How To Know if a Solar Power Bank Is Fully Charged

How To Know if a Solar Power Bank Is Fully Charged

If you’ve ever used a solar power bank to charge your devices, you might have noticed that your power bank doesn’t give you as many full charges as you’d expect. While they’re useful, power banks don’t always necessarily charge to full, even though we think they do.

You can tell that a solar power bank is fully charged by checking the LED indicators on the device. Each light indicates that the device has charged to a certain percentage — often in 25% increments. You can also test the amperage of the power bank at full charge.

Read on to learn how you can know for sure that your solar power bank is fully charged.

How Can You Tell if Your Power Bank Is Completely Charged?

Use the LED Indicators on the Power Bank

The easiest way to tell if your solar power bank is completely charged is to look at the LED indicators located on the device. These indicators are usually on the top or side of both solar and regular power banks.

Solar power banks may seem a bit complicated to use upfront, especially if you’ve never used one. Luckily, they all operate relatively similarly, making it really easy to tell how much charge the power bank has.

Solar power banks operate in much the same way as regular power banks. The only major difference is that solar power banks can be charged via solar energy or electricity. However, the indicators that it is fully charged will be the same in either case.

Most solar power banks have four LED indicators. Each light represents a 25% battery increase. For example, one light would indicate that the battery is charged to 25%, three lights would indicate 75%, and four would indicate 100%.

Are LED Indicators Accurate?

Solar power banks are made to have a battery percentage indicator that can be read with ease. The problem is that these indicator lights aren’t always as accurate as we’d like them to be. Instead, most solar power banks vary slightly between what the light displays and the battery percentage.

This can make it quite challenging to tell exactly how much charge the power bank has.

That doesn’t mean, however, that the LED indicators are completely inaccurate or that you can’t rely on them. The critical thing to remember is that the LED indicators are an estimation rather than an exact percentage.

One of the things you can do to ensure your power bank gets fully charged is to leave it charging for another hour or so after the last LED light comes on. Usually, this will make up for any percentage variations.

Test the Amperage of Your Solar Power Bank

Another way to test if your solar power bank is fully charged is to use a multimeter, which will give you the amperage of your power bank. You can purchase a multimeter from hardware or electronics stores or an online retailer. You can then use the amperage reading to determine your power bank’s mAh rating (more on that later).

You should find that the mAh rating of your fully charged power bank is as advertised.

Combining the Two Methods

We’ve discussed how to use the LED lights to see if your power bank is fully charged. We’ve also talked about testing the amperage with a multimeter. Using the amperage reading, you can calculate your solar power bank’s total mAh rating, or capacity, and measure that against your device’s mAh rating. Once you’ve established that, you will then charge the device with the power bank multiple times.

Let’s look at how to go about this.

This method requires the following items:

  • A voltage meter
  • Your solar power bank
  • A USB charger
  • A mobile phone
  • A timer

This technique is a little more complicated than just using LED indicators. However, it will give you a much more accurate picture of whether your solar power bank is fully charged.

We’ll go through the procedure step-by-step later on, but first, here’s a brief overview of what to expect from this method.


All batteries have what is known as a capacity rating. It’s also sometimes called the mAh rating because the capacity is measured in milliamperes per hour (mAh). Milliamperes are a unit of measurement for electrical charges.

What you’ll be doing here is finding the battery capacity of both your solar power bank and your phone. You’ll then measure that against how often you can fully charge your phone from a “fully charged” solar power bank.

This method will allow you to determine whether or not the LED indicators are actually accurate in their readings. You’ll see more about how this works when we get to the calculations.

First, you need to find out your phone’s battery capacity. This information is almost always found in your phone’s settings. You will likely find it in the “About” section (or something similar). You can also do a simple online search using your phone’s make and model number to determine the battery capacity.

Generally, your phone’s battery capacity will fall between 4,000 and 7,000 mAh.

You’ll also have to determine the solar power bank’s capacity rating. While you can find this in the user manual — or anywhere else where the specifications might be listed — the figure is rarely accurate, so it’s best to measure it yourself (we’ll get into that shortly).

Once you have both capacity ratings, you’ll charge your solar power bank via the USB and then plug in your phone to see if the calculations line up. Here’s an example of what I mean:

If the solar power bank measures 20,000 mAh, and your phone’s battery measures 5,000 mAh, then each full charge of your phone should take one LED light away if each light measures 25% of the power bank’s charge.

Now, let’s dive into the method in more detail.

