3 ms·

Powerbanks are rated that way because batteries are rated that way. For phones it _kind of_ makes sense, as the capacities are closer to the 2.8-3.5 Ah range, a

Powerbanks are rated that way because batteries are rated that way. For phones it _kind of_ makes sense, as the capacities are closer to the 2.8-3.5 Ah range, and that difference is fairly substantial. (No comment on why processors are measured in GHz rather than MHz). If you want to pick a gripe why do we measure capacity in charge per unit of time times hour, rather than MJ?

formerly_proven 5y ago
They're marketed that way because big numbers sound good. Lead acid batteries are generally rated in Ah, even if it's just a small stand-by battery with 1.2 Ah. On the other hand, ratings within a group of products are usually done without a prefix change. Perhaps because of defective sorting algorithms, which would e.g. place a 1.7 kV device at the end of the list, while putting a 1500 V device first. That's probably why you see e.g. 4700 µF instead of 4.7 mF as well, though sometimes the latter is used. > If you want to pick a gripe why do we measure capacity in charge per unit of time times hour, rather than MJ? They're equivalent, it's just that writing Wh makes it very clear that you can divide this figure by some discharge power and get the approximate discharge duration. OTOH, an EE likely knows [power W] * [time s] = [energy J] anyway.
- nybble41 5y ago
  Listing capacity in Wh rather than MJ is fine, but many batteries are rated in amp-hours which isn't very helpful unless you also know the voltage. This is particularly annoying when it comes to "smart batteries" such as this USB power pack[0] where you have specs listing "10000 mAh" and "5 volts", but the actual capacity is around 37 Wh because they measured amp-hours at the internal battery and the battery voltage isn't exposed to the user. (Yes, the product description halfway down the page does say "10000mAh/3.7V" in small print if you know to go looking for it, but this isn't noted anywhere in the title or the "About this item" section at the top.) [0] Example: https://www.amazon.com/10000mAh-Portable-Charger-Stylish-Compact/dp/B08D9J2NJP/ https://www.amazon.com/10000mAh-Portable-Charger-Stylish-Com...
  - fomine3 5y ago
    Almost all robot vacuums just tell only mAh without V on specsheet. It's useless but it seems that 14.4V is de facto standard. I wonder some crap manufacturer may tell only mAh meanwhile using 7.2V battery.
Y_Y 5y ago
I share this objection. If you want to get even crazier, what about fuel economy? Using miles per gallon (or km/l) can also be simplified to reciprocal area, so 40 mpg is just 1.7e7 m^-2. For an electric car you can change 250 miles per 100 kWh to the more perfect representation 0.001 s^2 /(kg m).

formerly_proven 5y ago

They're marketed that way because big numbers sound good. Lead acid batteries are generally rated in Ah, even if it's just a small stand-by battery with 1.2 Ah. On the other hand, ratings within a group of products are usually done without a prefix change. Perhaps because of defective sorting algorithms, which would e.g. place a 1.7 kV device at the end of the list, while putting a 1500 V device first. That's probably why you see e.g. 4700 µF instead of 4.7 mF as well, though sometimes the latter is used. > If you want to pick a gripe why do we measure capacity in charge per unit of time times hour, rather than MJ? They're equivalent, it's just that writing Wh makes it very clear that you can divide this figure by some discharge power and get the approximate discharge duration. OTOH, an EE likely knows [power W] * [time s] = [energy J] anyway.

nybble41 5y ago
Listing capacity in Wh rather than MJ is fine, but many batteries are rated in amp-hours which isn't very helpful unless you also know the voltage. This is particularly annoying when it comes to "smart batteries" such as this USB power pack[0] where you have specs listing "10000 mAh" and "5 volts", but the actual capacity is around 37 Wh because they measured amp-hours at the internal battery and the battery voltage isn't exposed to the user. (Yes, the product description halfway down the page does say "10000mAh/3.7V" in small print if you know to go looking for it, but this isn't noted anywhere in the title or the "About this item" section at the top.) [0] Example: https://www.amazon.com/10000mAh-Portable-Charger-Stylish-Compact/dp/B08D9J2NJP/ https://www.amazon.com/10000mAh-Portable-Charger-Stylish-Com...
- fomine3 5y ago
  Almost all robot vacuums just tell only mAh without V on specsheet. It's useless but it seems that 14.4V is de facto standard. I wonder some crap manufacturer may tell only mAh meanwhile using 7.2V battery.

Y_Y 5y ago

I share this objection. If you want to get even crazier, what about fuel economy? Using miles per gallon (or km/l) can also be simplified to reciprocal area, so 40 mpg is just 1.7e7 m^-2. For an electric car you can change 250 miles per 100 kWh to the more perfect representation 0.001 s^2 /(kg m).