Eggtimer Rocketry Eggtimer User Manual page 14

Eggtimer Battery Options
The Eggtimer requires a battery with a nominal voltage of 3.0v-4.0v. The typical current draw is
about 8-10 ma when operating, and 2 ma when is Reset/Hold mode. This means that you have
several battery options:
· 3v Lithium batteries, such as a CR123 or CR2
· 3v Lithium "coin cells", such as a CR2032, if they are used for a very short time
· 3v or 3.6v rechargeable Lithium batteries
· Two 1.5v alkaline batteries wired in series for 3v, typically size AAA or AA
· 3.6v rechargeable Nickel Metal Hydride (NiMh) battery packs
· 3.7v rechargeable Lithium Ion (LiPo) cells
IMPORTANT: DO NOT CONNECT A 9V BATTERY DIRECTLY TO THE
EGGTIMER'S "3V" PADS! YOU WILL DAMAGE IT IRREPARABLY!
Because of the wide variety of rocket sizes and the different types of installations that are
possible, the Eggtimer does not come with a battery. There are a wide variety of battery options
available, which one you choose will depend on many factors:
1) The size of the rocket – Small rockets may require a smaller battery than a large rocket. If
you are trying to put the Eggtimer into the smallest possible space, a 3v Lithium "coin cell"
battery may work, but see the notes below.
2) How often you wish to replace the battery – Smaller lower-capacity batteries will have to be
replaced (or recharged) more often than larger batteries, which is the trade-off for the smaller
size
3) How much room you have for a battery – If you have a 3", 4 pound I-powered rocket you
probably have plenty of room for a 250 maH 3.6v NiMH battery, and don't care so much about a
few extra grams. Not so true if you're flying it in a BT-55 "D"-powered rocket...
4) How much the battery costs – Rechargeable LiPo batteries are nice, but you may need a
special charger for them. NiMH or plain old alkaline batteries are heavier, but they cost a whole
lot less to use. Non-rechargeable Lithium cells (i.e. CR2, CR123) work very well, provide a lot
of current, are cheap and readily available, and are relatively lightweight compared to alkaline
batteries and NiMH packs.
Lithium coin cell batteries (i.e. CR2032) will work with the Eggtimer, however they have a
relatively small capacity compared to most of the other battery options. Specifically, a CR2032
battery is rated at 225 maH, but this rating is for an extremely low draw, 80 MICRO-amps,
which is what you might get from a very low power device like a digital watch or calculator.
The Eggtimer draws significantly more current that this, so you may only get 100 maH of
capacity out of the battery, and that will drop even less if the battery gets warm (as in sitting out
on the pad in the desert in 110° weather). If you want to use a coin cell battery, use the largest
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