Alkaline batteries, though very good at room temperature, cannot deliver much current at lower temperatures. Battery life can be as little as one third normal on a cold day and even less if they cold soak for any length of time. Life can be as little as just a few minutes at -20 F.

If you must use disposable batteries (non rechargeable) then lithium batteries are the only good choice. They have shorter life at low temperatures but are still much better than alkalines.

In the AA battery size, lithium and NiMh batteries are the best cold weather choice. At low temperatures NiMh have almost as much life as at room temperature and are rechargeable to boot. Our tests indicate that the NiMh AA batteries when used in an MM400 or SM transmitter have 75% of normal capacity at minus 15C (-15C) or +5F. We recommend the Eveready NiMh batteries and 15 minute charger that we provide with the SM, SMD and SMQ transmitters. One precaution is that the batteries cannot be recharged if they are cold. They can be used cold without any problem but must be at about room temperature to be recharged. The lithium batteries are also good at low temperatures but they don't like being cold for long periods of time. If cold soaked at 5 deg F for an hour, they still have 95% of their normal capacity. Allowed to cold soak overnight they only have 50% of their capacity. Here are some run times in a cold SM transmitter at different temperatures:

At room temperature:

• NiMh 4:02
• Lithium 6:01

At 5 def F cold soaked for one hour:

• NiMh 2:52
• Lithium 5:45

At 5 deg F cold soaked for 16 hours:

• NiMh 2:46
• Lithium 3:19

In the 9 Volt battery size, NiMh batteries perform as well cold as they do at room temperature but they don't have much battery life (capacity) cold or warm. At one time they were the only choice for very low temperatures but LiPoly rechargeable batteries are now available that have more capacity than alkaline batteries and perform very well at low temperature. They are currently sold under the iPower brand and are available on the internet, from some dealers and from Lectrosonics. (See FAQ#086-WIRELESS)