Sizing small solar panels for batt storage

[roland stockham]

Hi all. Need advice on size of solar panels. I am leaving the boat on hard standing for 6-9months and there is no electric available. In the Carribean so lots of sun. I don't have anywhere to put large panels so a full setup is out.
What would be the right size panel to keep 700Ah of lead acids up. No loads running but how much extra would you reckon the run the bilge pumps on a 35ft?
I have a little 5watt panel but that will only run a radio or GPS, gives about 150ma. I think I need 2-3a @13.4v, that's about what the charger runs at when everything is charged which is 25-40W. But, given that the 5W panel actually on produces about 2W I am a bit stuck on sizing! Don't want to overdo it and cook the batteries.

Thanks

[svlamorocha]

Originally Posted by roland stockham

Hi all. Need advice on size of solar panels. I am leaving the boat on hard standing for 6-9months and there is no electric available. In the Carribean so lots of sun. I don't have anywhere to put large panels so a full setup is out.
What would be the right size panel to keep 700Ah of lead acids up. No loads running but how much extra would you reckon the run the bilge pumps on a 35ft?
I have a little 5watt panel but that will only run a radio or GPS, gives about 150ma. I think I need 2-3a @13.4v, that's about what the charger runs at when everything is charged which is 25-40W. But, given that the 5W panel actually on produces about 2W I am a bit stuck on sizing! Don't want to overdo it and cook the batteries.

Thanks

When you say "lead acid", do you mean flooded (as opposed to AGM lead acids?)

Are you talking about panels connected directly to the battery, without a controller/regulator in between?

If so, ask your battery manufacturer. The panel will need to offset the self discharge, which is significant for flooded batteries and depends on the composition of the plates (antimony vs calcium). I estimate you will need in the order of one amp times 12 hours a day just to offset self discharge.

You can make your own estimate by first charging the batteries to 100% (ie when accepted amps at manufacturer-specified absorption voltage drop to manufacturer specified value, say 1or 2% of 700 = 7 to 14 amps. Then use a shorepower charger to keep the batteries at float voltage and measure the amps going in to the batteries. Multiply those amps by 13 or so and get the rating in W of the panel you want.

All that said, given that these days you can get a proper Victron solar controller for US$105, there is no need to walk the fine line between undercharging and overcharging the batteries by not having a controller.

[roland stockham]

Thanks for that and yest they are flooded deep cycle. I have measured the charge going in from the shore charger and it is about 20Ah/day which suggest a panel output of 3A for 6 hrs or about 40w. I was hoping to find someone who has used small panels like this and find out what thy got in real life. don't want to buy something and find the output is half what I expect. The ideal would be something I can temporarily hook up whenever I leave the boat.

[donradcliffe]

If you have room for 50 watts, just add a $10 regulator from Ebay and fugettaboutit.

[hellosailor]

700Ah of batteries might only need 150-200mAh of charging to offset the normal self-discharge of the batteries. You'd have to check out their specs to find out what those particular batteries need, and then look for a battery "maintainer" rather than a full powered charger, to simply maintain them.


Since you're on the hard, check for a bilge drain plug and consider installing one if it isn't there. No reason to run a bilge pump when you're on the hard, and using a drain plug also means you don't worry about the bilge pump failing, which they regularly do. Or starting a fire. If you must keep the bilge pump...I'd consider putting in a separate unregulated solar panel, just to power the bilge pump. That's not a totally reliable solution, obviously, but it should keep up with any routine seepage, without complicating the battery maintenance.

[Hybrid Power]

A good rule of thumb is for panel sizing is to design a setup that will yield at least 3A (Imp) per 100 amp-hours of battery capacity.

For small panels of less than 140 watts you cannot go wrong with a little Genasun controller.