How to Choose a Power Bank | June 09 2024, 01:43

If you’re planning to buy a power bank, here’s a lifehack to get a better one for the same money:

Firstly, pay attention to the type of battery – Li-ion or polymer Li-ion (LiPo). The latter have a higher energy density (yielding more charge for the same weight), and they are safer.

Secondly, look at the ratio between weight and stated capacity. For example, mine weighs 436 grams and is labeled 40000 mAh at 3.7V. Convert this into watt-hours by multiplying 40000 mAh by 3.7, which gives us 148 Wh, resulting in 148/0.436 = 339 Wh/kg.

The thing is, such a density in batteries does not exist. For LiPo, the range is 150-250 Wh/kg. And 250 Wh/kg is for the most advanced, expensive types.

For a battery weighing 0.436 kg and a voltage of 3.7 V, the capacity range would be approximately from 18000 mAh to 30000 mAh with an energy density of 150-250 Wh/kg. It’s more likely between 20000 mAh to 25000 mAh. Which is quite good, but definitely not the 40000 mAh as listed.

In other words, take the device’s weight in grams, multiply it by a number between 40 and 65, and you get a very probable real capacity in mAh. I would use 45 for certainty. But it’s very likely that you should use 40 if you’re also trying to get it for the lowest price.

Next, look at the charging time. On mine, it’s written that with a 30W power source, it should take 6 hours to charge from 0 to 100%. Typically, it’s about 9 volts (common for 30W), though it’s not specified. So, the average charging current would be 3.33A (30/9). The battery capacity can be calculated by multiplying the current (3.33) by the time in hours (6). That gives us 3.33*6=19.98A*h=19980 mAh. This is another hint that the battery is nowhere near 40000 mAh, but rather around 20000 mAh.

Is 20000 mAh a lot? The instructions, and on the battery itself, state that for the USB-C port, the charging current is 3.1A at 5V, 2.22A at 9V, 1.66A at 12V, and the same for the Lightning port at 5V. If all this holds true, a full charge from 0% to 100% of my iPhone 15 Pro Max with a 4400 mAh battery should take a maximum of 2 hours. You could charge 4.5 phones like mine with this power bank, or it should nearly fully charge a laptop (70Wh) to 100% and then be depleted. So at first glance, even 20000 mAh is not bad.