Non-interruptable power to a medical device drawing 1.3 amps

[MacConsultant]

Hi everyone !! This is my first post, and electronics is not my specialty.

I have a medical device which when used needs to run for up to seven hours without interruption in the event the house AC goes off. The device has a brick transformer taking 120V to 24V and the device draws 1.2 amps maximum but that might be measured at the device and not before the transformer.

I purchased a Cyberpower 1500 AVRT for $140 (CYBERPOWER 900 Watts: http://www.cyberpowersystems.com/products/ups-systems/avr-series-ups/CP1500AVRT.html

Others have recommended this Duracell $140 listed as providing 480 watts http://www.amazon.com/Duracell-DPP-600HD-Powerpack-Starter-Emergency/dp/B000TKHMWK/ref=sr_1_1?ie=UTF8&qid=1290647287&sr=8-1

QUESTIONS:

• 1. Which device of these two would you choose ?
• 2. I also heard about eserviceinfo listing some switching device to which any battery can be connected for seamless changeover from house AC to the power supply, but I can't find such a switching device here. There is a power inverter available here: http://www.cpap.com/productpage/resmed-150-watt-dc-ac-power-inverter.html
• 3. In the event the house AC goes off, how many hours of power would the Duracell unit provide this device ?
• 4. How many hours of power would the Cyberpower unit provide ?
• 5. Is there another setup that should be considered to accomplish the goal ?


Thank you in advance.

[torbjorn]

Before you make any decisions, make sure that you know more about the medical device's power requirements.
Does the brick transformer supply 24 V AC or DC? What are the equipment's requirements regarding voltage tolerance, ripple etc?

If we assume that the medical equipment requires 24 V DC and that it accepts "automotive" voltage tolerances, then the best solution would probably be to supply it from a 24 V sealed lead-acid battery, for instance 12 to 20 Ah. Connect a stabilized power supply in parallel to the battery and adjust it to a suitable voltage for trickle charging the battery (typically 27,6 V). In case the battery has became discharged, increase the voltage to 29,5-30,0 V for 24 h in order to get it fully charged within reasonable time. Make sure that the power supply has a rated current in the order of 3 to 5 A, and that it has a current limiting feature in case of overcurrent.
You should perform a battery test at least twice a year to make sure that the battery is in good condition. Discharge the battery by a suitable current (for instance, C/20) and make sure that the voltage does not drop below the limit stated by the battery manufacturer before it has delivered at least 80 % of its rated capacity (i.e. after 16 hours if you discharge it by C/20). Make sure that the medical equipment has another reliable power supply for the time of the battery test.