User's Manual

Power requirements: Batteries 33
RIM 902M OEM Radio Modem – Integrator’s Guide
should be obtained from the manufacturer. The RIM engineering development
team can help you determine whether a particular battery is suitable for your
application.
The cells chosen must be able to meet the load specifications of the RIM 902M.
Specifically, they must be able to provide 1.7 A (at 4.5V) for transmission.
Rechargeable cells vary considerably, because capacity varies with current
draw. Even if two cells have the same published capacity, one may not be as
efficient as another when the radio transmitter is turned on. This is because
some batteries have a higher equivalent series resistance (ESR) at high current
drain. The ESR should be low enough that the battery can supply the transmit
current required without a large voltage drop.
Rechargeable alkaline batteries are another option. These cells are typically
rated for about 25 discharge cycles, far fewer than NiCads, but they provide
longer life than NiCads. For the first five to ten cycles, you will get about 70 to
80 percent of the battery life you would expect from a single-use alkaline cell.
After 25 discharges, this number may drop to 50 percent. Some precautions
must be taken with this type of battery. These cells are also not intended to be
used to their full capacity, so the actual useful run-time of these cells is closer to
30 to 40 percent of a single-use alkaline cell, and requires the user to pay closer
attention to the state of the batteries. If you fully discharge a rechargeable
alkaline battery, you may only get five recharges before the capacity decreases
to the point where it is useless.
Single-use batteries
Among single-use cells, only alkaline and lithium cells provide the high
current necessary for transmission. In particular, alkaline AA’s are
inexpensive, widely available, and provide an excellent power source. Alkaline
cells typically run about four times longer than similar-size NiCad cells, and
about three times longer than similar-size NiMH cells.
The use of general-purpose carbon-based batteries is not recommended, as this
type of battery is unable to supply the power required by the transmitter. If this
type of battery is used, the voltage will drop below the minimum power
required under load almost immediately following a radio transmit, which
would reset the radio.
Since carbon cells are generally sold under names like “super heavy duty,” the
best way to be sure that a single-use battery is alkaline is to look for the word