Wall power adapter with battery backup?

I'm working on an electronics project that I need to make sure stays functional even if the power goes out. I need something like a wall power adapter that also has a battery backup inside. That way if the power goes out, the battery will supply voltage until the power comes back on. When the power comes back on, the battery charges back up.

Can anyone think of an off-the-shelf product that would do the trick? I need at least 3 Volts DC, and 50mA of current.

More than you need, and it would require a little bit of work: https://www.mini-box.com/OpenUPS Basically overkill.

KeepItSimpleStupid said:

More than you need, and it would require a little bit of work: https://www.mini-box.com/OpenUPS Basically overkill.

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Thanks for the info. Yes, it's a little too big and expensive for my application. I need to do about 100 of these and they are going to be placed all around my house. I was hoping to find something really simple.

It would be easy to build using a wall adapter and 3 rechargable batteries, but I've not seen one off the shelf.

What sort of time would you want this for?

Is this for LED emergency lighting?

Something like this https://www.sparkfun.com/products/8722 might work? I either have that or something similar.

If this is for short ride-through Linear Technology has a supercap backup supply.

How long a power outage?

Three NiMh cells would give you about 4.5V to 3.5V as they discharge, so a LDO regulator downstream is probably in order. A retired 5V cell-phone charger could be used to supply power to the LDO input while simultaneously trickle-charging the batteries. A couple of Schottky streering diodes would automate the switch-over...