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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: 0s | 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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1. mattmaroon ◴[21 Sep 25 22:55 UTC] No.45327410[source]▶
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.

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2. MobiusHorizons ◴[21 Sep 25 23:19 UTC] No.45327555[source]▶
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.