Battery capacity is always measured in amp hours not watt hours, because it’s telling you more than just capacity. The rating is a measure of how many amps the battery can emit continuously for one hour. You can estimate how long the battery will last at different loads, but it won’t match up 1:1 because of efficiency differences depending on how fast the battery is discharged, and if it has a chance to recover between discharges. Basically watt hour measurements are path dependent, and using a fungible unit like watt hours obscures the meaning of the measurement.

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mbesto 3 days ago | parent | next [-]

> The rating is a measure of how many amps the battery can emit continuously for one hour.

This is absolutely not true at all. 'Ah' is a measure of capacity and 'amps' is a measure of current. Batteries typically have three measurements: nominal voltage, capacity (Ah or mAh), and rated continuous current (amps).

> watt hour measurements are path dependent

Watt hour is a normalized measurement of the battery's capacity. For example, it lets me compare a 12v/100Ah LifePO4 battery versus a 3.7v/3Ah Li-ion 18650 battery in terms of each batteries capacity (in this case 1200Wh versus 11 Wh).

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MobiusHorizons 3 days ago | parent [-]

Batteries have different capacities at different C rate discharge. 1C is the amperage listed as the battery amp hours, but at higher discharge, batteries have a lower capacity. Some batteries handle higher discharge better than others (closer to the rated capacity). You can of course estimate the battery capacity in watt hours, but it’s not how the battery is classified (eg in a data sheet)

	▲	mbesto 3 days ago \| parent [-]
		Sort of. C rate discharge is just another (confusing) way of state CDR. You might as well just say the CDR in amps. C rate is really just to give a comparison on battery chemistries to illustrate how performant different chemistries can deliver power at a normalized value (to illustrate how the battery chemistry could in theory scale up) For example, 1C is rated continuous discharge amps, which means a 1C rated battery will provide 1 * Ah. So if a 20,000 mAh battery is rated for 20,000 mAh @ 1C, it will (in theory) discharge 20,000 mAh at 20A in one hour. > You can of course estimate the battery capacity in watt hours, but it’s not how the battery is classified (eg in a data sheet) You're right but this is irrelevant because real life usage highly varies. Data sheets are just guides.

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wkat4242 3 days ago | parent | prev | next [-]

Ah does not mean it has to be discharged over exactly one hour in some test. It's simply a unit just like kWh. And most batteries are not optimised for one hour discharge. There's batteries with insanely high C-rates for delivering a lot of power quickly, discharging in 10 minutes. And some with really low that can standby for years. They'll still indicate the Ah capacity the same way.

	▲	MobiusHorizons 3 days ago \| parent [-]
		The C rate still comes from the amp hour number on the battery. I’m aware you can discharge faster or slower than that, but the capacity measured will not be the same

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mattmaroon 3 days ago | parent | prev [-]

I can see why you would think that, but no, that’s not the case. My RV battery is 200aH but can discharge at 380 amps and drain itself in well under an hour. (That’s a peak rate which it cannot sustain for long enough to discharge itself but it can still be well above 200 the entire time.)

Most lithium batteries can drain themselves much faster than an hour.

	▲	MobiusHorizons 3 days ago \| parent [-]
		I understand that. What I’m saying is that the capacity is different depending on how quickly you drain it. Different battery construction can mitigate this effect to a higher or lower degree, but the battery is rated for 200A discharge in one hour. At 380A you will get less total watt hours out of the battery than you would at 200A, ie it will discharge in less than the 31.5 minutes you would otherwise expect.