Can A Solar Panel Overcharge A Battery? 12v Battery Charging

Updated on October 17, 2022
Can A Solar Panel Overcharge A Battery? 12v Battery Charging

A 12 volt car battery can be overcharged by a solar panel

Could you over charge a battery with a solar panel?

There are many varieties of batteries for many different applications, from powering a flashlight to powerful 12 volt car batteries.

Whatever battery under consideration, the two most important factors are the voltage of the panel output and of the battery you’re connecting to.

As a general rule a solar panel can overcharge a battery. The charge rate depends on the solar panel voltage, the output current and the battery voltage. Overcharging is eliminated by using a solar charge controller.

Can you overcharge a 12v car battery with a solar panel?

For this explanation, let’s take the very common car battery, with a nominal voltage of 12V. What does that mean?

Electrical circuits are designed for a certain voltage, which is called ‘nominal‘. The actual operating voltage may be more or less, depending on battery charge and load characteristics.

How to keep a solar panel from overcharging a battery

A lead acid battery (by far the most commonly used for auto applications) has a fully charged open-circuit voltage of around 12.73V to 12.8V. This voltage falls gradually as more current is taken out of the battery.

The voltage level can actually be used to determine the discharge level of lead acid batteries, if allowed to settle with no activity for two hours.

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Overcharging a car battery is eliminated when using a charge controller

Logically, any voltage over the battery volts at any given time will cause current to flow into the battery and charge it up.

During this process, the charging voltage is regulated to ensure efficient and safe charging, and this is the crux of the matter – direct solar panel connection is normally unregulated.

12 Volt Lead Acid Battery Open Circuit Voltage Discharge Table

Percentage Charge Volts Percentage Charge Volts
100% 12.73 50% 12.10
90% 12.62 40% 11.96
80% 12.50 30% 11.81
70% 12.37 20% 11.66
60% 12.24 10% 11.51
Swipe for More
12 Volt Lead Acid Battery Chart



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Solar charge controllers regulate incoming solar power to maintain batteries and prevent overcharging. We offer two models, the more advanced Maximum Power Point Tracking (MPPT) and the industry-standard Pulse Width Modulation (PWM).



12 Volt Lead Acid Charging Cycle And Voltages

All battery chargers use some method of voltage control to safeguard against battery damage. Such damage includes buckled internal plates and over-heating.

A 12 volt lead acid bank of cells may be charge initially at approximately 14.4 volts, if significantly discharged.

When the battery is fully charged up, the charge detects this and drops the charge voltage to 13.6 volts.

This is called ‘float charge’ and ensures the battery remains in tip-top condition ready for use.

What size solar panel to charge 12v battery?

Solar panels have a certain power rating depending on their physical dimensions, but the important characteristic for battery charging is the voltage.

For most panels open circuit voltage is about 22 volts, give or take a volt either way. Keep in mind that it isn’t the voltage that varies with light intensity, but mostly the available current generated by the panel.

Such a solar panel connected to a battery in bright sunlight will charge the battery and keep on charging it over and above normal recommended voltages. It would definitely damage the battery internally. It’s important to use a suitable solar charger.

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A Charge Controller Is Essential To Prevent Solar Battery Overcharging

What Does A Solar Charge Controller Do?

In the previous sections we learned about the importance of regulating battery voltage during charge and this is the job of the solar charge controller (SCC).

This electronic circuit detects the existing battery voltage and adjust the solar panel output voltage down to a safe level. At the same time it maximizes the current that the panel delivers, so that the charging time is as short as it can be.

When the battery voltage approaches around 14.1 volts, the controller reduces it’s output to suit. This is called float charge and maintains full charge for battery health.

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Battery over charging protection – MPPT or PWM charge controllers

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Connecting Solar Panels To Batteries

Before we take a look at the two basic types of solar charger commonly used, it’s worthwhile explaining how to connect them your panels and battery.

Solar chargers have detection circuits that instruct the components how to vary both voltage and current going out to the battery.

It’s important to note that solar chargers don’t have an inherent power supply – they detect the battery voltage and then use it’s voltage to operate it’s circuits. This is why the battery should always be connected first.

When connected, the solar charger will display the battery voltage, which is some indication of it’s state of charge (see above chart).

