Battery: Help me monitor battery life of solar powered Lithium ION temp project

I'm new to this, a novice. I conquered phase-1 of my project goal. I have a temperature sensor buried in my compost pile giving me great data 24/7. The adafruit load-sharing board is keeping the battery charged nicely and the battery continues to run my project now for many days. I presume it will last for a couple years or more. However, I want to not only see the temp every 15 minutes, but also the battery condition, so I can know, without having to walk out and test it. I'm putting the circuit to sleep for 15 minute periods, then reporting data back, and back to sleep again. I'm concerned that having another circuit connected to this same battery might cause the battery to drain fully, or at an unacceptably faster rate. I'm wondering if I should use a relay to manage this in some way. I've been reading about this and considering it off and on for several weeks, when I don't have other chores or work. I thought I'd post here... hoping to gain some wisdom from you all. Thank you.