Battery spec

[Judders]

I thought I just needed 12vt but apparently it's more complicated than that?

It isn't running a starter motor, just the lights, instruments and radios.

Can anyone tell me what I need?

 

[gavina]

Depends on how the battery will be cycled. Will the battery be run flat routinely or is it usually kept topped up in a marina?

 

[Judders]

Well I killed the old one by not topping it up for too long. There is an alternator on the outboard but of course that is not always run for very long. That said, I am planning to buy a solar trickle charger this time around so as to avoid it's going completely flat again. Since I am going to be on a swinging mooring this summer, the prospect of passing the battery down into the tender every week does not really appeal.

 

[Bodach na mara]

Not without some more information. Do you mean what is usually referred to as a "domestic" battery? How is it to be charged? What sort of load are you going to place on it?

For what it is worth, on my last boat I had two batteries; and engine start one (for which cold-cranking amps is the important value) and a domestic battery to operate lights, radio and a cold box. The domestic battery had a capacity of 80 ampere hours and was only just adequate. But I could not have got a larger one in anyway.

On my current boat I have more lights, heating, pressure pump for water and a fridge and find that a domestic bank of two 85 Ah "leisure" batteries are OK. These are supposedly deeep discharge type and should be better able to stand being drained regularly than the starter battery, which really likes to be kept up to charge all the time. The arrangement is a bit odd (to some people) in that the batterise are in series giving 24 V for the fridge, heating and water pump (the high demand items) and 12 V is supplied via a DC/DC 24 to 12 V convertor. I get the equivalent of about 160 Ah capacity in total.

 

[Judders]

This will be a very much less thirsty situation as the boat in question is a small cruiser racer.

It will be charged by the inboard outboard, when that is running, and a solar trickle charger to save it going flat. I will endeavour to take it home every now and then for a full charge.

In terms of what it is to power, it is just instruments, lights (both nav and domestic), the FM/AM radio and the VHF.

 

[maxi77]

I have kept a bank of 2 standard leisure batteries for several years using a small wind generator (one of the baby Forgens) with the occasional 'proper' charge. I think overall you need to be able to put back a bit more than the self discharge every day, plus what you use when you are sailing.

PS buy your battery from a caravan shop, usually cheaper.

 

[LadyInBed]

Sounds like your consumption is quite light, so on the face of it you might think that you don't need a high Amp/Hour battery, but that isn't so.
Using the metaphor of buckets for batteries.
Three quarter emptying a small bucket is far worse than quarter emptying a large bucket.
Another point to note is that several small buckets in parallel to give equal capacity (Amp/Hour) is better than one large bucket. Also if one goes fut (a technical term /forums/images/graemlins/grin.gif ) it can be isolated and you can still use the others.
Your main problem will be charging them. O/B's aren't great at filling buckets, so something like a Solar-Powered 12V 12W Battery Charger £40 would do a job. I wouldn't consider anything less than 10W.

 

[Allan]

I had a similar set-up on my previous boat, Etap 22i. I agree with the idea of buying at a caravan shop, I saved £20 by doing so. I bought a £9.99 solar panel from Maplins and never removed the battery again. The only other charge it got was a couple of times a year in a marina with a 6amp Halfords charger.
Allan

 

[seumask]

Judders on a similar boat also swing mooring we run an 85ah battery which is only topped up by a Forgen 500 (pretty low output) wind generator and a 5-7 W removable solar panel when we go away for about 10 days in July, current battery (Varta) from http://www.bardenbatteries.co.uk/ is 4 years old and appears healthy. I do carry a spare 85AH from a caravan shop as well but have never used it. We run depth, log, chartplotter, radio, cabin lights, autopilot, LED nav lights all fairly sparingly.