The AquaLINK GPRS and 3G units are using Polymer Lithium Ion Batteries.
Lithium polymer cells begin to degrade as soon as they are manufactured; in other words, they have a finite "shelf life". As Li-Po batteries age, their internal resistance rises. This causes the voltage to drop under load, reducing the maximum current that can be drawn.
You can slow the rate at which Li-Po packs degrade and maximize their usable lifespan by storing them correctly when not in use. Degradation is dependent on cell charge state and temperature. Store your Li-Po batteries at a low temperature, recommendation of 15°C and no more than 40% charged with humidity 45-75%. Warm them back up to ambient temperature before charging and/or discharging. Drawing high current from a cold Li-Po pack risks damaging the pack and causing it to balloon.
Besides common aging, a Li-polymer battery can also fail because of undercharge. This occurs if a pack is stored in a discharged condition. Self-discharge gradually lowers the voltage of the already discharged battery and the protection circuit cuts off between 2.20 and 2.90V/cell.
If a Lithium polymer cell or battery is kept stored for a long time without regularly cycle charging it (at least once per month) the battery will constantly discharge until it potentially will enter a deep-discharge and the voltage will eventually reach 0.0V. This will short circuit the cell and cause permanent damage which could also result in some swelling and possible leaking of the electrolyte.
The best way to keep a long battery life on your AquaLINK device is to store it in a working state. Keep the device in the sun even during off-season to allow the solar cell to keep the daily charging cycles on the battery.
If the battery of an AquaLINK device is not stored properly, it will loose it's warrantee.