The most important measure of a battery is how much power you can get out of it on a regular basis. That number is represented by Watt-Hour or Wh. Not Amp Hour or even Milliamp hour that most marketing people like to write in the ads and on the products, that is only part of the answer. (Also remember that older types of batteries, like lead batteries don't survive long if you use more than half of what they contain.)
Let's start by oversimplifying and looking at a battery like a box. You can add stuff, like charging a battery or you can take stuff out when you want to use it. Its capacity is its Volume which is equal to its Length time Width times depth. If any of these change then the total volume will change.
If we simplify a battery capacity we can use the same approach. You can see it as an energy box where the 3 important numbers can be seen as sides of a box. So we calculate the power capacity as Volts times Amps times hours equals capacity in Watt-Hours.
You can use the numbers printed on a battery to calculate a number for its capacity in Wh if its not already there. Most of the time the Amp and the hour have already been multiplied so the equation is just Volts times Amp hour equals Watt hour. That is often what you find printed on a battery.
Unfortunately counting the battery capacity number this way is always too high because the reality is that the battery will always deliver less than this number for several reasons we will look at in the second half of the video. This is the number that the marketing department normally like to print in the specifications because it sound better that the number that you measure getting out of the battery bank.
Ok so let's look a more nuanced view of the battery.
The first reason you don't get as much power as the simplified calculation would suggest is that the Voltage decreases as the battery is being emptied. This can be accounted for if we imagine the container is not shaped like a box but more as a slanting wall. The volume or capacity will decrease.So due to this fact the battery can deliver 10-25% less power than the simple estimation would suggest.
A second reason you get less power is if the battery has to convert its internal voltage level to another that is expected outside. This is the case for all USB power banks that are typically 3-4V internally and have to boost the voltage to 5V which USB requires. This leads to yet more loss of 5-15%.
A third reason for a battery delivering less than expected is when they are working at a high discharge rate. Then the voltage drops even more than normally and the hard work of the batteries causes them to lose energy as heat. In this case it is not unusual to only get 50% out of a battery. With the risk of shutdown due to overheating.
A fourth reason is aging or damaged batteries that do not have all their initial capacity left. When a battery can only reach 80% of the original capacity after fully charging a battery is considered spent, and you can assume that it will degrade fast after this point.
The only reliable way to know how much capacity a battery has is to measure it but that is for another video. For now remember to find out the theoretical Watt Hour capacity and know that the reality will be lower.
Here is an example of the capacity of two batteries
A cell phone on average has 10 watt hours battery capacity. If we let a lego block represent one watt hour it looks like this.
A Currentium Power Bank has a true measured output capacity of at least 65 watt hours when new. It looks like this. In marketing language it would be called a 20000 milliamp hour battery and the Watt hour rating would be higher but that number is not very useful unless you know how the rest of the battery performs.
With the real Watt hour capacity we know that this battery will give this phone over 6 full recharges. That is accurate information. Check out the Currentium home page for more about power banks and solar power.