15 watt solar panel

[trailrider]

I have a friend that bought a "15 watt battery solar charger kit" at a garage sale and he wants me to install it for him on a trailer with two 12 volt group 24 batteries. He is just going to use it to keep the batteries from going dead when it is parked out back.
The documentation says "charge 12v batteries with sunlight. Built in blocking diode protects battery discharge at night." I put it in the sun and it is putting out 24.5 volts. So I have some questions.

I was going to get a charge controller for it since they are fairly inexpensive. Do I need a charge controller for this small of a panel?

Is a 24 volt panel the correct panel to charge 12 volt batteries wired in parallel? Or does a charge controller handle the excess voltage?

[RCtime]

That solar panel will do a great job of maintaining fully charged batteries. My SIL uses that set up. 1st he went to autozone and purchased a simple disconnect switch he installed on his batteries to prevent any parasitic backflow. Hook the solar panel directly to the proper battery terminals.
Also he made some wood legs that propped the panel towards the sun when placed on top of his trailer.
He covered the end of the legs with foam and taped over to prevent scratching.
All you need is some sunshine.

[Almot]

It will keep it topped up if he disconnects all the loads. Which means - ALL. No detectors, no radio on standby, no anything. To do this, he needs to disconnect the load cables from the battery, and clamp the solar panel wires to the battery. So if he wants it to be "installed", such an install would have to include some loose dangling wires that he will clip and unclip when needed - or some switch for that purpose.

If he won't disconnect all the loads, the panel will slow down the drain but may not stop it - depends on how much load is there, and it won't - probably - overcharge the battery either.

What BFL said - overcharging (with all loads disconnected) is possible, so this is something to keep an eye on, especially with all the loads disconnected.

[trailrider]

Thanks for the help guys!!! 🙂

[pianotuna]

Hi,

It will be fine to direct connect it to the group twenty-four batteries with no controller.

[hmknightnc]

With 2 GP24 batteries there is no need for a controller with a 15watt solar panel. The 24.5 volts you are reading is open circuit voltage, not a problem. That panel probably does something like 0.9 Amps at 17.5 volts (i.e. 15 watts at Max Power point). 0.9 amps barely above 0.5% of battery capacity so no danger of damaging the batteries. Keep the batteries wired in parrellel and disconnected from trailer as that solar panel will not keep up with parastic loads.

[BFL13]

I am going to guess that his panel kit is a "12v" panel with a 23v+ Voc.

Some 12v panels that are poly have the usual 21v Voc, while some mono's have 22v. (result is the Isc on the monos is lower 😞 )

The 15w panel(s) is probably amorphous vice poly or mono. So if 23v Voc and still 12v,(most "24v" are 36 or so Voc),then you only need a controller if that 15w will get the batt bank over 15v during the available daylight. Situational-try it.

Not likely.

[KD4UPL]

I too am surprised it's putting out that much voltage. Either way you don't need a controller. just hook it up positive to positive and negative to negative. Be sure to put a fuse in the positive lead off the battery.
Unless all the loads are disconnected from the batteries they'll still go dead. A 15 watt panel is basically nothing into those batteries.

[time2roll]

No controller needed. Once connected you will be lucky to get them up to a normal 14 volts. Still need to disconnect all draw from the battery.

[RJsfishin]

No controller needed. Is it 24 volts? I
d thought those small panels were more like 16-18 volts