LTC4150 Coulomb Counter Hookup Guide
MikeGrusin 
Battery Basics
Before we talk about coulombs, let's talk for a minute about batteries.
When you buy a battery from SparkFun (or anywhere else), you'll decide which one you want based on two important numbers:
One of these is how many volts the battery provides. You'll of course want to pick a battery that matches your project's requirements (too much or too little voltage isn't good). Usually we'll recommend a specific battery, such as two 1.5V AA cells for our Simon game.
The other number is the capacity of the battery, or how "big" it is. The higher the capacity, the longer your project will run. Higher capacity batteries are larger and heavier than smaller ones, so you'll need to trade off size and weight vs. runtime -- you might want to use AA batteries for a more portable project, even though they won't last as long as D batteries would.
We measure battery capacity in milliamp-hours (mAh) for small batteries, or amp-hours (Ah) for large ones. This number indicates the theoretical amount of current a battery can provide for one hour before running out of juice.
For example, all of these alkaline batteries have the same voltage (1.5V), but different capacities:
- D: 12000mAh
- C: 8000mAh
- AA: 2700mAh
- AAA: 1200mAh
The AAA battery above has a capacity of 1200mAh, which means it could provide 1.5V at 1200mA (1.2A) for one hour. But that's just the current it could provide for one hour. It could just as easily provide:
- 600mA for two hours (600mA = 1200mAh/2h)
- 300mA for four hours (300mA = 1200mAh/4h)
- 150mA for 8 hours (150mA = 1200mAh/8h), etc.
Conversely, depending on the kind of battery you're using, it might be possible to get:
- 2400mA for half an hour (2400mA = 1200mAh/0.5h)
- 4800mA for 15 minutes (4800mA = 1200mAh/0.25h)
- 72000mA (72A!!!) for 1 minute! (72000mA = 1200mAh/(1/60h))
In reality, the chemicals in a battery can only react at a certain rate, so you can't get unlimited amounts of power even for a short amount of time. However, high-discharge LiPo batteries without protection circuitry CAN discharge breathtaking amounts of power for a few minutes, and are used in model aircraft for exactly this reason.
If you want to know how long a battery will last, the math is easy:
To determine the current a full battery can provide for a given number of hours, divide the total capacity by hours:
1200mAh / 10 hours = 120mA
To determine how long a full battery will last at a given current draw, divide the total capacity by your project's current draw:
1200mAh / 50mA = 24 hours