  1. Discharge your power bank completely. You’ll be measuring the amperage of the power bank both when it’s empty and when it’s supposed to be fully charged.
  2. Using the multimeter, measure the amperage of your solar power bank. This will give you a baseline reading.
  3. Start the charging process. You’ll want to know the amperage reading immediately after you begin charging, so be sure to record this. You’ll also want to start the timer at this step.
  4. Allow the power bank to charge fully. However, you’ll need to know the last amperage reading before a full charge, as the device won’t show any amperage once it’s fully charged. A good way to do this is to record the multimeter readings throughout the charging time.
  5. Stop your timer. Once your power bank is fully charged, stop the timer.

After completing the above steps, it’s time to do some simple calculations.

Find the Average Amperage of Your Solar Power Bank

You will take the initial amperage reading from the empty power bank and average it with the last amperage reading before a full charge.

Say your initial reading was 0.8A, and the final reading was 0.2A. Your average reading will be 0.5A:

0.8A + 0.2A = 1A

1A ÷ 2 = 0.5A

Convert the Amperage to Milliamperes

You’ll want to convert the amperage to milliamperes since this is what battery capacity is measured in. To do this, multiply your average amperage reading by 1,000 (there are 1,000 milliamperes in 1 ampere). Using our previous example, 0.5A equates to 500mA:

0.5A x 1,000 = 500mA

Multiply the Milliamperes by the Total Charging Time

Once you’ve converted the amperage to milliamperes, multiply the figure by the total time it took your power bank to charge to full. This will give you the battery capacity of your power bank. For example, say it took four hours for the power bank to charge fully:

500mA x 4h = 20,000mAh

According to your calculations, you have now established that your solar power bank has a battery capacity of 20,000mAh.

Note: This calculation requires you to input the charging time in hours. So, for example, a total charge time of 4 hours and 30 minutes would be input as 4.5h.

Bring Your Phone’s Battery Capacity Into the Calculations

Now, it’s time to bring in your phone’s battery capacity.

Divide your solar power bank’s average mAh reading by your phone’s mAh rating. The answer to this should tell you roughly how many times you should expect to charge your phone from empty to full using the power bank.

Continuing with our previous example, a power bank with a 20,000mAh rating should charge a 5,000mAh phone battery from empty to full about four times:

20,000mAh + 5,000mAh = 4

Let’s continue with the steps to the method:

  1. Begin charging your phone from empty. Charge your phone from empty to full. Pay attention to the decrease in LED indicators. Did the indicator lights move down by one from one full charge, or are they all still present?
  2. Repeat this process as often as it takes to drain the solar power bank fully. You could use multiple phones for this as long as they all have the same battery capacity.

The battery capacity of the solar power bank should last through as many charges as your calculations offer. As a reminder, it should look something like four total charges for a 20,000 mAh power bank and a 5,000 mAh phone.

If the power bank isn’t lasting through the number of charges it should be based on your calculations, then it’s likely that the LED indicators aren’t an accurate representation of how much charge your power bank has.

How Long Does a Fully Charged Battery Last in a Solar Power Bank?

Solar power banks typically last through at least three full phone charges when the power bank is fully charged. The actual number of full charges can vary between the type of device you are charging and the quality of the solar power bank.

On average, solar power banks last about as long as regular power banks for charging power and capacity. Exactly how much use you get out of any given power bank depends heavily on its quality and how much energy your phone or other device is pulling from it.

Can You Fully Charge a Solar Power Bank Using Solar Power Only?

You can’t usually fully charge a solar power bank using solar power alone because the sun’s energy often varies. Factors like clouds, shade, and more can affect access to solar energy. In addition, solar power banks take significantly longer to charge via solar energy.

Solar power banks are designed to be charged by either solar power or via USB. Usually, unless you are traveling with it, it’s best to charge it via electricity. This allows the power bank to charge faster and until much closer to full. You’ll want to use solar energy for charging only when necessary.

This isn’t to say you won’t be able to charge your solar power bank at all with solar energy. In fact, on a day with many hours of direct sunlight, you can charge it significantly. However, bringing the battery to full charge will take far too long.

Combine this with the battery discharge that solar power banks experience and you end up with a power bank that is never quite fully charged.

Final Thoughts

The easiest way to determine whether a solar power bank is fully charged is to look at the LED indicators on the side or top of the device. Usually, each light will represent 25% of the charge on a solar power bank.

However, LED indicators aren’t always accurate. For this reason, it’s a good idea to test your power bank with a voltage meter.

A solar power bank will usually charge almost fully with electricity. If you’re using solar energy, it can take significantly longer to charge, and, likely, the device won’t charge to full.

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