Hi,

I've experimented with using a computer Uninterruptible Power
Supply (UPS) in place of an inverter and charger. In theory this
seems appealing as the UPS already packages a charger, small
battery and inverter in one box. Modifying it to add battery capacity
from an extra camping battery is also easy. However in practice
thjere are some problems to watch out for.

Here are some experiences:
In general take capacities with a grain of salt. They are often
stated in VA instead of Watts, and they probably mean peak
instead of RMS. For resistive loads stay below 3/4 of stated
VA capacity, and for anything with a motor test it first, as
motors can draw 2-5x more current at startup, which can make
a UPS (or regular inverter) shut off.

The small consumer oriented APC models like BE500R worked
well. Inside is a 12V 10Ah gel cell battery. I bought my unit
on sale for CAD$39. It can be installed and connected to the
vehicle battery so that it will charge both batteries whenever it's
plugged in and when power is lost it will automatically switch
to inverter mode. As configured it will emit an annoying beep
when running off battery, but this can be turned off with
the supplied SW.

In summary it makes an economic alternative to buying
a small inverter and charger, and it is also small enough to
bring to say the camping table.

The larger server models (Smart UPS series) are not so
suitable. The three I tested would not turn on unless they were
plugged in, and the larger (1000 and 1500VA) ones are 24V
internally, so would only be compatible to wire into a 24V
heavy truck or bus electric system.

Martin