Introduction: Solar Car Battery Charger DIY

Picture of Solar Car Battery Charger DIY

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.

Step 1: Voltmeter Human Supervision

Picture of 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

Picture of 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

Picture of 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

Picture of 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!

Picture of 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.


BigL4 (author)2017-01-24

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

DrJoseph (author)2016-05-26

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?

gorustal (author)2015-02-11

Everything is completely explained on

kyrka (author)2015-02-09

_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:

auhs1 (author)2012-08-07

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

strangegg (author)2011-11-17

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.

Yerboogieman (author)2008-09-21

whats the peanut butter for?

Paulie151 (author)Yerboogieman2011-09-22

That was his lunch when he threw this thing together. ;)

ReCreate (author)Yerboogieman2008-12-23

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

ssisssy19 (author)2010-05-18

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

thorpenny (author)ssisssy192011-06-06

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

pennsteve (author)2010-01-01

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

juicepig21 (author)pennsteve2011-01-27

Or you could just read the text and it would tell you that his alternator broke.

Badlad220 (author)pennsteve2010-10-16

mayb his altonator broke or summin so he has to charge it manuly

Entropy512 (author)pennsteve2010-04-08

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.

pennsteve (author)Entropy5122010-04-08

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

Yes, I think he is driving with it.

abadfart (author)pennsteve2010-01-07

maby his died  

Unit042 (author)2008-07-13

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!

Unit042 (author)Unit0422008-07-13

Whoops, they are gremlins, not gramlins.

SuperJesus (author)Unit0422008-07-16

there gramlins if your from the south lol.

Unit042 (author)SuperJesus2008-07-16

Yeah. Half gremlin, half gram cracker with cinnamon.
Thus, gramlin.


TANZMEISTER (author)Unit0422010-10-27

They're only gramlins if they call you gramma or grampa

sabarblatoe (author)SuperJesus2008-08-20

From texas and felt the need to say "Gramlins" out loud just to see......and your right. Laughed my assoff.....and still laughing.

knektek (author)2010-05-03

Get a 15v solar cell,
Get 10 of them in parralel,
Check the output,
Regulate it,
And your now happy with a quick reboot of power in your 12v battery.

jimmy dean (author)2009-10-15

How many amp-hours does your typical car battery put out. I can onlyfind cold crank amps. Is that the same thing? Also, I'm not really sureon what wattage panel I should get if I just want to charge one battery.Could someone plese explain to me what a good wattage would be and why.

tammasus (author)2009-02-08

shouldn't there be a diode between the panel and the battery to prevent the battery from discharging into the panel?

ElectricMan1 (author)tammasus2009-04-13

No the charge controler does that for you.

Ausi319 (author)ElectricMan12009-06-19

reverse diode to be exact but yes the charge controller does that for you

Heliosphan (author)2009-04-20

Hi all.
This is a helpful instructable for me as I'm interested in buying this -
I already have a 1Watt solar cell which is doing a nice job of powering a salvaged PC Fan to do a reasonable effort of cooling my dining room as the sun shines! The amount of time the fan is on as the sun shines does bring the whole room temp down a good few degrees, esp if you're close to the fan.
Now I'm interested in doing a little more - ie getting hold of a big car battery and charging it up for duties such as charging my mobile phone, laptop, etc.
I have done quite a bit of circuit design in the past, including programming microcontrollers for controlling motors etc.
I'm somewhat unfamiliar though with the concept of charging a battery at the same time as using it. Sure cars do it all the time with the alternator, but if you think of it from the classic electronics and electrics standpoint its a little confusing - how can you power a battery, and at the very same time, take power from it, its akin to connecting two landline power sources up to each other, you'd never do that with traditional mains power supplies!
Any ideas on how/why this works, what caveats are there, eg is power from the battery actually going TO the solar cells too and potentially damaging them?
Thanks all.

greatscotmagic (author)2008-04-25

Hi, I use a handicapped scooter at festivals and I can't always charge up my batteries. is there a way for me to rig up a solar panel to run the motor during the day while i am using it? Could I charge the batteries while I was using it and charge the batteries while the scooter was not being used? I do like these ideas as I get to events that I can't get access to power but I would still like to use my laptop. Thanks.

Just follow the same steps in this instructable. It doesn't matter if you use your scooter while charging it. Probably you'll want the panel overhead as an awning. You probably want lightweight panels. Look for weight vs. power rather than wieght vs. size.

