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Your questions are all very subjective to a variety of operating conditions, so I can only give you some broad estimates of what we’ve experienced with the 300+ stations we’ve installed and maintained.
We typically put a 4400mah Lipo battery on the Mayfly boards because on standard logger configuration, they can be charged fully by a 3.5w or 5w panel in just a few hours of direct sun. It gets more complicated if you’re a shady location or a higher latitude, or by time of year.
The overall power draw of a CTD-only station with LTE cellular module will mostly be the affected by the time it take the cellular module to connect to the network and send the data. The CTD uses only a few milliamps of power for a couple seconds, and the Mayfly board sleeps when not taking a sample, so if you’re taking readings every 5 minutes, you could operate for several months without a solar panel if you weren’t transmitting via LTE. At 15 minute intervals, I would guess you could get 3 or 4 months of lifespan, especially if you upgraded to a 6600ma battery. With the new EnviroDIY LTE cellular module, we typically get at least 3-4 weeks of operation without a solar panel at 5 minute sample rates before the battery starts to get low. However, cellular signal strength and network availability at the monitoring location determines how long the cell transmitted is on during each transmission. I’ve seen some stations make their report in 5 seconds or less. Other will stay on nearly a minute each time before completing the transmission. (The old Digi brand LTE boards used 4 times more power during operation and took close to a minute for each transmission, so those boards really killed our battery life. We’ve since replaced them all with our own sim7080 EnviroDIY LTEbee which are much more efficient.)
So stations with long transmission times and shady locations will struggle with battery life sometimes, whereas stations in strong signal areas and sun will last almost indefinitely. We have some stations that were installed in 2016 that are still using the original Lipo battery pack. The lifespan of the Lipo pack seems to depend more on temperature extremes and moisture. The blue 4400mah packs we like to use (because they have the compatible JST-connector polarity) have open ends that allow moisture to get into the insides of the pack and cause corrosion/rust on the metal tabs that connect the two cells. When one of those tabs rusts through, the pack becomes unstable and soon dies. We’ve had packs last many years in dry conditions, but if a logger enclosure leaks or has high humidity inside that results in frequent condensation on the battery pack (usually at night when temps drop) then those packs will only last a year or two. Keeping a desiccant pack in the logger enclosure and making sure there are no leaks will greatly extend the life of the battery.
The default charge rate of the Mayfly v1.1revB is 500 mA. The 5w ETFE panel from Voltaic Systems can generate about 900ma in full sun, but unless you change the default charge rate, the Mayfly will be limited to 500ma of input current. You can adjust this by changing solder jumper SJ15 (and SJ14) as explained on the Mayfly Jumper Settings page, however see the note on that page that says using a charge rate above 500ma will result in very high temperatures of the charging ic chip on the Mayfly, which could damage or shorten its lifespan.