Charge small 12v battery from car?

[Robobass]

I have a small rig for tent camping. 10w panel, 12ah lead acid battery, charge controller. On cloudy days the panel isn't enough to do a full charge, so I'd like to be able to use the car battery sometimes. The controller says don't attach anything but a solar panel. If I put a resistor in the circuit would that make it safe? I also have a laptop power supply than can run from 12v and output a range from 12-24v. What if I fed the controller with say 15v and put maybe a 50 ohm resistor between the power supply and the charge controller? I don't see an off the shelf device which does what I want. Just a note: the car is parked far from the tent.

[Robobass]

Also, it's actually two 5w panels wired in parallel. Is that the way to do it? It seems to work well.

[Mike90250]

First thought is, follow the manufacturers instructions. Second, unless the car engine is running, it can't recharge a 12v battery, otherwise tow companies would be out of business. You either need more PV panel to be able to harvest in cloudy weather (about 100w would work, but only in cloudy, otherwise it will boil your little battery) or a small gasoline generator. Or a bicycle generator, it might take 2 hours of hard pedaling to charge up the small battery

[bcroe]

Also, it's actually two 5w panels wired in parallel. Is that the way to do it? It seems to work well.

The simple solution is make an extension cord to run off the car battery instead. I believe
you can find a charger for your 12AH battery that runs off your car battery, or arrange a
battery isolator (as RVers use) to charge it with the engine. Fooling with resistors is not
the answer. Bruce Roe

[SunEagle]

A battery instead of a solar panel is not be the correct way to use a Charge controller to charge another battery.

Either get more panel wattage or use jumper cables from the car battery (with a running engine) to charge your solar battery. It takes more than 13V to charge a 12V battery and the alternator should provide the correct amount.

[Robobass]

The laptop power supply has a range from 12-24v, so I thought 15v would be good. I could just charge directly without using the controller, but don't want to overcharge, hence the resistor.

I don't think it's necessary to have the car running. We're talking about topping up a 12ah battery with a 65ah battery. It wouldn't discharge the car battery all that much.

As I said, the parking area if far from the tent, so running an extension cord isn't an option.

[Robobass]

I figured it out. After a bit more research I think my laptop power supply and a resistor would work, but even better than a simple resistor is a "CV CC buck Step down Power Supply Module" which lets you dial in both output voltage and current. I will still need the laptop PS because yes, you need more than 12v on the input side.