In Chapter 7 you learned about various ways of supplying low-voltage DC to your electronics designs from AC. In this chapter you will learn how to use various types of batteries and photovoltaic solar cells.
8.1 Estimating Battery Life
Problem
You want to know how long your battery is going to last.
Solution
The capacity of a battery is specified in Ah (ampere hours) or mAh (milliampere hours). To calculate how many hours your battery will last, you need to divide this number by the current consumption of your project in A or mA.
For example, a 9V PP3 rechargeable battery typically has a capacity of about 200mAh. If you were to connect an LED with a suitable series resistor that limited the current to 20mA, the battery should be good for about:
Discussion
The estimate of time you get using the preceding calculation is very much an estimate and all sorts of factors such as the age of the battery, the temperature, and the current will affect the actual battery life.
If you are using a number of batteries in serial (e.g., in a battery holder that contains 4xAA batteries), then you do not get to multiply the battery life by 4, because the same current will be flowing out of each battery (Figure 8-1).
Figure 8-1. Batteries in Series
From Kirchhoff’s Current Law (Recipe 1.4) we know the current flowing ...
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