Most chargers have a liquid digital display for indicating parameters such as voltage and charging current.Finally, the panel can be connected to the charger input terminals.

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NOTE: If the panels are connected before the battery, the displayed voltage will be seen to fluctuate.

This is not recommended by manufacturers, who state that the controller may be damaged in this way. Solar charge controllers need the battery voltage connected to function properly.

Difference between MPPT vs PWM – which is best for battery charging?

Although it’s agreed that the differences between PWM and MPPT charge controllers is negligible for small applications such as charging just one battery, bigger batteries, and batteries connected together can be charge considerably quicker with MPPT.

Why is MPPT charge controller better?

PWM controllers are less efficient than MPPT, which can provide 25% to 40% more charging current. The method of delivering charging voltage is completely different.

The MPPT controller adjusts it’s internal resistance so that the Maximum Power Point of the solar panel optimizes the current output to the battery.

The voltage of a PWM controller is pulled down to just above the battery voltage, which determines the charging current. To draw maximum current from a solar panel the load resistance must equal the solar panel’s characteristic resistance. An MPPT controller varies the load resistance so that maximum current is drawn.

Series Or Parallel Connections – Solar Panels And Batteries

While normally we consider 12 volt batteries as a standard, certainly for gasoline auto applications, this isn’t the only configuration. Batteries can be connected in series or parallel.

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Parallel battery connections for 12 volts solar charging

In some applications, such as in the marine or RV leisure industry, more power might be needed to run everything. When considering lead-acid batteries, anything over 100Ah (amp-hours) is pretty heavy.

It’s often practicable to connect two batteries together in parallel as shown above.

The voltage stays the same at 12 volt, but the available current and overall amp-hour capacity doubles, assuming identical batteries. Simply make the normal connections to any positive and negative terminal.

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Series Battery Connections Double The Voltage

It’s quite common to have marine equipment rated at 24 volts. For example, electric trolling motors used for fishing are usually rated at 12 volts up to 40 lbs thrust, and then 24 volt or 36 volts above that.

Connecting two 12 volt batteries in series will give an output of 24 volts, while the current capacity remains the same.

Solar Panel Connections For Charging – Series Or Parallel?

The normal open-circuit voltage output from a so-called 12 volt solar panel is about 22 volts in bright sunshine.

In some cases, with certain types of solar charge controller, it’s more efficient to connect two in series for small installations such as motorized solar kayaks or small boats.

Two panels connected together in this way would produce double the voltage but the maximum current output would remain the same.

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Solar Panels Connected In Series and Parallel

The good news is that most solar charge controllers detect battery voltage and panel voltage, adjusting the charging characteristics accordingly. The charge nameplate normally gives details of the maximum voltages and current for that particular model.

Video Explaining Exactly How Not To Charge A Battery With Solar

Can A Solar Panel Overcharge A Battery? Summary

A solar panel without a solar charge regulator connected will almost definitely damage your battery. The only caveat is if the panel is so small it can only deliver trickle current and maintain a float charge.

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What power solar panel to charge 12v battery?

It is possible to directly connect a larger panel to a battery if you also have a load connected that’s in regular or constant use. That way the battery will never be in danger of over-charging.

If no load is connected, then the battery voltage must be monitored manually and the battery disconnected when the voltage reaches around 14 volts.

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Questions About Solar Panels and Battery Charging

What size solar panel to maintain 12 volt battery?

A small solar panel of 5 watt will maintain a 12 volt battery at optimum levels. Once fully charged, a small current will flow to compensate for natural leakage. Larger panels should not be used without a solar charge regulator.

Can I leave a solar trickle charger on all the time?

As a general rule, a solar trickle charger can be left connected to a battery indefinitely. The charge current of a trickle charger is regulated by battery voltage and will be limited to almost zero as the battery charges fully. Solar trickle chargers with overcharge protection provide complete protection.

What size solar panel to charge 12v battery?

Any solar panel with a voltage over 13.6 volt will charge a 12 volt battery. Open circuit voltage of an average ’12 volt’ solar panel is around 21 volts, so care should be taken not to overcharge the battery. A solar charger should be used at all times.

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The post ‘Can A Solar Panel Overcharge A Battery?’ was written based on the experiences of solar installers specializing in home solar applications.

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