TI makes a line of switching regulators. (switching for efficiency)
They're in the DIgiKey catalog.

A battery and a diode (perhaps 2 diodes)  for isolation.

Set it up so that external power is high enough to keep the switcher
supplying 0%.  THen when the external goes away the switcher will take
over.

george

In article <11mc80qd2cql632@corp.supernews.com>, 
mhsprang@NOcustomSPAMware.nl says...
> <steve.jones@scannex.co.uk> wrote in message
> news:1130763336.762647.230000@o13g2000cwo.googlegroups.com...
> > We have a small unit which has a external power supply of 7V. This unit
> > draws around 100mA. We have been asked to supply some sort of battery
> > backup for it. There is no ups to hand so what we are looking for is a
> > DC small DC supply 7 to 15V which can power the unit from the mains and
> > keep powering the unit for around 2 hours on power fail.
> 
> This can be setup really simple: feed your circuit from tha 15V (must be
> higher than battery voltage) supply through a diode. Also connect a 12
> lead-gel battery to the circuit through another diode. When the power fails,
> the battery takes over. You can trickle-charge the battery from the power
> supply as well though a suitable resistor.
> 
> Meindert


Depending on what the user means by 'small',   a 9V alkaline battery
(with about 600mAH capacity) and a diode might work.  An unstated 
specification is how often they expect the power to fail and whether
they want to build the battery into the same housing as the device.

Mark Borgerson

<steve.jones@scannex.co.uk> wrote in message
news:1130763336.762647.230000@o13g2000cwo.googlegroups.com...
> We have a small unit which has a external power supply of 7V. This unit
> draws around 100mA. We have been asked to supply some sort of battery
> backup for it. There is no ups to hand so what we are looking for is a
> DC small DC supply 7 to 15V which can power the unit from the mains and
> keep powering the unit for around 2 hours on power fail.

This can be setup really simple: feed your circuit from tha 15V (must be
higher than battery voltage) supply through a diode. Also connect a 12
lead-gel battery to the circuit through another diode. When the power fails,
the battery takes over. You can trickle-charge the battery from the power
supply as well though a suitable resistor.

Meindert

We have a small unit which has a external power supply of 7V. This unit
draws around 100mA. We have been asked to supply some sort of battery
backup for it. There is no ups to hand so what we are looking for is a
DC small DC supply 7 to 15V which can power the unit from the mains and
keep powering the unit for around 2 hours on power fail. This could be
a external battery that connects in line between the current power
supply and the unit, or a totally new power supply with integrated
battery. The batterys will need to be accessable to allow them to be
changed once a year. I know I could design and build something but
wondered if there is anything on the market. We would want several
hundred of these. Any advice/help would be appreciated.

Thanks