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Charging a Car Battery using a Solar Panel

Battery chargers enhance performance of batteries extend battery life and can extend the life of the battery. Car batteries will discharge even when not in use as devices like alarms and clocks will discharge the battery.

The standard car battery is rated 12v batteries can be recharged with a maximum of 10 amps at 13.8 to 15 volts. The recommended float charging is one amp at (13.2 to 14v for some). You can check the recommended settings with the batteries manual. This can also very depending on the type of battery; whether AGM, Gel or Lithium Batteries.

So can you charge a car battery with a solar panel? Yes, a standard 100 watt solar panel can provide around 5 amps to charge a battery and will take around 12 hours to fully recharge a battery with a battery voltage below 11.85. This can be two days depending on the amount of sunlight the panel is receiving. A smaller 5 watt panel can provide trickle charge to maintain the battery and prevent it from dying.

How to Set Up Solar Panel to Charge a Car Battery

Solar panels will only work during the day or in the presence of sunlight. (stating the obvious here) Because the solar panel has a voltage and the voltage will drop during the night this can lead to reverse charging (the battery charging the solar panel!). Draining all those precious amps from the battery. A blocking diode is a device that will stop the solar panel from discharging the battery. Luckily most charge controllers will also come with this feature.

  1. Ensure you have a Charge Controller

    First and foremost you should never use a solar panel (above 5 watts) to directly charge a car battery.  Because of sunlight intensity the solar panels voltage will always fluctuate and a really high voltage and drive the amps to your battery quickly and overcharge the battery. A solar charge controller will help you cope with the battery and prevent the panel from overcharging your battery.

  2. Cables

    Also ensure that you have a long cables that will reach the battery from the solar panel and the controller is mounted.  Long cables will ensure that you can mount the panel where there is sufficient sunlight and still reach the battery whether it is still in the car or in storage.
    At the end of the cables should be alligator clips or ring terminals to be able to hook up on the batteries terminals. The clips should be labeled positive or negative depending on where it the cable is hooked up on the panel.

  3. Access the Batteries

    Car batteries should be accessible once you open the hood or the trunk.  But if you need to remove the battery proper precautions should be taken.
    Once you gain access to the battery identify the negative and positive terminals. This is easy the negative terminal which is the ground cable will have a negative (-) and the power or positive cable will have plus (+) sign.

  4. Connect the Charge Controller to the Battery

    When using a Charge Controller the battery should be the first thing you connect to the Controller. You should NEVER connect a Solar Panel to the Controller without a Battery. The charge controller should be clearly labeled for ‘BATTERY + and -‘ make SURE the polarity is correct.

  5. Connect the Solar Panel to the Charge Controller

    Once you have connected the battery now connect the charge controller to the solar panels. The charge controller should also be labeled for a ‘PV INPUT + and -‘. This is where you hook up the panels. Make sure not to mix up the positive and negative Positive (+) is usually red, while negative (-) is usually black. If you are not sure you can check polarity using a multi meter.

3-Stage Charging Using a Charge Controller

A battery charge cycle describes the voltage and current relationship and can be broken down into stages. A charge controller will allow you to set how you want your battery to be charge. You can pick from the 3-stages depending on the level of charge of your battery or your battery type.

The three stages are; bulk, absorption and float and are applicable to Wet, Gel and AGM Lead Acid batteries.

Bulk Charging

Bulk charging is the normal battery charging that provides a fast charge. To provide a fast charge sufficient sunlight must be available and the battery must have been deeply discharged.

Absorption Charging

The Absorption stage is where the battery slows down the amps going in the battery once a certain voltage is reached.  It is conditioning mode to step down the charging but allow the panel to complete charging.

Float Charging

At the float stage the battery is full and is only being topped off with a small charge to compensate for the idle discharge.


Some MPPT Chargers will routinely (once a month) apply an equalizing charge. This is the intentionally overcharging of Lead Acid batteries to remove sulfate crystals that build up on the plates over time commonly referred to as sulfation.

Plug and Play Portable Solar Chargers

If setting up a solar panel is too much of a task for you and you can just buy a portable solar car battery charger that is easy to set up and use. Some come with Cigarette Lighter and OBDII connectors that you won’t have to open the hood of your car to charge.  You simply place the solar charger on your windscreen or dashboard then connect it to the cigarette lighter and you can leave it for hours without worrying about overcharging.

Ensure that before selecting a charger for your car battery whether a solar charger or a regular charger to check the manual for instructions. Charging voltages will vary according to the battery type, condition and state of charge of the battery.

Also ensure to follow all safety precautions as batteries contain sulfuric acid and can cause severe burns and can also produce dangerous gases.  Check safety instruction from the charger as well.