HACKER Q&A
📣 njpurchio

Why can’t all battery-powered devices operate when plugged in?


Many devices we own (ex: my partners breast pumps) consume a lot of power and take a long time to recharge their internal batteries to operational levels. If we ever forget to charge them or the charger didn’t work, etc, we’re unable to use them until while plugged in and have to wait until the batteries are sufficiently charged. But I can use my iPhone when it is on 1% no problem. Why isn’t every battery-powered device able to be used in a similar manner?

  👤 dusted Accepted Answer ✓
There are some complications to implementing a power supply that works both off of battery and from the input power as well.

Random list:

Your input supply needs to be stronger, because it potentially has to charge the batter + run the device, that means a more expensive supply.

Extra circutry needed for supplying the device from either the battery or the fixed supply, this is not exactly hard to do, but it still means extra engineering, extra components and extra testing.

Thermal design can also become a problem when you're both charging batteries and passing current to the device.

Then there are safetey issues as detaro mentions

👤 detaro
It's some extra effort to implement, so manufacturers might cheap out on it.

For some devices it's considered a safety feature. e.g. an electric razor that's advertised as watertight and usable in the shower, because then someone will try using it in the shower while plugged in, and that's not a safe idea. Also applies to some medical stuff.

👤 airbreather
And the duration to charge is often conservative, but can be substantially better - eg the charge rate for my Ryobi 18V hand tool batteries is phenomenal, charging a 5AH battery in less than 60 minutes and seems like half charge in 15minutes, but they have fairly elaborate circuitry built into the battery and seem to charge as hard as possible up to a temperature limit sensed on individual cells.