Something doesn't seem right

Anonymous user · ‎2021-06-28

I have a spare BE350G UPS that's rated at 200 watts output. I have a small networking switch that draws maybe 10 watts of power that I connected to the UPS to see how long it could be powered in case of a power outage. It came out to roughly 3 hrs.

I connected the same switch to the USB port on my laptop which has a 9-cell battery and ran the same test. Powered only by the laptop battery, the switch lasted close to 24 hours.

Why the huge difference in uptime? I would think the UPS battery would have much greater capacity but obviously not. Any explanation why?

voidstar_apc · ‎2021-06-28

There's a few differences between those two scenerios:

- The BE350G uses a sealed lead-acid battery. These are cheaper than the lithium ion battery in your laptop, but also have a lower energy density.

- When your USB hub is attached to the UPS, you have losses from DC->AC conversion in the UPS and AC->DC conversion in a cheap chinese wall-wart. Your laptop, by contrast, is doing a single DC-DC conversion.

- A typical laptop USB port provides 5V at 0.5-0.9A, so anywhere from 2.5W to 4.5W. The UPS is capable of providing 200W. All else being equal, power components that can deliver 200W consume more power than those that provide 2.5W.

See Answer In Context

voidstar_apc · ‎2021-06-28

There's a few differences between those two scenerios:

- The BE350G uses a sealed lead-acid battery. These are cheaper than the lithium ion battery in your laptop, but also have a lower energy density.

- When your USB hub is attached to the UPS, you have losses from DC->AC conversion in the UPS and AC->DC conversion in a cheap chinese wall-wart. Your laptop, by contrast, is doing a single DC-DC conversion.

- A typical laptop USB port provides 5V at 0.5-0.9A, so anywhere from 2.5W to 4.5W. The UPS is capable of providing 200W. All else being equal, power components that can deliver 200W consume more power than those that provide 2.5W.