There is no easy answer to your question for this kind of battery (Li-SOCl2 chemistry). It’s a professional battery, and there is no better kind for the very demanding, outdoors applications. It has some of the highest energy densities possible, can withstand a very wide range of ambient temperatures, and is uncommonly durable. But it is also notoriously difficult to estimate the remaining capacity after it’s been used. Unlike other types of batteries, the voltage does not change much with used up capacity until the very end-of-life, when it drops quickly and suddenly. The voltage will fluctuate with varying ambient temperatures, the amount of power drowned, and the length of time it was idle (when stored away without being used).

For example, this chart below, as viewed on the Zenseio Dashboard (https://app.zenseio.com/) which you have access to, shows the battery voltage (green line) and ambient temperature (purple line) over the last 1 year. The daily fluctuations are due to the day/night temperature variations. However, last summer the voltage was higher than now because the temperature was about 45F higher. Additionally, the battery lost some capacity last year and was stored idle for several months, so this contributes to greater voltage fluctuations then when the battery was brand new. But you could only determine the capacity by observing the magnitude of voltage fluctuations under similar temperature conditions, which is difficult here since the ambient temperatures are very different at different times of year.

As a rule of thumb, for CSMP, we see batteries last 1-2 seasons, depending on length of the active season, transmit periodicity, SM probe type, and the cellular signal quality. When the temperatures rise in the spring, if you see large daily voltage fluctuations or when the voltage is much lower compared to last year’s voltages at similar temperatures, it’s time to replace the battery. And I would try to be on the cautious side since the cellular radio module does not handle abrupt power loss due to very weak battery) which could result in its permanent damage. The radio module manufacturer (Multitech) refuses to cover such failures under warranty since they consider such behavior a property of the module that must be avoided by users, and not a defect.

This year, we are introducing an improved battery model that has an additional “booster” cell which compensates for the temperature variability and adds more capacity. As you can see on the chart below, the battery voltage does not vary much even at low temperatures (3.6V nominal), which allows the device to better handle low voltage shutdown of the radio module in a controlled manner. So, you should get longer battery life and safer device operation. But, if you prefer completely not to bother with battery replacements, another option is our solar charging kit (Instruction Manual for Zenseio Solar Charger Kit). It uses a rechargeable battery which is charged by a small solar panel.

For LSMP, we are seeing very long battery life on the order of several years. And there is no radio module concern as for CSMP. LSMP can be used all the way down to the battery’s final moments without any issues.