For example, a tv remote needing two AA batteries instead of four AAA batteries.
In: 2
They balance:
1. What voltage is required to get the gadget to work.
2. What is a reasonable expected time between replacing batteries
3. How big a device will users tolerate.
The main difference between your standard 1.5V AA VS AAA batteries is physical size and amount of energy. The voltage is the same.
So if you need 3V to get the gadget to run your choice is whether to use 2 AAA, 2 AA, 2 C or 2D. These will each combine to churn out 3V to make the device run, and will do so for progressively longer.
If they want the device as small as possible they’ll use AAA batteries, at the trade off of more frequent battery changes, if they are willing to go with a slightly bulkier remote they’ll use AA. A remote that’ll take C or D batteries should last a long time, but would be way bigger than tolerated by most customers so they are unlikely to choose these unless there’s a serious reason to.
It’s usually a balancing act between battery life and form.
You could make a remote with four D batteries that would probably outlast the TV, but it would be absurdly large.
You could also power it with a button battery and have a tiny remote, but you’d have to replace it 4x a year.
AA’s and AAA’s are right about the sweet spots for TV remotes, small enough to fit in the hand, while large enough to only need replacing a few times in its life. Each AA battery is going to have about 3x the battery life of a AAA battery. So between them, it often depends on what shape the lead designer want the remote to be. If they want it thinner, then AAA’s are better, but sacrifice a bit of battery life for it. If the remote is already large, then it’s not a big deal to have larger batteries.
Devices have both power and voltage requirements. Voltage is based on what the components need to function. It’s much easier if a chip needs 5v to just give it 5v than to convert. Power is based on the collective consumption. More power hungry components more power needed.
Batteries have two features, voltage and capacity. Voltage is the actual voltage one battery puts out. If you connect one battery to the end of another battery in a line you get double the voltage. This is usually why devices have more batteries, they need 6v instead of 3v (for normal 1.5v AA batts) power is how many amps a battery can put out for a given time. For example a 2000 milli amp hour battery (mAh can put put 2000 mA or 2a for an hour. But it could also do 20mAh for 100 hours. The bigger the battery size the longer it can run. This is the second reason for more batteries, a longer run time.
With a remote it only uses power when buttons are pressed and when they are pressed they only use a very small amount of power. But critically the little IR led might need more than 1.5v to work. So it gets two batteries, the capacity doesn’t much matter but bigger = longer run time so it’s usually a size constraint, bigger remotes get AA, smaller AAA, really small might use a coin cell. But basically it’s pretty much just voltage and capacity that impact it
They gadget people need a thing to do something, so they go to micro controller manufacturers and ask for micro controllers, the micro controller manufacturers provide specs on voltage and current requirements for their controllers, the gadget people design their gadget to use the batteries that provide that voltage and current.
As for how the micro controller designers figure it out, they go to college and get electrical engineering degrees.
What voltage is needed? Most batteries (AAA, AA, C, D) in consumer electronics are 1.5v each. Most microchips run on ~3v or ~5v, so they need either 2 or 4 batteries.
How long does it need to operate? The size of the battery tells you how much energy it stores. The more stored energy, the longer it lasts. 8 AAA will run a portable stereo, but for way less time than 8 D batteries will.
A little more technical. The capacity of a battery is measured in amp-hours (Ah). A battery with 1Ah of storage (same as 1000mAh) will supply 1 Amp (1000 milliamperes) of current for one hour. Or 0.5A for 2 hours. I’m f your device has a constant load current of 400mA and you want it to run for a minimum of 17 hours, you need a minimum total capacity of 0.4A * 17hr = 6.8Ah. Combine that with your voltage requirements and off you go.
Some circuits need 3v but one AA is 1.5v. So two batteries are placed in series, or end to end, because you add voltages in series. But maybe the circuit only needs 1.5v but it needs to run longer than one battery can last. You place the batteries side by side in parallel and now you still have the same voltage but you’ve doubled the available energy that can be used per second.
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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