Will A Solar Panel Charge A Dead Battery? (Must-Know)

One of the purposes of a solar panel is to charge batteries to give access to power when the sun is not shining. A solar panel is not charging a completely dead battery but only topping up the battery’s charge in normal circumstances.

Will a solar panel be able to charge a battery that is completely dead and restore it to full charge? A solar panel can charge a dead battery, provided certain conditions are met.

Here are some of the key points we’ll be looking at;

  • The battery is in serviceable condition.
  • The solar panel has sufficient light.
  • The panel and battery have the same voltage rating.
  • Use a solar charge controller to prevent damage by overcharging the battery

A solar panel can charge a dead battery because this is one of the intended purposes of a solar panel, but it is not quite as simple as hooking it up. Using a solar panel this way requires knowing how these components work to avoid further damaging the battery.

Charging a battery

Can A Solar Panel Charge A Dead Battery?

Solar panels can be used as a charger for a dead battery as long as you understand how a solar panel works, the output it delivers, and the voltage and amperage the battery can accept.

Solar panels do not output a standard amount of energy. The energy output varies throughout the day depending on the level of sunlight the panel is receiving. Solar panels also have different voltage outputs, from 12-volts to 48-volts.

Suppose the solar panel has been specifically designed for recharging 12-volt batteries such as car batteries. In that case, you can connect the panel to the battery, and it will safely charge the battery without damage.

If the solar panel has not been specifically made for charging car batteries, connecting a 12V solar panel to the battery will cause fluctuating voltages, and the light intensity increases and decreases.

The voltage could increase at some times of the day, pushing more than 14-volts through the battery.

This will cause the battery to produce gas, the electrolyte will bubble, and the battery will be ruined.

Hooking up a 12-volt battery to a 48-volt solar panel is going to cause damage to the battery. Does this mean you cannot use a 48-volt solar panel to charge a 12-volt battery?

Not at all; you can use this arrangement with an additional piece of equipment installed between the battery and the solar panel.

A charge controller can be used to regulate the amount of energy the solar panel is pushing through to the battery, limiting the energy output to exactly what the battery needs to charge.

What Size Solar Panel Will Charge A Dead Battery?

A 12-volt battery needs a minimum charge voltage of about 13.5 to 13.6-volts. This means the solar panel’s output must at least offer this voltage, but preferably up to 14-volts to successfully charge the battery.

Most solar panels rated at 12-volts will produce up to 17-volts without a load attached at peak power generation. This voltage is much higher than the safe charging voltage of the battery at 13.5-volts.

This is why a charge controller is necessary to regulate the charging voltage within safe parameters for the battery.

The charge controller should have a feature that will measure the battery’s charge, and once it is charged, it will stop sending a charging voltage to the battery. This will prevent the battery from being over-charged and sustaining damage.

Over-charging a battery can damage the battery, and it can be dangerous, causing the battery to explode.

To sum up the information thus far, you cannot directly charge a 12-volt battery with a solar panel rated at 12-volts unless the solar panel is designed for this.

All normal solar panels, even higher-rated panels, such as a 48-volt panel, can be used in conjunction with a charge controller to regulate the energy going to the battery.

So, where do the watts of the solar panel fit in? The wattage of the solar panel will determine how long the solar panel will take to charge the battery. The lower the wattage of the solar panel, the longer it will take to fully charge the battery.

See also: How to Charge a Battery with a Solar Panel: A Comprehensive Guide for Beginners

How Long Does A Solar Panel Take To Charge A Dead Battery?

A solar panel should take between 5 and 8-hours to fully charge a dead battery if the battery is in good condition and the solar panel is sized correctly. This is, however, not an exact time because of the variables involved.

The time t will take for a dead battery to charge will vary depending on the battery’s condition, the voltage drop on the solar panel due to varying light conditions, and losses in the system due to heat.

Most batteries will have their amp-hour rating stamped or labeled on the side of the battery. You can use this information together with the battery voltage and the wattage of the solar panel to work out an estimate of how long it will take to charge the battery.

If our 12-volt battery is a 70-Ah battery, it will take 12 X 70 = 840-Wh (watt-hours) to charge.

To estimate how long a 100-Watt solar panel would take to charge this battery, we divide the 840-Wh by the 100-Watts of the solar panel, and we get 840/100 = 8.4-hours to charge.

If we have a 300-Watt solar panel, the equation looks a little different. 840/300 = 2.8-hours to charge the battery.

These equations assume that the solar panel is operating at full capacity and that the battery is in good condition. If either of these conditions is not true, it will increase the estimated time for the battery to fully charge.

Why Your Solar Panel Is Not Charging The Dead Battery

A solar panel may not charge the battery if the battery is beyond salvaging or if the solar panel is not generating enough energy to charge the battery.

There will be some instances where the battery will not become charged from the solar panel. The cause for this will require some troubleshooting.

The first step is disconnecting the solar panel and using a voltmeter to check the solar panel’s voltage. If the panel is producing less than 13.6-volts, it will be insufficient to charge the battery. This could indicate an incorrectly rated solar panel or a faulty solar panel.

If you are using a solar charge controller, ensure you have wired the solar panel to the correct connectors in the correct orientation. Also, check that you have connected the battery to the correct terminals on the charge controller. Ensure all the wiring is firmly connected, with no loose terminals.

Short of testing each cell on the battery and measuring the specific gravity for each cell, you will not know definitively if the battery is bad.

If you have run through all the other checks and tried charging the battery with the solar panel in good light conditions, then, via the process of elimination, the battery is probably bad. It will not be possible to charge a battery in this condition, and you will need to replace it.


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Elliot has 20+ years of experience in renewable technology, from conservation to efficient living. His passion is to help others achieve independent off-grid living.

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