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Old 04-04-2015, 05:19 PM   #1
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Adding a solar charger for batteries.

I am interested in adding a solar charger to my unit to charge/maintain my batteries. I have two 12 volt batteries and the purpose is to keep them maintained when not in use. Maybe a second purpose would be to charge them up a little during the day, but not expecting much.

The question is does it matter what size panel you buy? Assuming that you get a smaller on can you get bigger if you want. At what point, wattage wise, does it switch from maintaining to charging?

Thanks.

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Old 04-06-2015, 03:47 AM   #2
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I have 2 deep cycle 6 volt batteries, 2nd year since newly installed. I use 1.5 watt solar panel I mount on TT tongue. It maintains the charge at 13.4 volts when stored. I tried a 4.5 watt earlier and it over charged the batteries. Just remember to check water levels in the batteries. Good luck


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Old 04-06-2015, 05:33 AM   #3
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It really does depend what you want to do. If you just want to keep a disconnected battery charged a 5w panel with a basic charge controller will work. The charge controller would just disconnect the panel from the battery at a set voltage so it doesn't over charge it. If you want something that will have different stages then you need a mppt type controller and a larger panel. If you want to charger your batteries then you need at least a 60W panel.
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Old 04-06-2015, 06:47 AM   #4
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Quote:
Originally Posted by 10Boomer View Post
I have 2 deep cycle 6 volt batteries, 2nd year since newly installed. I use 1.5 watt solar panel I mount on TT tongue. It maintains the charge at 13.4 volts when stored. I tried a 4.5 watt earlier and it over charged the batteries. Just remember to check water levels in the batteries. Good luck


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This is why a solar charge controller is needed to replace battery capacity lost.

Small panels without integral controllers can overcharge because current will always flow when the panel in in partial to full sun; whether the battery can handle the influx or not.

Your 1.5 watt panel is producing 13.4 volts at (.112 amps) 112 milliamps (1.5 watts) in full sun; less current if not. That is about what parasite draw plus internal loss is; so capacity remains about the same when the panel is connected. Much more continuous current than that will definitely overcharge and ruin the battery over time as you discovered.

A solar charge controller will reduce charging current to a trickle as the battery fills; a good one will also vary voltage output based on battery condition, like your camper's staged converter, to "make hay while the sun shines" and efficiently recharge a partially discharged battery.
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Old 04-15-2015, 10:10 AM   #5
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After doing some more research I have determined that what I want to do is to use the batteries during camping and then be able to charge them slowly over course of a week or two. If I need a quicker turn around, like camping two weekends in a row I can pull the battery and charge them at home.

I know I need a controller but the question stands for the panel. Does this additional info help narrow the panel size?
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Old 04-15-2015, 11:17 AM   #6
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There is a ton of information that is needed to size panels correctly. . I see a lot of misinformation on forums, and a lot of wasted money as well. My advice to you would be install a panel on your roof. 50-100 watts. Chase the cable down the fridge vent, Mount a charge controller in the back of the fridge ( go with a sealed charge controller, yes they exist), and tie into the DC Side of your panel. There is better, ways to install, that can maximize your DC out put, but this will give you your desired result with pretty good recovery time, for the best dollar for dollar result. . Do not buy these parts from an RV dealer. Buy on line or find a reputable electrical supply vendors. Panels should be at or about $1/watt. Basic charge controller about $60.
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Old 04-15-2015, 01:15 PM   #7
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There is a ton of information that is needed to size panels correctly.
Therein lies the nut.

Existing battery capacity in Amp Hours; the amount of that battery capacity you use per day; the amount you would like to replace each day of available/usable daylight; and how low you are willing to discharge your expensive deep cycle battery(s) will determine the wattage you will need on the roof.
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