Solar Car Battery Charger DIY




Tim Anderson is the author of the "Heirloom Technology" column in Make Magazine. He is co-founde...

Here's how to charge your lead-acid car battery with a solar panel.

The simplest way:
Get a voltmeter and a solar panel. Connect the panel to your battery and watch the voltage rise. When it gets near 14 volts your battery is charged. Disconnect your solar panel to keep it from overcharging your battery.

Your battery can be damaged if you charge it too fast, so don't get a panel that's too big for your battery. Consult the battery manufacturer's data to see how many amps it can handle, both while charging and discharging. If you happen to have a big panel and a little battery, there are tricks further along in this instructable.

Some solar panels are made just for the purpose of maintaining batteries in vehicles that are parked a long time. They don't go over 13.5 or 14 volts and don't ever produce enough current to damage the battery. These you can just hook to your battery and forget.

If you want to use more power you'll want to get a bigger, higher voltage panel than that.
Here I am running my truck's electrical system off a solar panel wedged in the windshield.
My alternater fell off the engine and the belt broke. I'm driving south so I just set up the panel and drove into the sun, peeking over the panel. Read more of that Haywire Mechanic Story or click on "step two" to read more battery/solar tricks.

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Step 1: Voltmeter Human Supervision

Here's the setup in the cab. I watch the voltmeter to see how things are going. If the voltage goes to zero I know something has gotten disconnected or shorted out. I then sniff to see if there's a burning smell, which means I need to pull over quick, open the hood, and put out an electrical fire.
If the voltage suddenly jumps to 17volts I suspect the jumper cables that carry the solar panel current to the battery have gotten disconnected.

The panel output voltage rises to 17 volts or so when it's not connected to a load. When current flows the voltage drops due to internal resistance in the panel.

If your battery voltage gets to 14 volts or if you hear a bubbling sound when you put your ear to your battery it means you're overcharging your battery. Disconnect your panel. You'll get to know your panel, battery, and loads, and maybe you'll disconnect at a lower voltage, like 13.5 volts.

This method works fine when camping and you can lay around watching to see how your battery is doing, or if your panels can't keep up with demand so you don't have to worry about overcharging.

Step 2: Overvoltage Disconnect

They call this item a "7 amp charge controller" at [ Camping World] and charge $25 or so for it.
All it does is disconnect the panel from the battery when battery voltage goes over 14 volts.
Home Power Magazine Has published schematics for circuits that do the same thing.

You can still overcharge a battery through one of these units if your panel is big and your battery is small. The bubbling sound you hear in your overcharging battery is water (H2O) being broken down into hydrogen and Oxygen, which then bubble out to join the atmosphere. Eventually the battery will run out of water. Eventually the exposed parts of the battery's lead plates will oxidize and the battery won't take a charge, even if you add distilled water to it.

Step 3: Low Voltage Disconnect and Inverter

Here's my inverter.
It takes in 12 volts DC from the battery and produces 110 volts AC that I can plug my sewing machine, laptop, etc into. You can get them for cheap at any truckstop or discount store now.

This model has a low-voltage disconnect feature which is a big help.
When battery voltage drops below 11 volts it beeps and turns off.
That protects your battery from over-discharging.

Sulfating and De-Sulfating
If you run your battery down further than 10.5 or 11 volts, or if you fail to charge it up again soon enough, sulfate crystals will form on the battery plates.
The sides of your battery will bulge from the crystal deposits on the plates, and your battery won't take a charge anymore. There are tricks to fix this. Look up "desulphate", "desulfate"(both spellings are ok), "pulse charger", "battery edta". According to one vendor of desulfating products, 75% of the lead-acid batteries removed from service can be rejuvenated with these techniques. The others are suffering from shorts, open circuits, and similarly less correctable flaws.

To measure your battery's capacity
Plug in an mechanical electric clock and a known load, like a 5 watt lightbulb. See how long it takes to run down the battery til it shuts off. For example: 5watts/12 volts = 0.42 amps. If it takes three hours to discharge your battery after it's fully charged, your capacity is 3 * 0.42 = 1.26 amp-hours. (not very much) A typical emergency-lighting gel-cell has 7 amp-hours capacity.

