Wouter,

The "usual" answer for 5 V systems is to use a 5 or 6 cell battery,
and a low-dropout regulator (LDO).

5 cells gives optimal efficiency for linear regulation (minimum
"wasted" headroom), but 6 cell pre-fabbed packs are common.  (The Ni-
Cd "9V transistor battery" size are usually 6-cell "7.2V" batteries).

Seiko and others make LDOs with very low quiescent current, which is
more capacity efficient than just dropping the raw pack by one diode
drop, because the dropout is less than 0.6 V (at low current draw)
when the battery is low.  I use the CMOS Seiko S81250-SG, but there
are others.

Look carefully at the discharge curves for the cells you choose to be
sure that the regulator will supply your load through the whole
discharge range.  Ni-Cds hold terminal voltage fairly constant until
near capacity, so its easier than, say, "alkaline" batteries, which
tail off slowly.

NiMH chemistries have better energy density (Watt-Seconds / Kilogram)
than NiCd, but are fussier to charge- check the spec sheets.
However, Cadmium is has to be disposed of (recycled?) carefully, so
some consumer applications are migrating away from NiCd even when
they don't need higher performance.  If you are planning around
commercially available batteries, you may be able to buy the
batteries and chargers off-the-shelf.

-Barry.
------------
Barry King, KA1NLH
NRG Systems "Measuring the Wind's Energy"
http://www.nrgsystems.com
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