←back to thread

Why your outdoorsy friend suddenly has a gummy bear power bank

(www.theverge.com)
258 points arnon | 2 comments | 21 Sep 25 12:31 UTC | HN request time: 0.517s | source

https://archive.ph/1G2Ut
Show context
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.
replies(8): >>45322494 #>>45322612 #>>45322781 #>>45322945 #>>45323050 #>>45323306 #>>45326979 #>>45328472 #
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.

replies(8): >>45323988 #>>45324108 #>>45324359 #>>45327092 #>>45327383 #>>45327422 #>>45327424 #>>45327597 #
invalidator ◴[21 Sep 25 23:25 UTC] No.45327597[source]▶
There's a technical reason for it: the voltage sags when the battery is discharged quickly. Ah is relatively constant, but Wh decreases significantly with faster discharge rates, so it can't specified as a single figure.
replies(2): >>45328169 #>>45331333 #
1. numpad0 ◴[22 Sep 25 10:15 UTC] No.45331333[source]▶
That's a bit cursed mental model tbf... The voltages of batteries, in the first place, is function of state of charge. 100% = 4.2V, 0% = ~2.7V, 50% is 3.7V(by volume or something. 2.7 is also technical absolute minimums, cutoff voltage is usually more like 3.2V. Please don't abuse the battery in the ranges between 3.2 to 2.7V, let alone below).

Charge/discharge current capacity is constant throughout, at least so battery manufacturers say, at 1-20x the amp-hour capacity depending on the cell. Usually 5x or less.

Since energy = voltage x current, instantaneous W capacity is higher at first, reducing as it becomes supply side limited rather than load side limited.

But all those is irrelevant to why everyone uses mAh, it's because products with biggest numbers sell fastest. Marking capacity in Wh is noble, but it's a clearance worthy sin if you ask the shelves.

replies(1): >>45377557 #
2. invalidator ◴[25 Sep 25 19:15 UTC] No.45377557[source]▶
> The voltages of batteries, in the first place, is function of state of charge.

It's also a function of the rate of discharge. Have a look at this:

https://marsen.com.au/wp-content/uploads/2021/08/Panasonic-N...

All that space between the black and green curves is energy being lost to internal resistance.