The Tempest device uses an integrated solar power charge module with four south-facing (north-facing in the southern hemisphere), vertically-oriented solar panels. This geometry provides optimal solar charging even in low sun angles at high latitude locations.
Internal Rechargeable Battery
The Tempest device contains a 1300mAh internal rechargeable battery that uses the LTO (Lithium-titanate) chemistry, ideal for use outdoors and in extreme temperatures. LTO batteries are safe on the environment and have a very long lifetime of almost 50,000 cycles - which is decades of recharging.
Your Tempest is shipped with a full battery and should give you about 2 weeks of operation without any additional power input, but you should put it in the sun as soon as possible. Note, it must be powered on in order for battery charging to occur.
From an empty state, it takes about 4 hours of adequate sunlight (350 W/m2 or so) on one or more of the four solar panels to reach a full charge. This is a rule-of-thumb and your mileage may vary. It will charge faster in direct sun but can still charge slowly with indirect sunlight. As long as your Tempest gets the equivalent of at least 4 hours of adequate sunlight every two weeks, it will continue happily running along. In the event of extended periods of low light conditions, power management code in the firmware will help your Tempest operate for as long as possible.
The LTO battery will continue normal charging up to about 113°F (45°C) and down to about -40°F (-40°C). And the battery will continue providing power well outside this temperature range for as long as it has a charge - usually long enough to return to a range where the battery can resume charging again.
Battery Level Indicator
You can check the battery level for your device by viewing the status page for your station: In the app, go to Settings > Stations > (choose a station) > Status
Battery Card Display
You can display a device's battery voltage in the app. To enable a battery card indicator on the data display, go to Settings > Stations > (choose a station) > Advanced > toggle Enable Battery Card
If you're watching the voltage level (because you are a true weather geek!), it may appear the Tempest is not charging even when the sun is out. That's because there is a "start-charge" threshold that must be reached before charging begins. This prevents the battery from going through a multiple charge cycle "yo-yo" during the day. Don't worry, that's normal. Every battery is a little different, but once it starts charging it will continue until the sun is no longer shining or it gets to a maximum of around 2.7 or 2.8 volts. From there, with no further input, it will decrease relatively quickly (over a couple hours) to around 2.6v, and then very slowly from there until the cycle starts again when the sun comes back out. This is perfectly normal too!