What type of battery would you recommend for a situation where you need
    about 9V, 20mA, for about one minute at a time (followed by about 30
    minutes of off time), repeated several times? The battery must be as small
    and light as possible (this is for a model rocket altimeter). Are there
    coin cells capable of this current level? If so, several could be stacked
    to get 9V.

Someone already mentioned the 2032's 50mA-ish short circuit current.  A 2016
is about half the thickness, weight, and total capacity, but has similar
SSC.  Traditional wisdom for model rockets is that you only use batteries
for one flight (since dying batteries can cause catastrophic failures.)  You
could cascade small 1.5V alkaline watch/toy batteries (186 or A76 sized.)
These are pretty cheap when you find the right sources.

Why 9V, anyway?

BG Micro USED to have some nice 3.6V 60mAh NiMH rechargables, about .4inch
diameter by .4 inch.  Two would be the eqivilent of a typical 9V NiCd (7.2V)
I don't know "normal" sources for NiMH button cells, but All Electronics has
some 7.2V "strip batteries" of similar size.


    Secondly, I remember small (smaller than AA size) 12V batteries being
    discussed here some time ago, but I can't find a supplier or the name of
    the type of battery. I think they might be used in wireless controls (such
    as car door lock transmitters).

They're typically called "lighter batteries." Radio shack has a couple
varieties of 12V as well as a 9V in a similar form factor,  IIRC, current
capability is pretty small; 20mA would probably be pushing it.

BillW

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