Solar panel overcharging?


Gold Subscriber
I've got a solar panel that will be used in a SHTF situation to charge a 12V car battery, which then is used for my HAM radio. I've got an inverter as well so that we can use AC power to charge other things.

The panel has alligator clamps that would go directly onto the battery. I heard recently on a podcast that some solar panels can be damaged if they are connected directly to batteries without some device in between to prevent overcharging. I don't have anything connected to the battery to show me what the charge is like.

I'm not a solar panel expert - but is there some type of device I should have attached between the panel and the battery to prevent overcharging and damage?

If so, can you give me a make/model that you'd recommend?


Brass Subscriber
Called a Charge Controller. Needed if the Panel makes a lot of Power, and the battery is relatively Small. Wastes excess power as heat.

Not needed if You have Small Panel and relatively Large Battery.

If You need a Charge Controller, it will have to be sized according to the specific Panel, Battery Size, and Solar input. Is a Three-(or Four!) Variable Equation to solve. Easiest way for non Electrical Person to be safe is to look at Panel Spec, look at Full Current, and select a Charge Controller that can handle Equal or more Current.

So, real world, if You have a 100-Watt Panel, charging a Motorcycle Battery, You need a Charge Controller.

If You have a 25-Watt Panel, charging a Automobile Battery like a Group 24, Charge controller is not necessary.

CAVEAT!!! If You plan on leaving things hooked up and working with little/no supervision, excess Charging can boil away the water, cause Hydrogen Outgassing, until a Spark! KABLAMMM! MONITOR YOUR SYSTEMS!


"Fly birdies!!"
Got it - the panel is 18v, so I'm guessing the charge controller isn't needed for a 12v battery. It would not be hooked up all the time.
The smaller charge controllers are pretty cheap. I'm not familiar with brands at the moment, but I would really suggest you put one between the solar cell and battery. Couldn't hurt. And it protects your battery. Especially if you leave it all hooked up all the time.

And some fuses!


Brass Subscriber
SOLAR panel specs always list OCV (open circuit voltage) which is the highest voltage a Brand New Panel can make, in Exactly Noontime Sun, at 65 Degrees F Ambient, EXACTLY at 90°Angles to the Sun Position, with Surgically Clean Face, etc...

You get the idea...

As soon as there is any currect flowing, the output voltage is less.


Brass Subscriber
Harbor Freight has Charge controllers from China, INC. that are good to 5-15? Amps, for Cheep. Azamon prolly has seventy-leven differnt ones, from mouse to MOUNTAIN Current specs. Ya Payz Yer Munnie, an Takes Yer Chances...

The Branch Manager

Winter is coming. Forever.
Gold Subscriber
I assumed based on my own interpretation of the problem described in the title, skipped reading the OP, breezed past the replies and give you this-

The sun is white. Its not supposed to be white. Or this intense. My eyes burn. I have new freckles. It's probably not skin cancer. All the fault of the white sun. Bring back yellow sun please.

Red dog

Active member
This is what happens when the battery is overcharged and there is a spark, I got lucky, normally sitting on the backhoe this is between my feet, that day I did what you're not suppose to and reached in to start it, ended up with just some acid holes in a pair of old jeans, could have been an acid bath and probably blindness.