Battery backup for 5v DC ~ 2amps?

[Noozer]

I need to put a battery backup on a small device. The device is powered by a
5v DC wall wart rated for 2 amps. I doubt that this actually draws near 2
amps though.

Any suggestions on how I could put a battery backup here efficiently? I'm
only looking for something that would keep it running for 5-10 minutes tops.

Thanks!

 

[Jack]

You could use 4 Ni-Cd AA cells in series across the 5 volt supply lines.
These give a nominal voltage of 4.8 in the event of power failure and the AA
cells have a 600 mah rating which should supply 1.2amps for 30 minutes or
1.8 amps for 20 minutes. The 5 volt supply should be perfect for keeping
these cells charged. One precaution - make sure the cells are fully charged
before you put them in the circuit as they might draw enough current to
damage your wall wart if you started it up with fully discharged cells.

 

[Noozer]

There's no problem with the NiCads overcharging if left connected? They're
just active like a BIG capacitor?

 

[Jack]

Overcharging should not be a problem if the output of your wall wart is less
than 5.8 volts while under load.
It would be a good idea to check this while it is connected to the device.

 

[Noozer]

Sounds good... Thanks!

Overcharging should not be a problem if the output of your wall wart is
less than 5.8 volts while under load.
It would be a good idea to check this while it is connected to the device.

 

[PeterD]

I need to put a battery backup on a small device. The device is powered by a
5v DC wall wart rated for 2 amps. I doubt that this actually draws near 2
amps though.

Any suggestions on how I could put a battery backup here efficiently? I'm
only looking for something that would keep it running for 5-10 minutes tops.

Thanks!

A lead-acid battery is not really good (not enough headroom for the
regulator...) but if you used a bunch of large ni-cads (or NI-MH) or
other 1.2 volt cells in series you could get 7.2 volts and that would
give some room for the regulator. Of course you could go to a LDO (low
drop out) regulator but that's really close to the line (6 to 5
volts...)

You may want to see if the device in question:

1. Has an internal regulator, and if it does, then a 6 volt battery
may work just fine,

and/or

2. What the true, under load, voltage from the wall wart is... (bet it
is more than 5 volts if it is not a switching version).

If the wall wart is really providing about 6.8 to 7.0 volts, and the
unit is internally regulated, and you use a lead-acid battery, you
could probably even use it (the wall wart) to charge the battery.

I'd also measure the current draw from the wall wart when in
operation, to help size the battery, and to determine if there is
enough reserve capacity to charge the battery.