Edit: I am talking about EVs
In: Physics
Car batteries are usually rated in amp-hours and cold cranking amps. But the only difference between Watt-hours and Amp-Hours is the battery voltage. If you are given a battery rated in Amp-hours, just multiply by the nominal voltage and now it’s in Watt-hours.
Different standards of different industries.
Batteries, for the most part, tend to care more about amperage ratings than the actual amount of power involved, so everything ends up being rated in amps (and in related terms, like amp-hours) than in watts or watt-hours.
However, the automotive industry measures engine and powertrain components based on power, and so they use power-based units for rating capacity. That means using watts and watt-hours.
Car starter batteries are measured in Ah.
Phone batteries are measured in mAh (which is, you know, just 0.001 Ah – because phone batteries are ridiculous small in comparison)
Electrical car batteries are measured in Ah too. Because batteries always are. But the data is *presented* to the user as kWh, because that is the unit that your power meter in your house measures your usage with; it’s used on electrical cars because it avoids confusion and makes it easier to correlate cost of driving to cost of charging.
Amp Hours are a more useful measure for cheap DC devices
If you’re building a dirt cheap electronic device then you’re using linear regulators, not switching regulators. For linear regulators the current is preserved while for switching regulators the power is preserved. Linear regulators are simple but hot, you set up a transistor to keep the output at a fixed voltage, in order to get the output to that voltage it has to burn off any extra input voltage across itself as heat.
If you have a little DC circuit that takes 1V and 30 mA and you’re feeding it from a battery through a linear regulator then if you feed it from a 9V 450 mAh battery then it’ll last 15 hours, if you feed it from a 2700 mAh AA battery then it’ll last 90 hours. The 9V and the 1.5V AA battery both technically have 4.05 Wh of power, but the extra voltage is just being thrown away as heat
For your phone we now use switching supplies so you’re right that it would be better to measure in Watt Hours, but since all phones use a single cell battery that runs at 3.7V you don’t run into any conversion issues.
For laptops which are multicell and run at different voltages we often label them as Wh to avoid having to do the conversion, and because larger batteries may have more small cells so they end up with a higher voltage and a lower mAh rating while still having more total power available.
Cars are an extension of the laptop problem. If you look at the Tesla S battery packs you’ll notice that the Amp-Hours of both of 85 kWh and the 60 kWh batteries are the same, this is because they’re multiple identical cells wired in parallel so the Amp-Hour rating of the battery is the same as the Amp-Hour rating of one of the cells, but with the 60 kWh battery pack running at 352V and the 85 kWh battery pack running at 402V, that same current rating turns into more total power. If you were to just list the Amp-Hour rating you’d have no real change, you *have* to list the Watt-Hour rating for Multicell batteries to make sense to a minimally informed consumer
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