Recommendation for LONG run time UPS for small load

[squishem]

I have a special need for a long run-time for a very small load. I need to maintain a 50W load for 18+ hours in case of power failure.

I have done the math and figured out a cheap solution using an automotive lead acid battery and wiring it into the power supply with the required extra components. While this will work, it is not ideal as this means every device has to be modified in this manner and device modifications are not on the table right now.

The alternative is to place a UPS on the 120 VAC supply to the device and keep it alive instead. I have found units like the APC Smart-UPS 5000VA but they last for 14 hours for a 50W load and cost $3425 USD! These are designed for a different purpose than what I require and are overkill. Kind of like using a big 16 cylinder engine to idle along compact car when a little 3 cylinder would suffice.

I could connect external batteries to a small UPS and maintain the run time that way but I was hoping for a commercially available product. Any suggestions?

TIA

 

[jfrantz]

Have you considered bypassing the power supply for the device you're trying to keep running? The 24vdc to 120vac conversion is fairly inefficient. You may be able run your device directly from batteries and save yourself a lot of money. It may take a bit of engineering to add a charging circuit and a voltage regulator between the battery and your device.

-Jeff

 

[squishem]

Thanks, but like I said in my post I can not change the device hardware. The device to be powered for 18 hrs has undergone certification and if I change anything in it, it will have to be re-certified incurring extra time and expense. The customer is well aware of the extra costs incurred by using the external only fix, but for now it is the only option since time is short and re-certification is not an option. If there are any future mods to the device, then modifying its power supply to use external batteries is a no-brainer.

 

[MacyH]

Several UPS manufacturers make small systems that are rated for extended run times. Most of these have XL in their name and also connectors for extra battery packs. Generally the mfr will provide a table or graph for computing battery size vs run time needs.

To be more specific, I've used XL units from APC and Minuteman for this purpose.

 

[FredWagner]

Radio Shack sells a small 12V-115V inverter that will handle up to 75W, for about $30. I use one of these with a portable car starting kit with an internal gel cell battery to run a camcorder for an hour or more at a time. Use the radio shack inverter with a larger battery - like a marine battery for a trolling motor, and you'll get the runtime you need. Use an appropriate charger for the marine battery - the whole solutions should cost less than $200. You can get bigger inverters designed to be hard-wired for $50-$80 from radio shack - the little one I use plugs directly into a cigarette lighter socket on my portable jump starter, which has a built-in charging setup.

Fred Wagner

 

[squishem]

Thanks all. I considered that approach too but stayed away for 2 reasons.

1) Need a solution which provided a sinusoidal output, the connected equipment is sensitive and does not work well with non-sinusoidal AC supplies.

2) I need hundreds and hundreds of these from a commercially available supplier that already have the properly certifications in place. The end customer would deal directly with them and order as necessary.

Thanks to all that replied. It looks like a solution from APC will have to suffice for now, where I can add up to 10 external battery packs if necessary. Not the most elegant solution and overkill, but still a solution I can use today.