Calculating How Long a Battery Will Last

98 Hacking Electronics

This actually takes all the difficulties away from using a LiPo in a situation where you want to charge the LiPo battery in situ. Your 5V microcontroller circuit or whatever you are using is just attached to the VCC and GND connections, the battery is clipped into the socket, and to charge the device, you just plug in a USB cable.

Calculating How Long a Battery Will Last

Figure 5-9 Combined LiPo charger and booster

We have already touched on the capacity of a battery—that is, the number of mAh it can supply. However, other factors come into play that we should think about when deciding if the batteries we are considering for a project are going to last long enough.

It’s really just a matter of common sense, but nevertheless it’s easy to make false assumptions about what you need.

As an example, I recently built an automated door for my chicken house. It opens at dawn and closes when it gets dark. It uses an electric motor, and electric motors use a lot of current, so I needed to decide what kind of batteries to use. My first thought was to use big D cells or a lead–acid battery. But when it came to do the math, I found this wasn’t really necessary.

Although the motor uses 1A each time it is in operation, it is only in operation twice a day, and each time only for about three seconds. I measured the control circuit as using 1mA all the time. So, let’s work out how many mAh the control circuit and motors each use in a day, and then see how many days various types of battery will last.

Let’s start with the motors:

1A × 3 seconds × 2 = 6As = 6/3600Ah = 0.0016 Ah = 1.6mAh per day

On the other hand, the controller, which I had assumed was the low power part of the project would consume:

1mA × 24 hours = 24mAh per day

This means we can pretty much ignore the power consumed by the motor since it is less than a tenth of the juice required by the controller. Let’s say the total requirement is 25mAh/day.