Can I Leave A Solar Trickle Charger On All The Time?

Updated on November 7, 2022

What is a solar trickle charger?

A solar trickle charger is a tool that uses solar power to charge various devices. These low-power charging devices are excellent for charging smartphones, laptops, or tablets.

They can be left plugged in all the time, providing an outlet for these devices at home without using  grid power.

It’s also a great option for battery charging and can be used with or without overcharge protection, although it is generally recommended to use a solar trickle charger with overcharge protection.

In general, a solar trickle charger can be left connected to a battery all the time. Most commercial chargers are designed to either generate very low power or incorporate over-charge protection. It’s always preferable to use a simple solar charge controller with any solar charging application.

Video – Car Battery Solar Trickle Charger Tips


How does a solar trickle charger work?

A solar trickle charger is powered by a solar panel, which is composed of multiple solar cells – panels used to charge 12 volt batteries have 35 cells and Voc (open circuit voltage) of 21 volts.

A solar cell generates an electrical current from the sunlight and transfers it to the charging device, which then charges the devices connected to it. The typical power rating for a solar trickle charger is 0.5-10 watts.

How many watts does it take to charge a battery with a solar panel?

A solar panel output is measured in watts, and it can vary from product to product. Solar panels can generate up to 350 watts, but solar trickle chargers are very low power, just a few watts.

In order to charge a battery, you will need a panel to generate more voltage than the battery, and also have a minimum power rating, which is determined by your battery size.

The big factor affecting solar panel output, assuming it’s unshaded, is the irradiance which is a measure of the amount of the sun’s energy reaching the panel measured in kWh/m2/day or year.

Incidentally, this value if also known as the peak-sun-hours for a particular location and is used by solar installers to assess solar panel output energy in kWh.

How many watts does it take to charge a battery with a solar panel?

Very low power solar panels can be left connected all the time

How much energy does it take to charge a battery?

Batteries are rated in ampere-hours (Ah). On the face of it, any calculation seems simple – if you have a battery of 50Ah, then you should be able to draw 25 amps for 2 hour. Unfortunately, this is wrong.

A normal lead-acid car battery should only be discharged to 25% maximum, so it’s effective capacity is much less than its rated capacity in Ah. Even a deep-cycle marine battery should only be discharged between 50% and 80%.

We need to express battery capacity in energy in Wh rather than Ah – this is done using the following formula:

Wh = Ah x V

Chart showing Ah to Wh conversions 10 to 160 amp-hours

See the chart below for conversions for various Ah ratings (assumes 12 volt battery):

















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How much energy does a single solar panel produce?

Energy produced in watts depends on irradiance, which is location dependent. A 10 watt solar panel based in  Lexington, Kentucky would produce:

10 x 4.05 peak sun hours = 40.5 Wh of energy per day

If we had  a battery with 20 Ah discharged, that would equal 240 watt-hours so it would take 240/40.5 days to fully recharge = 5.93 days.

As you can see, trickle chargers are really just for maintaining a battery fully charged, not for recharging when it’s been heavily used.

Do I need a charge controller?

A charge controller is a tool that prevents overcharging. The controllers ensure that the battery does not exceed a safe charge current, therefore protecting it from overheating and damage. Charge controllers are generally recommended for charging batteries over 20Ah capacity.

What is the difference between MPPT and PWM solar controller?

MPPT stands for Maximum Power Point Tracking, while PWM stands for Pulse Width Modulation. Both of these solar controllers are designed to regulate battery charge current from solar panels.

However, they do so in a different way. MPPT controllers have the ability to scan the internal resistance of a solar panel and determine which load resistance will result in maximum output from the panel.

MPPT controller technology then matches it’s resistance to that of the panel to optimize the output. This is the Maximum Power Point of the panel.

PWM controllers simply maximize this voltage by switching on and off to output as much power as possible using a momentary maximum voltage during on state. They are much less efficient, often delivering up to 30% less charging current than MPPT.

What is the difference between MPPT and PWM solar controller?

Solar charge controllers – battery overcharge protection

Renogy Solar Chargers – Innovation Meets Quality – Shop Now!

How to connect solar trickle charger to a battery?

If using a solar trickle charge without a charge controller, connection is very simple. Place the solar panel in the sun and attach the crocodile clips to the battery terminals, black to negative and red to positive.

A charge controller is generally recommended, and connection is not much more complicated. Simply connect the positive and negative solar panel leads to the controller and the controller red and black lead connect to the battery as previously described.

What size solar trickle charger do I need for a battery?

The size of the solar trickle charger you need depends on the size of the battery, its state of discharge and how much power you can get from your solar panel.

It is important to know that it will take longer to charge larger batteries. So, if you have a battery with a larger capacity (more Ah), make sure that your solar trickle charger can match its power requirements.

How to measure lead acid state of charge – quick method

Lead acid battery state of charge chart

The battery terminal voltage is a pretty good indicator of depth of discharge (DoD) for lead acid cells. The battery should be left with no activity for a few hours before. See the chart below for details:

State Of Charge % (12 V Lead-Acid)

Battery terminal voltage





















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Video – Renogy Solar Trickle Charger


Can a solar trickle charger overcharge a battery?

A solar trickle charger of low power normally cannot overcharge a battery – it naturally keeps pace with natural discharge rates over time.

However, it is important to make sure that your trickle charger has both overload protection and overcharge protection built into it, so that you will never encounter this issue.

A good solar trickle charger will also have an auto-shutoff feature that will cut off power to the unit when the battery reaches a certain temperature.

Some trickle charges have both overcharge protection and blocking diodes to eliminate discharge through battery drain, such as the Suner Power model.

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

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You can leave the Suner Trickle Charger connected all the time

Can a trickle charger ruin a battery?

A solar trickle charger can ruin a battery if left on too long without overcharge protection.

However, it is unlikely that a solar trickle charger will ruin a healthy battery if used correctly.

In fact, there are solar charging devices specifically for marine batteries designed to be left on all the time.

Just remember to use a solar trickle charger with overcharge protection.

How long do I leave a solar trickle charger on?

The amount of time that you should leave a solar trickle charger on depends on the size of the battery and the capacity of the solar panel.

If you have a small battery with minimal capacity, it will take less time for your device to charge. A larger battery with higher capacity will take longer to charge.

Question relating to battery charging with a solar trickle charger:

Are solar trickle chargers safe?

Trickle chargers of the proper wattage are safe. Most trickle chargers on the market these days have overcharge protection built into them, so it is not likely that you will encounter an issue with them.

You should make sure that your solar charger has both overcharge protection and automatic shut-off protection.

Can I connect solar panel directly to battery?

It is not a good idea to connect all types of un-controlled solar panel directly to a battery, although many products are available to do just that.

It is always best to use a solar charge controller, even if you plan on connecting only one device. The charge controller will help protect the battery from overcharging and will allow you to connect multiple devices.

Can a solar trickle charger drain a battery?

Solar trickle chargers do not drain a battery. They incorporate blocking diodes which stop any current flowing from the battery to the charge.

Using a solar trickle charger can actually help keep a battery in better condition by keeping it at an optimum level of charge.

Resources relating to Solar Trickle Chargers:

Best Solar Trickle Chargers For Motorhomes

7 Best Solar Car Battery Charger (Solar Panel) | 2021 Reviews


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