Not real-world useful if connected to smartphone. Have it connect via bluetooth directly to bluetooth capable headphones. Automatically begin audio readouts when descending below 100 feet. "100 - 80 - 60 - 50 - 40 - 30 - 20 - 15 - 10 - 9 - 8 - 7 - 6 - 5 - 4 - 3 - 2 - 1."
A pilot would not turn on a smartphone app, connect to the altimeter, watch it during landing, etc. Especially during a hairy landing, which is when it would be needed.
Use a good high capacity USB rechargeable lithium battery, one that's good for many hours. For battery life, include an altimeter chip and maybe an accelerometer in the electronics. Then go low power mode when ascending and holding altitude. Full power when descending only. Turn off when at no altitude change for more that 10 minutes (i.e. on the ground) or no vibration (i.e. engine off). KISS and make it useful, not a novelty that will not be used in practice. If you get the details right, I would buy one of your LIDAR altimeters!