Eggtimer Rocketry Eggtimer User Manual page 15

one you can fit in the rocket (CR2450's are the best since they have the largest capacity), and test
them with a Digital Voltmeter while the Eggtimer is powered on BEFORE EVERY FLIGHT.
General Battery Procedures
1) TEST YOUR BATTERY WITH A DVM BEFORE EVERY FLIGHT!!!
If you do not have one, buy an inexpensive digital voltmeter and check the battery voltage while
the Eggtimer is powered ON, before EVERY flight. The voltage across the two Battery
terminals (B+ and B-) should be between 2.8v and 4.0v. (It may be a little higher if you have
just recharged the battery, i.e. LiPo batteries may show up to 4.2v when fully charged; that's OK,
the Eggtimer will be fine with it). If your battery reads under 2.8v, recharge it or dispose of it
and use another battery. NEVER fly with a battery under 3.0v if you are using electronic
deployment. The Eggtimer draws very little current, but you don't want to risk having the
battery run low at 10,000'.
Similarly, test the deployment batteries as well if you are using electronic deployment. Check
your igniter vendor's guidelines for recommended voltage, and replace the battery if it's close to
the minimum value, or if the voltage is less than 80% of its rated value, whichever is highest.
For example, a Quest Q2G2 igniter will fire with as little as 3.7v (a fully-charged LiPo cell), but
if you're using as 9v battery you should replace it if it's under 7.2v (80% of its rated value)
because its voltage drops rapidly as it begins to run down.
The Eggtimer's built-in battery tester is designed to be used as a quick check to see if your
battery MAY be getting low. It is not a substitute for checking the battery with a DVM, it does
not have the accuracy that you're going to get with a dedicated instrument. You NEED a digital
voltmeter in your tool box anyway; if you don't have one, run out to Radio Shack and buy one,
they're under $20.
2) Balance your model with the batteries in place.
AAA and NiMH batteries are fairly heavy, as are some of the larger Lithium batteries such as a
CR123. Alkaline 9V batteries are VERY heavy, almost 50 grams (nearly 2 oz!) You should
always weigh your rocket and determine the CG with the intended flight battery in place. We
strongly recommend using a program such as RocSim or Open Rocket to determine the flight
path of your rocket with the intended payload configuration (batteries, sled, etc.) and motor.
If the rocket was designed for egg lofting or multiple deployment, chances are pretty good that
just about any configuration with an Eggtimer isn't going to cause any stability problems, since
those payload configurations are generally at least as heavy as a fully-loaded Eggtimer sled using
a 3v battery and LiPo battery for deployment. However, if you are building a rocket from
scratch or adding a payload bay onto a kit, particularly with a BT-60 or smaller body tube
diameter, you may end up with an over-stable rocket that wants to weathercock into the wind or
dance around on its tail after clearing the launch rod, due to excessive nose weight. Using a
larger motor or one with a higher average thrust (i.e. E20 instead of E9) to get more speed off the
rod will help that situation somewhat, as will using a longer launch rod. If you do the latter, you
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