How long should a smartphone battery last? How well should it hold up to heavy usage? Perhaps you're not familiar with the benchmark tests and the battery ratings of the new smartphone you're buying; there is one little detail you need to note - the size of the battery. And it is measured in mAh (S.I unit).

For an in-depth look, here's an excerpt from androidauthority.com :

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The A is capitalized because, under the International System of Units, “ampere” is always represented with a capital A. The term mAh is an abbreviation for “milliampere hour,” and it’s a way to express the electrical capacity of smaller batteries. With larger batteries, like car batteries, we usually use ampere hours, or Ah. There are 1000 mAh in a single Ah. mAh is calculated by multiplying the amount of time the battery lasts by the amperes of the discharge current.
That may sound complicated, but it’s really not that bad. If you have a battery and you don’t know what it’s capacity it is, all you have to do is hook it up to supply a 1000 mA discharge and see how long it lasts. If it lasts an hour, hey, you’ve got a 1000mAh battery. If it lasts 7 and a half hours, well then you’re holding a 7500mAh battery.
In real life, discharge rates vary not only from device to device, but also user to user. How efficiently your smartphone manages its battery power plays a role, but so too does the amount of active screen time you typically experience as well as how resource-hungry your apps are. So, while the mAh can give you a reasonably good idea about how long a battery will last, the figure doesn’t tell the whole story. That’s why it’s always a good idea to check reviews and see what other users’ experiences have been like before buying a smartphone.
Battery life is effectively inversely correlated to how frustrated you will be with a mobile device. One of the biggest complaints we hear about a smartphone or tablet is that the battery dies in the middle of the day. If you want a positive handset experience, mAh size is one of the first figures you should look at.
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With that said you get the drill: the higher the mAh the better the battery. But not so with all devices. Why? It is because some manufacturers may bump up the battery size but run a high quality display, a super fast processor, and premium specs which saps rapidly the juice from the battery. If you're to pick up a new device and are battery focused, make sure to relate it with other specs to find out how overwhelming the battery drain might take effect.

Source : http://informationfromjay..co.ke/2018/02/mah-battery-explained.html?m=1