Specifications

30.8.3. Battery capacity rating
Imagine a jug which holds one gallon. If you drain this jug at one gallon per hour, then the
bottle will be empty in one hour. If you drain the jug at half a gallon per hour, the jug will be
empty in two hours, and if you drain the jug at two gallons per hour, then the jug will be empty
in half an hour.
Similarly, a 2100 maH battery can supply 2100 ma of current for one hour, or 1050 ma of cur-
rent for two hours, or other combinations.
30.8.4. Internal resistance/discharge rate
Each battery has an internal resistance which determines the maximum continuous discharge
rate.
For NiCad/NiMH batteries, the internal resistance is measured in milliohms. The lower the
reistance (milliohms), the more current the battery can supply.
A good rule of thumb is to estimate the maximum discharge capacity at 125 divided by the mil-
liohms of internal resistance. For example, the Sanyo RC2400 cells which have about 4.5 mil-
liohms of internal resistance can supply about 125 / 4.5 or about 27 amps of continous dis-
charge current.
For LiPo cells, the maximum discharge rate is usually given as a C-rating, such as 10C or 12C.
In the RC world, the C-rating specifies the current rating as a mutiple of the battery capacity.
So a 2100 maH battery pack would be able to supply 2100 ma * 10 or 21000 ma (21 amps) of
current. Note that this 21 amps can be either a maximum continous dicharge rate or a maxim-
um burst discharge rate.
Note that reputable companies rate their cells conservatively for maximum current and their
batteries will last hundreds of cycles at the maximum current rating. Less reputable companies
will aggressively over-rate their cells, and if you continously draw the maximum current from
the battery, it will only last for twenty or thirty charge/discharge cycles. Therefore, if you buy
the cheapest batteries, you will spend more on batteries over the long term.
30.9. Battery care and maintenance
Different types of batteries have different requirements for care and maintenance. In order to
ensure maximum battery usability, you should carefully follow the recommendations for each
battery type.
30.9.1. Nickel-cadmium (NiCad) batteries
Nickel-cadmium batteries are generally the most robust of all the battery types. They're the old-
est and most reliable battery type, (often over 200 charge/discharge cycles) and are fairly res-
istant to overcharging, vibration, and other forms of abuse.
NiCad batteries can be charged at a maximum rate of 2C.
NiCad batteries should be left discharged at the end of the flying day. When stored long-term
(over two months) they should be stored in a cool storage area. After storage, you should do a
formatting charge (which overcharges the pack slightly) to ensure the battery pack is properly
balanced.
Warning
NiCad batteries contain cadmium which is toxic and can cause lung and kidney
Technical Appendix
208