I don't think the ZTS do well with RCR (Li-Ion) cells. For lithium primaries, NiMH, and alkaline cells, it works fine.
The owner of ZTS told me that it was something with the RCR cell chemistry that makes it difficult to obtain accurate readings with these testers (Could be Li-Ion in general).
I usually gauge my RCR cell cell capacity using the resting voltage using a Digital Multi-Meter. While not exact, it gives me an approximate and allows me to ensure that my cells are paired at = or near = resting voltages in multi-cell lights. There is a post somewhere in this forum providing a rough idea on gauging Li-Ion cell capacity based on resting voltage. Again, its a rule of thumb and not an exact science, but enough for my purposes.
BTW, us "weird battery" people learned about batteries because of our lights. I agree with the previous post, neither cool nor clever when you're asking for help . Don't bite the hand that can feed you.