2S is not required. I use a 1S, 25C, 325mAh Li-Po, with a JST connector.
This topic has been discussed may times and as others posted, if you search the form you may find some threads recommending a 2S, however you may also find some threads that provide the math to show that a 1S is an option as well, and a better choice with this altimeter. I am not overly knowledgeable on this topic, however I trust the person that provided me the advice based on math, not simply an opinion, and I consider his as close as a subject mater specialist on this topic as it applies to rocketry and electronics.
I was not fully on-board with the advice when I was given it either, and I know not many, if any, of the people I repeated the advice to opted for the 1S, in fact I am sure that most decided to ignore it and go with the 2S. I get this because a 1S is non-intuitive being 3.7 volts, and the altimeter's voltage requirements are 4-16 volts, however a 1S Li-Po fully charged is 4.2 volts. I also get that .2 volts does not seem like a lot of margin. Keep in mind that Li-Po chemistry is very different from traditional alkaline battery chemistry, so things like voltage drop and internal resistance are also very different.
However I decided that part of this hobby for me was to be open to advice. Based on this I did some extensive bench testing with four e matches per test (2 per channel), multiple back to back tests and I was very surprised at the results. I cannot recall the number of tests it was a total of 4 or 5, but below is what I recall without any exaggeration to make the point and I would encourage anyone to perform the same tests.
- charged 1S 25C 325 mAh Li-Po to 100 charged, 4.2 volts
- connected two e matches per drogue channel and two e matches on main channel
- connected the battery to the altimeter via the featherweight magnetic switch, which also consumes some power although it is minor
- connected the DT4U to the altimeter, laptop and started up DataCap and fired both channels one after the other with only the time required to click the options between firings
- powered down the altimeter and repeated the test one or two more times, after the 3rd or 4th test I checked the voltage of the battery and it was 4.0-4.1 volts
- left the batter connected to the magnetic switch, but powered off, over night and performed the test for a 4th or 5th test, measured the voltage and it was 4ish volts.
I left the altimeter overnight connected to the magnetic switch because I had concerns with how much current it would deplete the battery while still prepping the rocket, from getting to the launch, getting the rocket prepped, altimeters off and waiting for my launch window.
I do not recall the exact voltages or number of tests, the point of the bench testing was to load up the battery with an unrealistic simulation in order to see how it responded with recovering as well as netted out with voltage. In my opinion my tests validated the configuration and I am pretty sure I could have repeated the test at least one more time before it dropped below 4 volts, but for my usage I was already running a scenario that would load the battery much more than any practical application.
From a practical real-world experience, I have prepped the rocket, left the rocket sitting on the pad for 45 min waiting for launch, launched, retrieved the rocket an hour later, left the altimeter beeping during the drive back to the prep-table, at the prep-table turned off the altimeter, connected the DT4U turned on the altimeter, pulled the data and the voltage on the battery was 4.1 volts. I have repeated this same scenario possibly a dozen ish times now over three flight seasons, with the same batteries.
So two sets of data to draw conclusions from, one a worst case no-realistic bench testing scenario, then a real application. Based on this regardless of the differing opinions on the topic, I can assure you that a 1S is plenty of battery for this altimeter as well as its older brother the SL100.