12v maintenance battery.

[Mudisox]

I know, a basic question. =

How big a battery is needed for a basic boat, small solar panel, alternator on 6HP outboard.
Internal led lights.
LED tricolour and running lights.
Log and depth gauge.
Tiller pilot. ST1.

But ?single 110AH needed/enough?

 

[Boater Sam]

Very small because with a little outboard you will never get it charged properly unless you can rely on a lot of solar which in the UK you cannot.

 

[Refueler]

First - try to find the power demand for each item and add up the total.

Now consider how long you will be running engine and what alternator output will be ..

Solar - what is the output assuming about 50% of its spec output and that during night / bad days - there will be zero charge from it.

How long will you be using the gear with no charge going in ?

Take care that you consider only to use 40% of the Lead Acid battery ... so your 110 A/hr will only deliver 44 A/hrs ... if you use more than that - you will risk shortening the life of the battery.

Lets just say ... assuming 5hr sailing etc.

Cabin lights at 10W total ... 5hrs per day ... = 50W
Nav lights and Tri ... say 15W at 5hrs ... = 75W
Log and Depth at 30W at 5hrs ... = 150W
Tillerpilot up to 20W on auto at 5hrs .. = 100W

Thats ... if I have this correct a total just over 31 A/hrs ... of course spread over the day / night session as not all will be on at same time.

Lets say you have a 40W solar panel ... possibly averaging 1.5 to 2.5A during daylight hours ... that's maybe 15 to 25 A/hrs put back into battery.
Outboard charging is usually pretty poor at about 1 - 2A ... so lets say another 5 A/hrs at most as you say you have a Tricolour - so you are a sailboat.

I've had a couple of Guiness - so my calcs may be a bit 'wonky' .... but I think you get my drift ...

 

[Tranona]

Depends on you estimated daily consumption minus what power you can generate during a day times the number of days you want to be independent of shorepower or not using the bot and having net charge from the solar..

So if your net consumption is 10 amps a day then to keep within the 50% SOC normally advised your 110ah will give you 5 days.

I would guess for typical weekend use a 110 would be ample.

 

[B27]

With no fridge, the tiller pilot is the potential villain.

Some people use them for brief periods while hoisting/reefing/handing sail, others use them for hours on end.
Some boats are heavier on the tiller and use more amps to steer.
More so in weather.

Some people do a lot of night hours running instruments and lights, or spend a lot of time at anchor on moorings with lights on.

Also people's engine hours vary a lot.

The real question though, is are you looking to be 100% bullet proof, or do you accept that if you have several days of poor sun, little motoring and high use, you might have to change your plans a bit to avoid running out of anchor light before dawn?

Personally, we mostly only use the autopilot while motoring, so if we didn't have a fridge or a heater, something like 10Ah a day would be plenty, and a 50W panel provides more than that in half-decent weather, so we'd be fine with a moped battery.

Also do you have HH VHF, tablet navigation or something, a few torches, so if your boat battery goes flat it's mildly annoying rather than life threatening?

Back last century in small boats, loss of power was just one of those things, keep some torches, a HH VHF and GPS, a camping lantern and a pack of AA's from Screwfix.


Also, the battery charging you actually get from your outboard should be measured. Some used to run 12V lights OK, but not get up to 14.4V or so to maximise charging a '12V' battery. Small engine flywheel alternators can be mysterious subtle things.

 

[LittleSister]

I had similar demand but only the outboard for charging years ago (before the days of solar panels on boats).

The 8hp outboard's charging was minimal. It was really only for powering lights directly. Perhaps outboards' electrical output has improved since then, but even so I very much doubt it's actually an alternator as such, and I doubt you'll get a lot of electrical power out of it, nor a high enough voltage for proper full charging of the battery (but it will help slow depletion). In those days I had to take the battery home to charge every time, and when on a week or two's cruise had to trudge around ports I visited, carrying it, trying to find a boatyard or garage that would charge my battery for me.

