giving it a try

[kleven626]

Just bought a 15w panel and two 12v marine batteries. Put them on my camper and going camping this summer. I think between the 12v charging on the truck, while towing, and the panel sitting out while set up for a couple of days. I should be able to maintain the batteries. What do you think? :-/

[Anon001]

A 15w panel isn't much. Also, it depends on what you are going to try and run off of it.

[johnjmw]

kleven, I am not sure, but 15w solar panel wont do much more than maintain the charge in those batteries. Charging while driving will easily recharge them depending on how long your drive is.
I don't remember the formula right off hand. The info you will need is the Amp hour rating for the batteries and the voltage (are they 6vdc or 12vdc) 12vdc they will be wired in Parallel. A 15watt panel at 12vdc is only supplying aprox 1amp per hour. If your batteries are only 75 amp hours each that would give you 150amp hours total.
If you only discharge the batteries 50% then it would take 75 hours of daylight to recharge them and at 4 or 5 hours of peak daylight that would be 15 days to recharge them.
I hope someone who understands this stuff better than me and can explain it to make more sense. As long as your not running to much power off the batteries for to long the drive between camps should recharge them. There are a number of meters available you can have connected to them to tell you how much usable power you have left in them. Jeff Yago even has an article on wiring the circuit.
John

[Anon001]

John is right about the meters. Some inverters (such as mine) have a digital readout showing the status of your batteries. I also have a digital voltmeter that I can use to check each individual battery or the entire bank as a whole. You also need a charge controller which you didn't mention.

[kleven626]

I think the batteries are 95amp hrs each. I should jump my system up to 3 panels from what you are saying. I do have a charge controller and it is rated to 7amps. I can hardly what. Thanks for all of you inputs. :)

[kleven626]

I also plan to replace the light bulbs with LEDs. This should extend the battery life. The only other thing that is going to run is the water pump from time to time.

[Watonga_Jim]

All good advice. During my first experiment to set up a small remote power system, I had an 18W solar panel connected to two 100 amp hour batteries (12v string), thinking that the extra storage would allow me to use a decent amount of power when I needed it and then charge during the day when I wouldn't be pulling any load. Big mistake! By not having enough power to keep the batteries charged after a minimal amount of discharge, I almost destroyed the batteries. Since then, I've dropped back to a single 100 AH battery and added an additional 50 watts of solar with a charge controller and it's now working fine. I learned to not have more battery capacity than you can effectively keep charged.

[12vman]

Realistically, large marine deep cell batteries have around 60 aHr. reserve. Two connected in parallel would give you around 120 aHr. reserve. (total)

You don't want to cycle your battery any deeper than 25% of its total reserve to avoid damage to the battery. In this case, a 30 aHr. margin is where you will be working at.

You have plenty of battery for 45 watts of panel but the amount of panel will designate how much energy you can actually use.

45 watts of panel will provide 3.75 amps of charge during good conditions. A 4 hour window (10am-2pm) is considered prime charge time. A little will be collected before and after this time frame but that just covers the normal internal losses of the battery. (20%)

Within the 4 hour window, you will collect 15 aHr. from the panel @ 45 watts. That's only half of the allowed margin in the 2 batteries that you have.

I suggest only using 1 battery and keep your usage within 15 aHr per day. (24 hr. period) This will assure that the battery will achieve full charge. (Or very near, depending on conditions)

To properly maintain the 2 batteries together, you would need around 100 watts of panel and your usage margin would increase to 30 aHr./day.

[kleven626]

Thanks 12vman. I am already looking to get a couple more 15w panels. I already have the charge controller in place and it can handle 7amps. Just a matter of how to set the panels. I was thinking of making a tri-fold plywood case. Should be able to just set it up when i pop-up for the day. The truck will charge it in transport.