Tim Thank you. I was planning to use the panel like an awning, not only will I be able to run my cart, but I can keep the sun and the rain off too! Another question: when you are using your laptop with the solar panel and the batteries, do you use the power inverter and the laptop's AC Adapter to power the laptop? Thanks again.

yep, I used an inverter to regular powersupply. one advantage is that inverters usually have a low-voltage cutoff so save your batteries from getting totally drained.

netbuddy (author)TimAnderson2008-10-11

That is wasteful way of powering a device designed for low voltage. I am aware that not all laptops use 12 volt DC input but you can get DC to DC converters that output 24volts which then can be regulated to the laptops preferred voltage requirements. The upshot it that you get more than a few hours worth of operation over an inverter method. It is pointless to take 12 volts and have it converted up to 110 / 240 volts only to be converted back to 12 to 18volts. The losses in such a circuit are huge and your battery's will serve you better.

ac-dc (author)netbuddy2009-03-26

Actually the losses are not huge as a switching circuit, and the distinct advantage is you have 110/220VAC available should you ever need to power something else. You cannot expect to boost 12V to 24V then reregulate again and have it be much better efficiency. The high efficiency option would be a purpose specific laptop car adapter that goes straight from the 12V-13.8V DC to the laptops required input voltage.

static (author)TimAnderson2008-05-29

I understand that some laptop battery chargers don't like modified sine wave inverters and one may be better off use an auto power cable to run the laptop. YMMV

greatscotmagic (author)static2008-05-29

Is an auto power cable a cable that will run a laptop directly from a car's battery? can all laptops do that? Thanks.

Thanks again, Tim.

Good girlfriend just got me Solar Gorilla Solar Laptop Charger. I was able to run my MacBook with solar panels

ReCreate (author)GadgetGuru012008-12-23

oh wow

netbuddy (author)greatscotmagic2008-10-11

I would say that you are a candidate for the next generation of solar PV. Amorphic cells give more output at lower light levels monocrystaline gives higher output per cm² polycrystaline is a mix to produce a cell that gives good output at all light levels. You are best off investing in several cells over one large cell. reason being that if the big boy gets damaged or is obscured, the overall output is lower or non existent.

deerfeeder (author)2007-06-15

Can someone give me a recommendation on a solar charger controller combination that will keep a 12 volt car battery charged when used to run a deer feeder once a day. I have a small panel (8" X 8") on the battery right now but it is not keeping the battery charged. Any help would be welcomed. Thanks

netbuddy (author)deerfeeder2008-10-11

Unless you know what current it outputs you won't know how many hours light is needed to charge it. If you took a discharged 12 volt car battery and hooked it up to a single solar panel like the ones on offer in maplins, it would take about 6 months to fully charge the battery to a usable capacity. I know because I have put it to the test, one 12 volt 1.5watt cell charged a total of about 0.5 volts a month. So the higher the wattage of the panel the quicker it will charge. Your cell voltage potential needs to be significantly higher, typically a solar cell needs to out put at least 15 volts to charge a battery. If that is not happening then your not charging just maintaining a trickle charge. If you have a 15 volt output from your cell and it outputs 15 watts, your current output is 1 Amp per hour, on a small cell like a 7 Amp cell will take about 7 hours to charge assuming that your cell gets adequate light.

explosivemaker (author)2008-08-29

"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." ....that just made me laugh....though I hope you haven't actually had it catch on fire....

gadget1961 (author)2008-06-12

OK I have a solar panel for a vw and it has a plug for lighter, its 3.2 watts and 18.8 volts max, I just want to run lights for night in the computer room which would be suitable for the wattage, , i am going to use a deep cell marine battery for storage , but i need to regulate the battery as i don't want to over charge it but to charge ok what would i need for this

TimAnderson (author)gadget19612008-06-12

If it's a flooded lead-acid battery you don't have to worry about overcharge, just disconnect it when the voltage gets to 14 volts or the battery bubbles. When the water gets low add distilled water. 3.2 watts isn't enough to dry out a marine battery very fast. Try it that way before worrying about a charge controller.

gaman (author)2008-03-05

Hi, Could you setup a solar panel to completely or partially run a trolling motor? If not, could you have a large enough solar charger that would double the Amp hours of the battery to increase trolling time?

btop (author)gaman2008-04-19

Yes, that would be possible. As for the amp hours of a batter, that wouldn't really change. What the solar panel would do is just keep charging the battery as the motor used it.

Dr.Paj (author)2008-04-09

This would be great for charging a motorcycle battery because you can't jump it with a car (too small) so you have to buy a charger that plugs into a wall. I know this from experience. (I don't know if you can jump it with a car or not, but I wouldn't advise it)

depayton (author)2008-04-07

Ahh... Serenity.

About This Instructable




Bio: Tim Anderson is the author of the "Heirloom Technology" column in Make Magazine. He is co-founder of, manufacturers of "3D Printer" output ... More »
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