I'd use a lead acid.  The high current capability of lead acids won't be
present because current doesn't change unless voltage or resistance
changes.  They are also usually cheaper.

A good source for them is TV stations, they have to pay to throw out
thier batteries, so after some grumbling from a lazy tech about howe
he's to busy for this, he'll happily hand them over.  These are also in
good condition as a tech once told me the batteries they throw out are
always good.  Because they are not going to risk using a bad battery in
a camera and miss a story.

As for capacity no idea without knowing more about the duty cycle of
your relay and it's current draw.  I'd I'm guessing a camera battery
should be sufficent.  If it isn't, go bigger.  :)  Or get two....

Aaron

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