Determining battery capacity for your project

For portable projects, choosing the right battery is not always easy. There are many choices e.g. alkaline batteries (AA, AAA, 9V block, etc.), lead acid, Li-ion, etc. With all different characteristics, different voltage, capacity, weight, size etc. This tutorial will guide you to choose a battery  in 4 steps and will end with an example.

  1. Determining voltage required for your project e.g. 3.3V, 5V, 12V or a combination of these.
  2. Measure current. You can use a lab power supply e.g. on a desk of the E-lab. When using this power supply you can see the current being drawn by your circuit. At the tutorial ‘how to use a DC supply’ you can find more info how to use this supply.
  3. Calculate required battery capacity by multiplying the amount of current being drawn with the hours you need your circuit to function e.g. 0.45 A equal to 450mA multiplied by 10 hours gives 4500mAh (milli-Ampere hours) or 4.5 Ah (Ampere hours). The capacity of rechargeable batteries is mostly given on the battery itself. See some links for the capacity of Alkaline batteries.
  4. Choose your battery and battery configuration.  You can add batteries in series. Adding them in series adds up the voltage. It is not a good idea to add different types of batteries in series.

Here is an example for the case in step 3:

Say we got a motor that draws 0.45A of current and works at 12V and we want to run it for the duration of 1 hr. We can use different battery configurations. Let’s take a standard AA alkaline battery. The nominal voltage of this battery is 1.5V and the capacity around 1500mAh. So to create a 12V battery we need 12/1.5=8 cells in series. For the capacity of 4500mAh we need 4500/1500=3 cells in parallel. To create this battery we need 8*3=24 cells in total. Another option is to buy a 12V lead acid battery with a capacity of 5Ah.

Your final pick of a battery depends on your project and must be a trade off between battery capacity, cost and, size/weight.

In case you need a 5 Volt supply, consider the use of a USB power bank. These are available in many different capacities. Charging and discharging is controlled via an internal circuit, improving safety. Be aware though that modern power banks tend to automatically shut down to save energy when providing low currents for some time period (typically tens of mA). The most simple solution to avoid this is using a suitable value power resistor as continuous load but this of course affects the useful capacity of the powerbank.