Solar panel for starter battery

[deltaevo]

Morning all,
My boat will be going on a swinging mooring this year. I am planning to have a solar panel to charge the starter battery. Any recommends on the size of panel needed
thought about getting a 100 watt, would this be overkill ? Or would it be advisable if buying a 100 watt panel, use it to charge both starter and leisure batteries.

Thanks

 

[peteK]

A 20w solar panel is sufficient to keep both batteries charged but if you are going to fit solar you may as well fit the largest that you have room for as prices have come down over recent years.

 

[BruceK]

What you need to do is work out your draw, work out how much charge you're putting back in, work out the deficit, then supply a panel capable of meeting that deficit in half the period between scheduled trips.

e.g. For my house batteries I will on an average weekend trip use about a third of my house battery capacity or 120 Amp/h
Amp-hours × Volts = Watt-hours
120x12 = 1440

If as an example my house batteries were not charged by an alternator and I went out every weekend then I'd need

1440/ (5/2) = ~500 -600 W solar panel

However my house batteries are on an alternator charge and on average a 2.5 hours journey (return plus next weeks outbound) and the alternator capacity is 30amps so 75 amp/hr is returned leaving a deficit of 45 a/hr or 540 W/hr
540/(5/2) = 200 W panel


If you pump back as much or more than you take out and just want to keep the batteries in peak condition then a 20W to 30W panel is more than sufficient. It's what I have on my starter battery

 

[deltaevo]

Thanks for the replies very informative. I think I will fit the 100 watt panel and split the load , I’m not running anything with a big power draw.

What you need to do is work out your draw, work out how much charge you're putting back in, work out the deficit, then supply a panel capable of meeting that deficit in half the period between scheduled trips.

e.g. For my house batteries I will on an average weekend trip use about a third of my house battery capacity or 120 Amp/h
Amp-hours × Volts = Watt-hours
120x12 = 1440

If as an example my house batteries were not charged by an alternator and I went out every weekend then I'd need

1440/ (5/2) = ~500 -600 W solar panel

However my house batteries are on an alternator charge and on average a 2.5 hours journey (return plus next weeks outbound) and the alternator capacity is 30amps so 75 amp/hr is returned leaving a deficit of 45 a/hr or 540 W/hr
540/(5/2) = 200 W panel


If you pump back as much or more than you take out and just want to keep the batteries in peak condition then a 20W to 30W panel is more than sufficient. It's what I have on my starter battery

 

[freddyuk]

Make sure you use a charge controller with that size panel. I would go for a 30w panel unless you leave any lights on. The battery is fully charged when you moor up and will not lose much charge whilst idle. A cheap 5 amp controller will ensure it is kept topped up and not drain out overnight.

 

[DavidJ]

Re freddy’s comment
From this forum (Hurricane I think) I bought this controller which does both battery banks
12V solar panels charging kits for caravans, motorhomes, boats, yachts, marine

also bought the solar panel from them and a remote meter display which tells you exactly what’s going on.

 

[BruceK]

Thanks for the replies very informative. I think I will fit the 100 watt panel and split the load , I’m not running anything with a big power draw.

Please note I have used days rather than hours in the calcs based on my own observations that the panels do not operate at 100% and power generated only peaks for a couple hours a day. I effectively ignore early morning and evening hours of daylight when power generated is low. i.e. it's a conservative calculation

 

[Hurricane]

No, my controller just charges the house batteries.

I don't agree with the comments about buying just any old cheap 5 amp charger.
You would be missing a trick if you don;t buy a proper MPPT controller,
Just Google MPPT controller and you will see what I mean.
Only a little more but way more efficient.
The latest Victron MPPTs are 20% even more efficient.
Also spend a bit of time and money sizing the wires - it would be a shame to install all this and not get the best out of the system - whatever panel you use.
A point to note is that the Victron controllers need 5v more than the battery bank's voltage before they start so the output voltage from the panel is also important to consider.

Because I have my panels connected to the house bank, I like to look at the maths differently.
I like to look at how much power the boat uses each day and how much is harvested from the solar panels.
I like to think of it in KwHours.
Our boat uses about 6Kw Hours per day and my solar panels can harvest 3Kw Hours on a good day in the Med.
So that means that I have to put the balance (3Kw hours) into the batteries using another method (the generator).
And for me, I choose to put that in when the batteries are at their least full - that way, they charge quicker.
As a lead acid battery gets full, it takes less current - exponentially so no point trying to charge a nearly full battery - it would take too long.
Thats my theory anyway.
But that is all for the house set.

 

[deltaevo]

Once again thanks for everyones helpful comments. I intend to run the panel via an mppt controller,
have always thought that was a better solution.

Just got to figure out where to fit the panel. The boats a Sea Ray 220 DA, so not exactly massive lol.

 

[Scubadoo]

Your going to have fun finding the best position and space for a 100watt panel on that size boat. I fitted a 11w solar panel to trickle charge my battery during the week to keep it topped up ready for the weekend. This size flexible panel fitted nicely on the raised helm console behind the windscreen on my 25ft boat.

 

[BruceK]

I got a 200W 2mm thick semi flexible with micro lenses on that act just the same as anti slip and sikaflexed it to the topdeck. Looks like it's painted on and is totally unobtrusive and actually compliments the boat's look.

They can also be attached to the canvas canopy with bespoke straps

 

[tillergirl]

I have a 24v system for the 'Start Bank' - 2 x 12v 110Ah batteries in series. Two 80w solar panels in series. The light (charge indicator on the controller) has been flashing all winter - meaning that the Bank is full.

 

[deltaevo]

I got a 200W 2mm thick semi flexible with micro lenses on that act just the same as anti slip and sikaflexed it to the topdeck. Looks like it's painted on and is totally unobtrusive and actually compliments the boat's look.

They can also be attached to the canvas canopy with bespoke straps

Have yuo a link for the flexi panel BruceK?

 

[BruceK]

You said you were after 100W. If so this will suite. Get a good controller with it

12V solar panels charging kits for caravans, motorhomes, boats, yachts, marine

Sorry cant find the straps for it anymore. Just the fasteners. I suppose it wouldn't be too much effort to fit them to some canvas straps

 

[freddyuk]

Please see this information. MPPT controllers are wasted on single 100w module. Once you moor up your batteries should be already charged up so you are trickle charging unless you are running some background loads constantly. Also the modules will be moving about so maximum power point will be a moot point!
Which solar charge controller: PWM or MPPT? | Victron Energy