The solar panel is potentially a game changer from the scenario above, and should mean you start each trip with a full battery (if it's not too large) but as others have suggested, it is very dependent on weather and time of year.

In terms of drain, it is the autopilot that really eats up the power, unless you generally only use it only briefly and occasionally. The LED lights' and the log & depth gauge's consumption is minimal in comparison. Even so, I'd advise having the speed and depth instruments on switches (if they're not already), and keep them off unless you are actually using them.

I am unable to be confident in suggesting a battery capacity, but suspect that 110 ah may be too large. The greater the capacity the longer before it is seriously depleted (which affects the length of its working life), but also the more difficult it is to fully charge with your limited inputs. Another consideration is the weight and bulk of the battery. (Presumably the boat, having only a 6hp outboard, is quite small.) If I were you I'd be tempted to get something like a 60ah to 75ah and see how you get on. If you find it's always fully charged when you set off you could always add another 40ah or 60ah or whatever in parallel to the first battery.

 

[William_H]

So OP will see there is no easy answer to the question. I think ease of managing to fit and remove the battery and weight in the boat might be significant factors.
I set up charging from my old 6HP Johnson but found it only charged at about .3 amp and that with schotky diodes in the rectifier for min volt drop. I never used it in anger relying on solar. Likely modern O/b will do better but as said yours needs to be checked.
I think Nigel's figures might be on the high side. Fit LED of course. As said auton helm might be biggest drain. I guess in reality all you can do is buy what you fancy and modify your real requirements with your own experience. A 40AH deep cycle might well suit you. Do fit a volt meter to monitor what is happening ton battery.

 

[KompetentKrew]

Secondhand car or van battery from the breaker's yard - £10 or £15.

See how that goes - a brand new battery of same Ah will have a bit more capacity. Maybe 50% more.

 

[Refueler]

Secondhand car or van battery from the breaker's yard - £10 or £15.

See how that goes - a brand new battery of same Ah will have a bit more capacity. Maybe 50% more.

Absolutely ..... when I was UK .. I never bought a new battery for my boats - I always trotted off to breakers yard for battery.

I had excellent life out of them ... one I remember ran for over 7 years ... while average was 5+ yrs.

My Alacrity 19 and later Snapdragon 23 - both had single batterys of 75 - 90 A/hr ... I had long weekends with those boats but they had typical 70's minimal gear.

My Sunrider 25 has two 90 A/hr Lead Acids but she's a Motor Sailer so about 50 - 60% of sailing time - engine will be assisting .... I have enjoyed 10 - 14 days jaunts easily.

 

[B27]

I'd be considering either something light enough to take home to charge, or a bigger solar panel for reasonable recharging between outings.

 

[Mudisox]

Thanks to all, I have taken the old battery out, not really used as the boat sat on the owner's drive since 2009, and reads at present 10.3v. It is/was 110Ah.

 

[Aquanaught]

We use a folding solar panel 120W that works really well. There is a 30W panel permanently mounted for trickle charging the battery when away from boat, but as soon as we moor up we deploy the folding panel which runs fridge and on a good day fully charges the battery within a day. Takes up little space in back of a locker.

 

[Refueler]

Thanks to all, I have taken the old battery out, not really used as the boat sat on the owner's drive since 2009, and reads at present 10.3v. It is/was 110Ah.

Pulsators say they can recover old batterys ... not usually ! BUT there is a faint chance if that battery says 10.3V - you may be able to get it back up again ... but not for serious work ... often such can be used to power lesser demand stuff like LED lights ... etc. Backed up by solar could still be useful.

 

[William_H]

Fred drift. When I was young 50 years ago breakers yards or "Wreckers" as we called them were common. Seems now that they are very scarce. I guess the real estate got too valuable and new cars are are so cheap compared to the cost of repairing old ones. Even the smallest accident damage seems to result in write off. I suppose someone must be crushing the old cars but retail s/h parts seem far away now. Just a small lament from antipodes. ol'will