For example, a tv remote needing two AA batteries instead of four AAA batteries.
In: 2
Voltage is the first and easiest consideration – you need enough batteries to get above the minimum voltage for the components to work. If you need 3 volts the you’re either going to need 1x 3 volt battery or 2x 1.5volts
Next is maximum draw – batteries can only supply so much current at a time so if you need to draw – lot of current you need a bigger battery or it will either fail faster, supply less voltage or overheat. This is a bit of a soft limit as you can either overdraw very briefly or use capacitors to slowly charge and supply the spike loads. For a car start battery, this is usually the governing factor
Now that you know what batteries can make your device work at all, you need to figure out how long you want it to run for. This may increase your battery size if your peak power draw isn’t already pushing you to a large enough cell.
Finally, you need to figure out what size fits in the case. You may need to compromise on power requirements if the device is very small or you may end up going for something more expensive like a custom lithium ion battery
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