Recent searches have showed that a number of radio batteries failed in cold conditions.
The aims of testing were to:
- Gain an understanding of the temperature characteristics of the various battery types used in radios
- Assess the usefulness of the new low self discharge NiMH batteries for field use
A series of tests were completed on the different battery types in temperature controlled conditions using a standard resistive load, equivalent to the nominal load of a handheld radio on receive. The testing was complimented by a review of manufacturer specifications for the various types of batteries and their characteristics.
In the tests conducted, no batteries met their nominal rated capacity at room temperatures, nominal 20°C.
Alkaline cells drop to ~40% of their nominal capacity at 4°C and are down to ~15% of their capacity by -10°C. Hence alkaline cells must be kept warm in order to operate in cold conditions. Due to their very low self discharge Alkaline cells have a nominal shelf life of five years.
NiMH Rechargeable Cells
Standard NiMH cells drop to ~50% of their nominal capacity at 4°C and are down to ~40% of their capacity by -10°C. The self discharge is in the range of 20-30% per month at room temperature. Hence, some months of self discharge in combination with low temperature use makes their use problematic, particularly in cold conditions.
Low Self-discharge NiMH Cells (such as eneloop, enekeep, combi, etc) These batteries have less self discharge than standard NiMH cells, typically only 1-2% per month. They hold their capacity at ~80% down to 4°C and only continue to drop to ~70% at -10°C. Battery recharge can be 1-3 hours, provided they are at room temperature.
A limiting factor for all NiMH batteries is that the nominal cell voltage is only 1.2V when compared to alkaline cells at a nominal 1.5V. Hence where NiMH cells replace alkaline cells in a radio the voltage is lower, hence reducing the transmit power. In addition, the NiMH voltage is also much closer to the low-voltage-shutdown within the radio, reducing the expected usage duration.
Use of Low Self-discharge NiMH cells could be considered in cold conditions providing room temperature charging facilities are available for the number of radios in use.
Li-Ion Battery Packs
The Uniden 5W Radio Li-ion battery pack provides 85% of its rated capacity at room temperature, ~80% at 4°C and <20% at -10°C. In addition, when cold the battery does not provide sufficient current for the radio to transmit at 5W. In addition, these batteries require 3.5 hours to fully recharge from flat, at room temperature. It is recommended that these radios and batteries must be kept warm and the radio should be set to only transmit at 2W in cold conditions.
Other radio Li-Ion battery packs tested held 80% of their capacity and their transmit power down to -10°C. Their recharge time is typically 2-4 hours at room temperature.
Li-ion batteries generally have a low self discharge rate of 2-3% per month.
To prevent cell damage Li-Ion batteries require special circuitry that cut off their charge and discharge at their terminal voltages.
The over riding problem for all rechargeable batteries is that they must be charged at room temperature to fully recharge, a difficult requirement in a search environment.
Author: Rik Head, 23 July 2012
Radio battery capacity testing 1r1.doc 3