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Why your outdoorsy friend suddenly has a gummy bear power bank

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258 points arnon | 2 comments | 21 Sep 25 12:31 UTC | HN request time: 0.541s | source

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cenamus ◴[21 Sep 25 13:07 UTC] No.45322381[source]▶
20Ah for 23 bucks? Seems like it's almost too good to be true. Wouldn't surprise me if it was just half that, would explain the price and weight.
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thegrim33 ◴[21 Sep 25 14:56 UTC] No.45323306[source]▶
Man it drives me crazy when people/products use Ah instead of Wh as a way to specify battery "capability".

Without knowing more details about the battery, "20Ah" alone does not convey enough information to determine how long the battery could power a given load for. If I need to power a 100 watt lightbulb, will a 20Ah battery power it for an hour? 10 hours? 10 days? No way to know.

Wh is the unit of stored energy, Wh is what I want to see. Even the official Amazon product page for it doesn't list a Wh figure.

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MobiusHorizons ◴[21 Sep 25 22:03 UTC] No.45327092[source]▶
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 ◴[21 Sep 25 22:35 UTC] No.45327291[source]▶
> 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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1. MobiusHorizons ◴[21 Sep 25 23:13 UTC] No.45327522[source]▶
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)
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2. mbesto ◴[21 Sep 25 23:35 UTC] No.45327645[source]▶
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.