Solar panels ,how large

[30boat]

What size solar panels should I have for keeping two 85Amp batteries topped up?The boat is in the Algarve and even in the winter there's quite a lot of sunlight.I'd like them to be the smallest possible useful size for ease of installation.

 

[pvb]

What sort of batteries are they? Different types of battery have different rates of self-discharge. And do you have any constant-drain electronics?

 

[lw395]

Strictly to combat self discharge of the batteries, 1W will do it. If the batteries are good and you get 25% of that watt on average over 24 hours.
I've had a 100Ah battery on a PSU in my garage for ages, the current drawn to float it at 13.6V drops off to about 10 or 20mA after a week or so.
But this figure varies a lot from battery to battery. A little motorbike battery that is past its sell by date (but still starts the bike) can use more.
It depends what you mean by 'keeping topped up'. Is that replenish the battery after occasional use of the odd light now and then, or running the eber for an hour a week or what?
Any amount is better than nothing.
1W will help your battery a lot.
5Wwill be better. (and maybe allow the eber now and then?)
Diminishing returns set in somewhere...
Think in terms of what is easy to install at sensible cost.

 

[BAtoo]

20w keeps my 140AH nicely topped up on the Orwell between too infrequent use of the boat and was fine over winter too.

Got mine off ebay, size about 600x400mm ish iirr.

 

[VicS]

5 watts more than enough to keep my 60AH leisure battery topped up and recharges the battery after a weekend's use no trouble.

about 300mm x 250mm

 

[Mistroma]

What sort of batteries are they? Different types of battery have different rates of self-discharge. And do you have any constant-drain electronics?

Spot on.

I have Trojan T105s 450Ah which have a high self-discharge rate, if they were AGMs then I'd get away with a much smaller panel. I also have an alarm and can monitor battery state remotely so that takes more power.

My 65W panel just kept up with self-discharge and alarm during January near Vigo. I calculated a loss of 5.25Ahr/day and "guessed" a charge efficiency of about 80% to get a required input of 6.5Ahr/day. Calculations based on historical data indicated that average daily output would be around 6.3Ah/day using my panel.

About 50% of my output was used run the alarm & monitoring system. On that basis, your 170Ah bank would be easily manage with a 12.5W panel. So you could use my figures for a worst case sanity check as your banks won't have higher self-discharge.

BUT, you are in the Algarve and probably have batteries with lower self-discharge and also may not have anything connected to drain them over winter.

You can work it out yourself using the figures below for a 10W panel lying flat on deck with no shading and located in the Algarve.
Month Avg. Ahr/Day
Jan.......... 1.5
Feb.......... 2.2
Mar.......... 3.0
Apr.......... 3.8
May......... 4.3
Jun.......... 4.7
Jul........... 4.7
Aug......... 4.2
Sep......... 3.4
Oct.......... 2.5
Nov......... 1.7
Dec......... 1.4

A 10W panel will provide approx. 1.5Ahr/day during the worst months (in an average year). Just work out the Ahr you need to allow for battery self-discharge plus any gear being run.
If it's 1.5Ahr/day then a 10W panel will do and if it's 0.75Ahr/day then a 5W panel will be fine.

N.B. Data obtained from PVGIS model http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php (might not be latest version).

 

[30boat]

Spot on.

I have Trojan T105s 450Ah which have a high self-discharge rate, if they were AGMs then I'd only get away with a much smaller panel. I also have an alarm and can monitor battery state remotely so that takes a little more power.

My 65W panel just kept up with self-discharge and alarm during January near Vigo. I calculated a loss of 5.25Ahr/day and "guessed" a charge efficiency of about 80% to get a required input of 6.5Ahr/day. Calculations based on historical data indicated that average daily output would be around 6.3Ah/day using my panel.

About 50% of my output was used run the alarm & monitoring system. On that basis, your 170Ah bank would be easily manage with a 12.5W panel. So you could use my figures for a worst case sanity check as your banks won't have higher self-discharge.

BUT, you are in the Algarve and probably have batteries with lower self-discharge and also may not have anything connected to drain them over winter. So you are likely to be fine with something around 5-10W

If you can provide more detail on battery type (plus any devices connected) then I can plug in new figures and re-calc for the Algarve.

The batteries are 85 amp leisure type(don't know much about batteries to be honest)one is 3 years old the other brand new.They're not maintenance free.There's usually nothing left connected.

