J-Go Tech Support Team - October 1, 2020

Why Milliamp Hours (mAh) Isn't Important When Charging a Laptop with a Power Bank

People often ask us, "How much mAh do I need to charge a laptop?" Here's why this specification can be misleading, and more importantly, how to truly calculate how much longer your laptop's battery can be extended with a power bank.

After the discovery that you can charge your laptop using a USB C power bank, the next question you may have might be, "How do I know how many hours of additional battery life or full recharges a power bank will give my laptop?"

The answer to this question is certainly not a "one size fits all" approach. There are some factors that only you will know that will ultimately determine the correct answer to this question. However, we've put together this quick guide to help you quickly determine about how much longer your power bank will extend the life of your laptop. 

To get the best estimate, here are the steps to take: 

  • 1. Find the capacity of your laptop battery and the power bank.
  • 2. Understand the power conversion efficiency of the power bank you're using
  • 3. Determine how long your laptop’s battery lasts under typical usage.
  • 4. Think about how you intend to use a power bank to recharge your laptop.
  • The Wrap-up

1. Find the capacity of your laptop battery and the power bank.  

It’s important to know the capacity of both batteries, measured in “Watt Hours” (Wh) instead of “Milli-Amp Hours” (mAh).

As an example, the internal battery capacity of the Apple MacBook Pro 16” can be measured at 8,790mAh (100Wh). This does NOT mean that a 10,000mAh (37Wh) power bank can fully recharge it up to 1 or more times. However, a power bank with a capacity of 27,000mAh (100Wh) would be more comparable.


You may be wondering how the “mAh” is comparable while the “Wh” is very different. This is because the nominal voltage of both batteries is different. While laptop batteries come in a variety of different nominal voltages, power banks typically use lithium-ion cells that have a nominal voltage around 3.7V. If you’re curious, here’s the equation to figure this out:

(milli-amp hours) * (nominal voltage)                                         (10,000) * (3.7)
________________________________  = Watt Hours          so:  ______________ = 37 (Watt Hours)

                        (1,000)                                                                          (1,000)
  

2. Power will be lost while it is transferred from the power bank to your computer. Here's why "conversion efficiency" matters.

"Conversion efficiency”, is basically a comparison of how much power reaches the device being charged compared to how much power is transferred. There are many things that can impact this. Some of the most common ones are cable length, high charging voltages/currents, heat and the battery health of the computer.

Most laptops charge at a current around 20V, which is significantly higher than the nominal voltage of the power bank (3.7V). As the voltage from the power bank is increased to the 20V current that the computer needs, heat is generated and some power will be lost. As the power leaves the power bank and passes through the cable to your computer, you can anticipate that some power will be lost. For computers that use a large amount of power (over 65W), this will also cause more heat to be generated, resulting in more power loss.

The conversion efficiency will vary greatly. Most experts say that the average conversion efficiency range is between 60% to 80% for charging small devices (phone, tablet, headphones, etc.), so this could be slightly lower for larger devices (such as a laptop). Power banks that use high-quality internal cells will perform toward the higher end of this range, but these products are usually more expensive because premium internal cells are considerably more expensive.  

In our previous example using the MacBook Pro 16”, a 100Wh power bank would be comparable. Since the laptop can charge at up to 96 watts and has a 100Wh internal battery, it would be more realistic to anticipate that a power bank with premium internal cells would provide up to 0.7 charges (70% more battery life). 

3. Determine how long your laptop’s battery lasts under typical usage.

This will give you an idea of how much battery your computer typically uses per hour.

Although it is quite common for manufacturers to list the estimated battery life of a laptop, this number isn’t accurate for every user in all situations. The type of programs you use, the number of applications open simultaneously, screen brightness, power settings and many other factors will impact battery life. Many people also notice that their battery doesn’t last as long after several months of use as it did when it was brand new, as this does happen over time.
 

Let’s refer back to our original example. If your computer has a 100Wh internal battery and it typically lasts 10 hours, this would indicate that your computer will use about 10Wh over the course of 60 minutes. This would likely mean that your computer battery drops by 10% after each hour of use.  

4. Think about how you intend to use a power bank to recharge your laptop.

This is something that is easy to overlook, but it’s very important to consider.

If you plan to shutdown your laptop when the battery is low before connecting a power bank to recharge it, this will help the power bank quickly and efficiently recharge your laptop battery. The first reason is because none of the power is being used while the computer is recharging. Another reason being that computers tend to recharge at lower currents when they are shutdown because additional power isn’t needed to operate the computer, which will help the conversion efficiency between the power bank and laptop.

If you plan to use the power bank as a substitute for a wall charger so that you can recharge your laptop while still using it, the power bank won’t last as long before it needs to be recharged. One thing that’s easy to overlook is that your laptop may switch to a different power mode when it’s connected to a charger, as it doesn’t notice the difference between different types of external power sources (power bank, car charger, wall charger, etc.). A laptop switching to a different power mode can be a somewhat obvious event because the screen brightness will typically increase and/or backlit keyboards will illuminate, but it isn’t always this apparent so be sure to check the power settings on your laptop to see how it changes when it’s plugged in and when it’s using battery power.
  

The Wrap-Up

Let’s refer back to our original example for the last time and put everything together:

In point #1, we determined the example laptop’s internal battery size (100Wh).

In point #2, we determined that the conversion efficiency of a 100Wh power bank with premium quality cells would likely drop to about 70% conversion efficiency due to the high charging current (96W watts) of our example laptop, resulting in about 70Wh being transferred to the example laptop battery (70% more battery life).

In point #3, we learned that our example laptop battery drops by about 10% after every hour of use, as it uses about 10Wh every 60 minutes under normal circumstances.

For point #4, let’s say that you have updated our power settings so that they do not change when the laptop is plugged in.

If you want to use the 100Wh (27,000mAh) power bank to recharge your MacBook Pro 16” while still using it, we have to factor the amount of power that is being used. If you charged your laptop for 1 hour, about 10Wh would be used to power the laptop, which is about 10% of the laptop’s battery life. Since we estimated that 70Wh would be transferred from the power bank to the laptop, now we have to deduct the 10Wh that were used to power the laptop. This would result in the laptop having about 60% more battery life, or 0.6 recharges.

We hope that this is helpful as you look for portable charging solutions for your laptop. If you have any questions or need some assistance, feel free to reach out to our team of experts! 




At J-Go Tech, we focus on developing innovation charging solutions that primarily support USB-C charging. As USB-C charging becomes a standard way of charging, we understand that there will be many opportunities for people to charge their devices faster, easier and with less equipment. With this, we are on a mission to educate consumers on the advantages on USB-C charging technology.

Have questions or need a personalized solution? Want us to cover another topic or model?
Reach out and let us know at support@jgotech.com

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