Really? That's surprising, though it makes sense from a cost POV. It must be a much better modified sine than I get with my other inverters. There's no noise, buzzing, or overheating at all, which is why I like them for my low-draw, sensitive gear.
Yup, I was surprised to find out it put out the same waveform as my cheap 12V inverter. If anything, the inverter modified sine was a little bit cleaner. On most devices I don't notice any difference either way, but connect an electric motor or LCD TV to them and you can hear the difference immediately.
I'm not entirely convinced of that, though I confess my research is incomplete. For example, I have two older ones (a Belkin and an APC) and have plugged them into a modified sine inverter in an attempt to clean up the power on the non-UPS outlets. That side works, though not all the noise is gone.
Both UPS units have dead batteries, so I'll need to swap them out to see if a modified sine will charge the internal battery. It doesn't matter much since it's easier pull straight DC from an external battery and solar panels. The only issue is a reset if I lose a connection (IIRC it's the APC that locks up).
I had no luck chaining APC units together (I also have a belkin but I haven't tested it). There is a sensitivity adjustment on them, but even at the lowest sensitivity I couldn't get it work. Older ones might be different, but I think currently only the APC smart UPS have true sine wave. During normal operation it just passes the regular line voltage straight through. Not until it's on backup power does it change to MSW. So if the first one is plugged into an outlet it would probably work, but as soon as you lose power both of them would stop charging.
During a windstorm last year I lost power for one week. I had to learn a lot about backup power very quickly.
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