This was a critical decision both from technical point of view as well as from the user experience point of view. Initial versions of our dipsticks had 3 AA batteries that are universally available. Pros being easily available and low cost. But there were three challenges with it:
1. Waterproofing the internal components when the battery chamber has to be easily accessible for the user
2. Battery life was lower compared to better options like Lithium polymer batteries. In fact, the nature of these batteries is such that there is a slow but constant voltage drop. Now because our microcontroller works only in a specified voltage range(3.6v), this meant that the batteries would be rendered unusable even before they were completely drained.
3. Three AA batteries made the dipstick bulkier, making it occupy more real estate where precious plants could grow.
Lithium polymer (LiPo) batteries, on the other hand, are the ones used in mobile phones. They are small and rechargeable. Plus their nature is that there is a voltage drop only towards the fag end of their lives, so it means the microcontrollers could function till every bit of their charge was used up. But the challenge here was that they required a separate circuit for charging them and managing them. That also meant increasing costs.
But in the end we decided to go ahead in favour of reliability and better user experience with the Lithium Polymer batteries. Now your dipstick is rechargeable and by our estimates can easily last a month per charge 🙂