Interesting idea, Martin. I've used APC products for years at work, and we have many of
them at work ranging from small 500VA units for wiring closets to 16KVa units and in
between. You are correct that the larger units generally use 24V or 48V. Also, the smaller
units are not only rated rather optimistically, they tend to have the worst quality AC output,
using a stepped square wave or similar. You can get good sine wave AC from units in the
1000VA range, but they are much larger (often >100lbs with batteries), and usually 24/48V
DC on the battery banks.

The older APC units (or generally any unit today with a metal case) tend to be better quality in
the smaller sizes (all the large units are generally very reliable, as they should be). We have
seen very high failure rates on the BK500 series, for example, with >25% failure after two
years in a batch of 20 units. By comparison, we have several early 600VA units from the
early 90s still in service. Perhaps I will resurrect one of our parts units for a battery
charger/inverter too... :-)

On 28 Mar 2007 at 16:12, Martin Jagersand <jag@CS.UALBERTA.CA> wrote:

> 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

Shawn Wright
http://members.shaw.ca/vwdiesels
'88 Westy 1.6TD 5 speed
'85 Jetta Diesel 1.6NA