had been wondering whether 2 batteries on such a small boat was necessary but have been led down that path because Cobblers already has a VSR and cabling for 2. I haven't tried starting the engine with the cranking handle but, if I can do it, that would be my fall back. I have limited electrics on the boat - just aircon and a fridge really (yes I'm joking) and have a 20w solar panel so would expect the battery (ies) to keep charge fairly well and I'm not intending do much night sailing. Maybe reverting to one battery would be OK.
 
Not sure if I'm talking rubbish but how about a motorbike battery? Modern bikes have electric start and some have quite big engines so could be worth looking at. No first hand knowledge as my bike days are long past and only had a kick start at the time!
 
JD. That's not as much as i thought worth considering. Am i right in believing they can be mounted in any orientation? The other possibility is a larger single battery. But i think my MD1 manual specifies a max size of 65ahr.
 
ghostlymoron said:
JD. That's not as much as i thought worth considering. Am i right in believing they can be mounted in any orientation? The other possibility is a larger single battery. But i think my MD1 manual specifies a max size of 65ahr.
Click to expand...

The Red Flashes can be mounted on their sides or upright, according to an ad I've just read. I suspect the 900 won't be powerful enough for starting your engine, though.

The maximum battery size specified for your engine probably relates to the limitations of the alternator/dynamo output capability.

The starting capability is normally specified as a minimum. Note that traditionally this would be given in ah (amp hours = roughly how much output it can give in total, whether fast or slow discharge, before it's flat) as a proxy indicator, but the critical thing for starting is the CCA (cold cranking amps = how much wallop it can put out for a very short period). This CCA requirement might be given in your engine manual*. Engine starting takes a lot of wallop, but only for a very short time, so not much ah. The Red Flash batteries are designed provide a very high CCA for the amount of ah (and physical size) compared to a traditional battery, so they are ideal for engine starting, but not so suitable as a domestic (lights, etc.) battery.

We have a Red Flash battery as an engine battery (1100, I think, for 14hp twin cyclinder engine), and a ludicrously large AGM battery (which lives on its side) for domestics. When the Red Flash battery that came with the boat failed, we ended up buying another as it was the only thing that would fit in the space available without a major reorganisation of the lockers and wiring, or replacing the big domestic battery. The Red Flash has perfomed fine for years since.

I think if I were you, and given you already have the wiring and VSR for two battery working, I would consider trying to find the space and cash for a Red Flash suitable for the engine, and a small conventional domestic battery of max 65ah. If you can't find that space and money, I'd practice starting the engine by hand (you can almost certainly gets some tips on hand-starting that model on this forum) and buy a single conventional battery of 65ah that best fits the space you have, and disconnect the VSR.


*PS I found a reference to a battery recommendation (by the manufaturers, Vapormatic) for the MD1 of 70 ah and 540 CCA. Don't know whether that much CCA is actually needed.
 
You must log in or register to reply here.

Share this page

Top