# How to compare mAh to Watt hours?

I have a laptop with a battery described as:

3 cell Li-Ion 57Wh

I would like to know how this compares to a powerbank with 26500 mAh capacity.

How do you perform these kind of comparisons? What is theory behind it? AFAIK:

Why are some batteries measured in mAh? In my view, a battery has a certain amount of energy, not a certain amount of electric charge.

Lithium ion cells have a nominal voltage of 3.7 volts, so the pack has a total (nominal) voltage of 11.1 volts. Power equals voltage time current. 57 Wh/11.1 gives a current capacity of 5.13 Ah, or 5130 mAh.

A powerbank normally has a single cell voltage, so a 26500 mAh bank can supply 26.5 x 3.7 = 98 Wh.

Choice of units is largely a matter of tradition. For batteries, since any "normal" load will run at battery voltage, rather than some arbitrary voltage, power and current are exactly equivalent. Therefore, total charge and energy are likewise equivalent. However, it's easier to measure current than it is power, since after all, power requires measuring both current AND voltage. So, for a known, more-or-less-constant voltage such as a battery it's just easier to measure only the current and be done with it.

• I am excited to read how the post continues... A capacity of what? munches pop corn Sep 5, 2018 at 12:38
• Great, thanks. I assume then that the power bank is "bigger" than the laptop battery, and should thus last longer than the laptop battery (which is basically what I am after) Sep 5, 2018 at 12:43
• And another comment: laptop battery gives 11.1 V directly on the motherboard, and the powerbank will give 5V (?) via the USB-C connector (Power Delivery). I assume this will have no impact (discarding system loses) on the total time that the energy contained in the powerbank will be able to power the laptop (that is, longer than the laptop battery) Sep 5, 2018 at 12:45
• He won't tell us. He's a Messiah. Monty Python - Life of Brian - The Prophet: youtube.com/watch?v=9czBBKof7Yo Sep 5, 2018 at 12:45
• @MarkoBuršič - Some of us do not sit in front of the computer all day. It's called "having a life". You should try it some time. Sep 5, 2018 at 16:25

1C = As; AsV = W*s = J; So if you multiply capacity Wh, mAh, ... with voltage you get energy. They are labeled differently because the voltage on battery drops while draining, but these days the converers will output constant voltage regardless on battery voltage, so the energy is more appropiate unit.