I recently made a DIY solar car battery charger.
Check it out:
The circuit has auto cut-off to prevent overcharging the battery. And you can use whatever size solar panel you want depending on whether you want a fast charge or a slow trickle charge.
Here’s how I did it:
Materials & Tools
- 12V car battery — or just a standard 12V lead acid battery
- Renogy Wanderer 10A charge controller — or any cheap PWM charge controller
- 12V solar panel — I used a 5W 12V solar panel for a slow trickle charge. I’d use a 20W 12V solar panel or greater for a faster charge.
- Wires, connectors, and fuses — I used the NOCO GC018 12V adapter, which comes with ring terminals. If you want battery clamps, use the NOCO GC017.
- MC4 adapter cables — Only needed if your solar panel has MC4 connectors. These aren’t needed for the 5W and 20W panels linked above.
- NOCO battery box (optional)
Step 1: Connect the Car Battery to the Solar Charge Controller
I used the NOCO GC018 to make it super easy to connect my 12V battery to my solar charge controller. (the GC018 is a 12V plug adapter that comes with an inline fuse and ring terminals.)
First, I cut off the 12V plug socket with my wire cutters. Snip, snip!
Then I peeled the wires apart and stripped them with my wire strippers.
Just like that, my “battery cables” are ready. I can now connect charge controller and battery.
To do so, I connected the positive and negative ring terminals to their respective battery terminals. Red to red, black to black! (I also swapped out the inline fuse for the right fuse size given my solar panel wattage.)
Then I connected the stripped wire ends to the battery terminals on my solar charge controller.
My solar charge controller turned on, indicating it was properly connected to the battery.
At this point, consult your charge controller’s manual to see if you need to program it for your battery’s chemistry. Mine defaults to sealed lead acid, which is the type I was using.
Step 2: Connect the Solar Panel to the Solar Charge Controller
Connect the solar panel wires to the solar panel (PV) terminals on your charge controller.
Note: If your solar panel has MC4 connectors, you’ll have to use MC4 adapter cables to be able to connect it to your charge controller.
Now the solar panel is connected to the 12V battery via the solar charge controller.
Here’s what mine looked like:
This means my solar car battery charger is complete! 😲
Yep, that’s all there was to it.
Time to test it and see if it works…
Step 3: Test Your Solar Car Battery Charger
To test my solar car charger, I simply took the solar charger outside and put the panel in direct sunlight.
Then I looked at the PV current display on my charge controller. It read 0.2 amps, meaning my solar panel was in fact charging my 12V battery.
Technically my solar car battery charger is done. I can just leave the panel out in the sun and the charge controller will cut off the charging when the battery is full.
That’s right — this solar 12V battery charger has auto cut-off built right in. Pretty cool. 😎
If I want to solar charge the battery while it’s in my car, I can just put the solar panel and charge controller on the hood of my car.
But I decided to take it one step further…
Step 4: Make Your Solar Car Charger Look Nice (Optional)
I picked up a NOCO battery box and plopped my 12V battery right inside.
Then I used a drill and the mounting screws included with my charge controller to mount the controller to the top of the battery box.
Here’s how it turned out:
Next, I drilled some holes in the top of the box to feed the battery cables through.
Tip: Drill the holes angled slightly away from each other so the leads won’t touch when fed through the lid. You don’t want to short the battery!
Then I reconnected everything…
…And I was finally done:
I’m happy with how it turned out. The box makes the whole system more portable and cuts down on cable clutter. It also looks nicer. 😄
DIY Solar Car Battery Charger Circuit Diagram
Here’s the circuit diagram for a solar 12V car battery charger with auto cut-off.
And here’s what it looks like when built:
Some things to note:
- Safety best practices are to place a fuse between the charge controller and both battery and solar panel
- For most charge controllers, you connect the battery first and then the solar panel. Consult your controller’s manual for the manufacturer’s recommended connection order.
- Make sure to get a 12V solar panel and 12V charge controller for your 12V car or lead acid battery
- I recommend a PWM charge controller because it’s a lot cheaper
Tip: This solar charging circuit diagram would work for any battery chemistry and voltage as long as you got a compatible charge controller and solar panel.
How Long Does It Take to Solar Charge a Car Battery?
A typical car battery is a 12V lead acid battery with a capacity of around 50 amp hours (Ah).
Knowing this, we can use our solar battery charging calculator to estimate how long it’d take to fully charge a depleted car battery using a PWM charge controller.
We just plug in everything to the calculator like so:
Here are the estimated charge times for 5 common solar panel sizes:
- 5W solar panel: 154.8 hours
- 10W solar panel: 78.4 hours
- 20W solar panel: 40.2 hours
- 50W solar panel: 17.3 hours
- 100W solar panel: 9.6 hours
A 5W or 10W solar panel is a good size to pick for a slow, trickle charge. Some people use these sizes to keep their car, camper, or RV battery topped up when not in use.
Want to solar charge your car battery in less than a week? Go with a 50W or 100W solar panel.
You could also make your charger faster by swapping out a PWM charge controller for an MPPT charge controller. MPPT charge controllers are much more efficient.
For more help, check out my post on sizing a solar panel to charge a 12V battery.
Add-ons and Upgrades
Here are a couple ideas for making your car battery solar charger even nicer:
- Add suction cups for attaching the solar panel to inside to your windshield: I’d recommend doing this with a 10W panel. Solar panels work behind glass, but with a reduced output. A 5W panel behind a windshield might not output a high enough voltage to charge the battery.
- Add a 12V plug to charge the battery through the 12V socket: Some car batteries can be charged through the 12V socket inside the car. Instead of connecting the solar charge controller directly to the battery, you could connect it to a 12V “cigarette lighter” plug and then put the plug in the 12V socket. Just remember not to connect the charge controller to the solar panel until you’ve connected it to the battery.
2 More DIY Solar Chargers You Can Build Now
You know how to solar charge a car battery…
…why not try building solar chargers for the other batteries and electronics in your life?
Here are two more solar charging project ideas for you:
You can upgrade the 12V solar charging system you just made into a solar ebike charger simply by adding an inverter. An inverter converts DC to AC, letting you plug in your ebike charger like you would into a wall outlet.
Here’s a portable solar charger for charging handheld 5V electronics such as your phone, tablet, Kindle, and USB battery pack. You’ll need a soldering iron for this project!