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Date:         Wed, 28 Mar 2007 16:12:27 -0600
Reply-To:     Martin Jagersand <jag@CS.UALBERTA.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Martin Jagersand <jag@CS.UALBERTA.CA>
Subject:      Using a UPS unit as inverter
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

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


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