Step 4: Christmas Lights As Current Regulators

If your panel is too big for your battery, you'll want a current regulator to keep from boiling your battery.
One way to do that is by putting an incandescent light bulb in series with your battery as shown here. Here I'm using christmas tree lights. Play with an ammeter and try putting multiple bulbs in series or parallel til you get the current limiting you want. Signal lights from cars can also work.

The clothes iron seen here is nothing but a big resistor, that can work also to reduce current to your battery. The small DC motor seen here is useful for testing scrap batteries. You can immediately feel how much current a battery can deliver.

Step 5: FREEDOM!

Here I am set up on the banks of fisheating creek, Florida. I'm charging my batteries with a 20-watt solar panel made by Solarex.

My batteries are a couple of 12 volt gel-cells scrapped by a hotel from their emergency lighting system. That's plenty to run my laptop, fluorescent work-light, or sewing machine.

Thanks to these simple tools I was able to do my work in a swamp.
In fact, here in a swamp I found a much better work environment than I'd had at our most prestigious technical university.

For some reason the buildings there were grossly overheated in the winter while the windows stood open and gigantic airconditioners ran at the same time, making a roaring sound. Sound crazy? it is.

Anyway, if you can charge a battery with a solar panel you can run away from the craziness to a paradise swamp like this and be happy.

The second photo is my boat set up with a bug net and chaise-longe folding chair for sleeping. The solar panel is across the bow of the canoe to charge the batteries during the day.

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    68 Discussions


    2 years ago

    Does said vehicle have to be on during the process of charging ones battery


    3 years ago

    here's MY question. I own a 600,000 maH solar charger. It has a 5v output at 2amps.

    Can I modify/connect a normal USB connector to a 12v cigarette lighter connector and charge my car battery?


    4 years ago on Step 3

    _most_ inverters such as the one you have shown here have an internal low voltage disconnect that is supposed to prevent this. if yours doesn't, you might want to DIY a low voltage disconnect with a diode. some info available over her:


    7 years ago on Introduction

    how are u??
    what equipment thaht we need 2 do this thing..


    7 years ago on Introduction

    I want to charge an extra battery in my van, so I can run a car heater or cooler in my cab at night while the engine is off. I would also like to use my computer and light if possible at night. I want to charge the extra battery while i drive during the day from solar panels. I was thinking of mounting the solar cells on my side windows. Do you think this is possible or wise? I also wondered if i could hook two batteries into my alternator and charge both while driving.


    Reply 10 years ago on Introduction

    I think attracting bears? id put a big electric current through the car so that when a bear comes ZZAAPP and leaves


    9 years ago on Introduction

    hi everyone.i am new to this solar stuff.i have a 12v die hard battery i want to use.i would like to charge it using solar panels but i dont have any idea on how to hook it up and what i need to do i need a battery charger to hook up to the solar panels or do i hook the battery right to the panel and how do i do that.i told you i am new to all this sorry everyone.if somone can help a dummie i thank you lol

    1 reply

    Reply 8 years ago on Introduction

    A good idea would be to invest in a charge controller which would keep your batteries from getting overcharged. Do some research!


    9 years ago on Step 1

    Why would you need this to charge your battery if you are driving?  Thats what your alternator does.

    5 replies

    Reply 9 years ago on Step 1

    Do you really think he's driving with a panel propped up against his windshield?

    This approach is usually used for keeping the batteries of rarely driven vehicles float charged - I use a 5W panel on my convertible with great results.


    Reply 9 years ago on Step 1

    He says if he smells a burning smell he has to pull over quick..........etc.......

    Yes, I think he is driving with it.


    11 years ago on Step 1

    Whoa! It looks like an army of bears and gramlins ate the insides of your car, then got themselves eaten by a dragon that was too fat to fit into a black hole, then shifted through time and interdimensional space to barf it back up, minus the fuzzy stuff that goes on the floors. Anyway, cool 'ible!

    2 replies