 

[Mistroma]

It took me a bit of digging to get self-discharge figures for my T105s and of course the figure also varies with temperature. It will be warmer in the Algarve in winter than Vigo but I'd guess that 0.2%-0.3% per day wouldn't be an unreasonable est. for leisure batteries. Hopefully someone with more knowledge will be able to confirm this guess.

That would mean you need 0.3 - 0.5 Ahr/day to cover self-discharge in Dec./Jan. period and that's a 2 - 3.3W panel (based on figures I gave for a 10W panel producing around 1.5 Ahr/day).

It looks as if a 5W panel would more than keep up with self-discharge in Dec./Jan. You might even get away with a smaller one as output would be quite a bit higher in Nov. and Feb.

I'm not an expert in this field but did do a lot of digging last year before leaving the boat for winter. I was able to monitor the batteries at different times and compare actual vs. forecast (using various models). It all seemed to work out and figures were close to those I expected (total Ahr generated and battery voltage).

I hope that this is of some help and you might get something better if a battery/solar expert appears.

 

[TQA]

1 w is not enough 5 w will do the trick.

 

[ghostlymoron]

5w might be enough for replenished self discharge but if you can fit larger you'll benefit when you're on the boat by replacing the power you use as well.

 

[30boat]

This is good news .I thougt I'd need large expensive panels.I'll get something around the 10w mark.I've been spending far too much on the boat as it is.

 

[GrahamM376]

5w might be enough for replenished self discharge but if you can fit larger you'll benefit when you're on the boat by replacing the power you use as well.

+1

What kind of sailing do you do and what electrical loads? Assuming one battery is for engine start and the second domestic, if intending to power a fridge then you need a reasonably large panel or the domestic battery will soon be discharged.

 

[30boat]

+1

What kind of sailing do you do and what electrical loads? Assuming one battery is for engine start and the second domestic, if intending to power a fridge then you need a reasonably large panel or the domestic battery will soon be discharged.

My largest loads are the car radio and some times the inverter when recharching a laptop.I've been too lazy to install a fridge until now .And cabin lihting of course but I'll fit some led bulbs soon.

 

[TQA]

You did not mention the fridge before. The 5 w wil take care of self discharge while you are away but if you are talking about using solar when you are aboard you need to do an accurate energy budget before setting it up. I have 400 w of tiltable solar and that is enough to run the fridge etc with power sto spare most days.. I could get away with 300 w on the sunny days.

 

[forelle541]

I use a £10 job from Maplin, more than keeps up with my three batteries and that's in Suffolk.

 

[30boat]

You did not mention the fridge before. The 5 w wil take care of self discharge while you are away but if you are talking about using solar when you are aboard you need to do an accurate energy budget before setting it up. I have 400 w of tiltable solar and that is enough to run the fridge etc with power sto spare most days.. I could get away with 300 w on the sunny days.

Sorry I frased that wrong.I still don't have a fridge so my loads are quite low.

 

[30boat]

Sorry I frased that wrong.I still don't have a fridge so my loads are quite low.

What about one of these?

http://www.maplin.co.uk/solar-powered-12v-2.4w-battery-trickle-charger-223251

 

[Richard10002]

You did not mention the fridge before. The 5 w wil take care of self discharge while you are away but if you are talking about using solar when you are aboard you need to do an accurate energy budget before setting it up. I have 400 w of tiltable solar and that is enough to run the fridge etc with power sto spare most days.. I could get away with 300 w on the sunny days.

My understanding is that he is merely trying to keep the batteries topped up while he is away from the boat for a few days or weeks - i.e. . something to keep up with any self discharge and, over time, get those last few Ah in so they get as close to 100% full as possible.

Whatever his daily useage whilst on board, he will need quite a substantial bit of solar kit if he wants to keep them topped up and keep up with useage whilst onboard.

A 100W panel might give 5A in good sun at the right angle on the Algarve. Mine, laid flat in Manchester, give about 3A at the very best.

 

[ghostlymoron]

What about one of these?

http://www.maplin.co.uk/solar-powered-12v-2.4w-battery-trickle-charger-223251

Too small and not protected to marine standard. I'd go for one of these http://www.sunstore.co.uk/12v-20w-Polycrystalline-Solar-Panel-Semi-Flexible.html

 

[30boat]

Too small and not protected to marine standard. I'd go for one of these http://www.sunstore.co.uk/12v-20w-Polycrystalline-Solar-Panel-Semi-Flexible.html

That's good but I was trying to do it on the cheap.I guess I'll have to bite the bullet eventually.The price is not bad actually.