Batteries also vary their voltage over time, temperature, and with load, and tend to have fairly high self-resistance and limited charge storage so you need a lot of them (or a lot of cells, or very big cells) for high-current components.
Most UPS units purchased are off-line so the battery is not doing anything (except being trickle charged) until you lose power, then the UPS switches to using the battery (and DC to AC converter). As @Prana Ferox said, the usual types do not provide very clean output, often 10% distortion or more (sometimes much more). True-sinewave converters are available but cost much more money and are usually bigger and heavier as well. There are some new versions that offer 1%~5% or so distortion that are not terribly expensive so I have a couple of those.
On-line UPS units run off the DC source all the time so never switch (no switch-over lag). An AC to DC converter supplies power and charges the batteries for the DC to AC converter as long as AC input power is supplied. These tend to be much more expensive than off-line units.
Toroidal transformers are usually built without an air gap in the core and that makes them much more sensitive to DC input. Conventional transformers are also sensitive to DC but much less so to the point of being an almost nonexistent problem for consumers.
FWIWFM - Don
Most UPS units purchased are off-line so the battery is not doing anything (except being trickle charged) until you lose power, then the UPS switches to using the battery (and DC to AC converter). As @Prana Ferox said, the usual types do not provide very clean output, often 10% distortion or more (sometimes much more). True-sinewave converters are available but cost much more money and are usually bigger and heavier as well. There are some new versions that offer 1%~5% or so distortion that are not terribly expensive so I have a couple of those.
On-line UPS units run off the DC source all the time so never switch (no switch-over lag). An AC to DC converter supplies power and charges the batteries for the DC to AC converter as long as AC input power is supplied. These tend to be much more expensive than off-line units.
Toroidal transformers are usually built without an air gap in the core and that makes them much more sensitive to DC input. Conventional transformers are also sensitive to DC but much less so to the point of being an almost nonexistent problem for consumers.
FWIWFM - Don