Solar panel for charging 2nd battery

[the evil twin]

OK... first thing is that the Solar Panel mentioned by the OP is not a great choice for 12 Volt application IE vehicle or camper as it is a 24 Volt (36 Volt open cct) panel. Most domestic installations I am familiar with run on higher voltage Panels and indeed most panels over 160 watts are 24 Volt so not suprised.

You will need a more expensive regulator to reduce the up to 36 Volt Output to 12 volts and then the useable power will only be around 60 watts as the maximum current that panel can produce is around 5 amps. The next option is to regulate to the designed 24 Volts and use a DC DC converter to charge the batteries which will give you about 100 to 120 watss depending on losses. Its all doable but I would price out the costs of a Reg and possibly a DC/DC Converter as the Panel needs to be a bargain to make it worth it

About the only trap with power budgets with Solar Panels is they are rated at the Panel and are current limited IE a 120 Watt Solar panel doesn't deliver 12 Volts at 10 Amps (or 10 amp hours if you like).

Rounded out to make the math a bit easier it works like this. The Solar Panel max voltage output of a 12 Volt panel (depending on technology) is close enough to call it 20 Volts so a 120 watt panel is capable of delivering 6 amps (6 amps X 20 Volts = 120 Watts). They will usually deliver a little more than that but the losses thru the cables and regs etc means its ballpark.

The Regulator will reduce the voltage so it doesn't fry any electronics or overcharge the battery but the Reg doesn't increase the current past the Panel capability. This means a 120 watt Solar Panel in a 12 Volt application will give you approx 75 to maybe 80 watts of useable power under max sunlight and optimum conditions (12 volts X 6 amps = 72 Watts). The rule of thumb is approx 2/3rds the rating IE 60 watt panel 40 watts power 120 watts panel 80 watts power.

[JoeG]

Yes I fully agree, it also gets worse in the tropics as the panel temperature rise causes the rating to fall and the load (if you have a fridge running) gets higher.
We recently (last winter) went through central Aust and up to Darwin we had a 100w system (2 of 50w BP Solar panels) these worked great till we hit the tropics then woefull!
we ended up adding another 100w panel in Darwin (and 20amp regulator) and then bliss, fully charged batteries with 2 hours sunshine!
so what works fine south of the tropics dosn't necessaraly work north of them.
However will never think of not having solar battery chargers as we can stay rough for days and not need to hit town till we run out of water or food
Joe

[Dark 1]

80 watt panel

110L Waeco, all led lights and electric pump we can camp for 5 days but batteries are loosing charge slowly over that time, the panel is just not quite enough. Once we start charging a laptop via its 12v charger we start to chew into it pretty hard. I did my calcs and arrived at around 120watt minium and 180 to be really safe. Maybe my panel has lost a bit of its output due to its age. I mount it on the camper on a 3m pole that i can rotate and align. I usually move it 3 times a day. I do have a spreadsheet on my home pc that allows you to calculate your useage over a day or week etc. Works extremely well i cna post later tonight.

Your 110ltr waeco would be pulling a lot of power, my old 80ltr waeco pulled about 7-8 amps an hour in recirc mode my 2 newish 40 ltr engels pull a max of 5 amps even in cool down mode, a 120-130watt panel would probably siut you better.