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Correct size solar panel for maintaining charge?

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doublereefed
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Initially Posted - 03/22/2018 : 14:58:02

I'd like to plug in a solar panel when I leave the boat, of the correct size to keep the battery charged between outings. What is the correct size for one 12V battery?

This might do the trick?
https://bit.ly/2pwVR9J

'95 C250 WB #61
Midway, UT

doublereefed
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Response Posted - 03/22/2018 : 14:59:22

Or this one? This one is 15W, the other is 5W. Are there safety issues with unattended charge rate, etc?
https://bit.ly/2HWYmti

'95 C250 WB #61
Midway, UT

Steve Milby
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Response Posted - 03/22/2018 : 15:47:50

I have a little 10w solar panel that easily keeps my battery topped off all summer. It came with a small charge controller that protects the battery from being over charged, but I've heard you don't need one for 10 w or less. Even if a charge controller isn't needed, it's cheap insurance to protect your battery. A 10 w panel should cost about $25.00 and a charge controller about $10.00. Check ebay.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Edited by - Steve Milby on 03/22/2018 15:49:02

islander
Master Marine Consultant

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Response Posted - 03/22/2018 : 16:00:58

I too have a 10 watt that does the job of trickle charging and maintaining. 5watts is very small and will maintain the charge but not much in the way of trickle charging a low battery. When it comes to solar panels, Bigger is better. Any reason you want a semi flexible? I don't think they have much weight to them so it might need to be tied down so it doesn't blow away. I use a charge controller for the insurance even though a 10watt panel doesn't require one. A 10-20 will do the job for you.

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

OLarryR
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Response Posted - 03/23/2018 : 06:34:15

You can do the math regarding the charge that a 5 watt, 10 watt or larger panel will put out on a daily/weekly basis to decide if you need to have a controller to prevent cooking the battery(ies). But there are factors that can vary quite a bit: The loads you generally subject on your batteries each week and the number of cloudy or non-charging days/hours each week and this can also vary depending on your locale. If you were rarely using your batteries for any loads and just want to keep the batteries trickle charged - 5 watts without a controller is generally okay. 10 watts may be okay without a controller if you regularly use the batteries each week besides having to maintain a trickle charge.

In the summer, a battery will lose approximately .5A/day just sitting around. In the winter, approximately .25A/day. I believe I saw this in a book for flooded batteries. For AGMs and Gels, it may be similar or a bit less loss per day. For summer months, anticipate losing 7 x .5A = 3.5A per week or maybe 7 x .4A = 2.8A per week.
My 20 watt panel puts out ideally 1.2A/hour but averages less than that ...more like ranging between .2A - .8A/hr for ..let's say 7 hrs per day since sun is not always directly overhead and a panel may lose some efficiency during it's lifetime. My panel constantly for past 10 years puts out a max of .8-1.0A but mostly .8A w/sun overhead in my locale. So, for average charging rate over the whole day, I figure it around .5A for 7hrs = 3.5A/day. Then, my locale, estimate 5 sunny days out of the week...so that's 3.5 x 5 = 17.5A/week. So, a 5 watt panel would be putting out about 1/4 of that for a week = 4.4A/week and the battery loss for the week would be between 2.8A - 3.5A per week. So, even if my estimates are slightly off +/-, you can see that a 5 watt panel is sufficient to trickle charge a battery - 4.4A vs approximately a 2.8A - 3.5A loss for the week.

A 5 watt panel will provide some but not much juice for running loads on the boat over and above keeping the battery trickle charged from expected daily battery loss. It's charging rate is also fairly close to the battery loss rate and so a controller not needed. A 10 watt would be able to handle some addl loads besides maintaining the battery against daily loss but without a controller, may cook the batteries to some degree - Depends on just how many charging amps above a fully charged battery state would result in cooking the batteries - Not sure. Definitely, a panel from 10-20 watts for a single battery should have a controller to avoid cooking the battery. (Also - If running the panel during the winter, the batteries lose less amps during the cooler months...so overcharging could be more of a concern during cooler months.)

I do not run many loads off of my 2 batteries - I regularly use my Fishfinder and for starting the outboard (though, the outboard if run for a time will charge the batteries). I occasionally use the nav/cabin lights and infrequently use a mini-boom box usage, vacuum, etc. My 20 watt panel is sufficient to keep both batteries fully charged throughout the year - My batteries never come off the boat and the boat is in the water year-round. I use a Morningstar Pro controller which is an overkill but I like the digital readout it provides for charging rate.

Larry
'89 Robin's Nest#5820, Potomac River/Quantico, Va
http://catalina25.homestead.com/olarryr.html

Edited by - OLarryR on 03/23/2018 06:50:16

doublereefed
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Response Posted - 03/23/2018 : 15:28:58

Folks, thank you very much for the info and advice. I just need to top up the battery after day sailing, so I will go with a 10W panel and controller. Rigid is probalby better, I will set it on the lazarette when I leave the boat... strapped down of course.

'95 C250 WB #61
Midway, UT

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