Those testers are good if you want to know the voltage.
Most chemistries have a linear decline in v when the cell/battery is used or charged.
This is no more than a handy onboard v meter to tell you a ruff state of charge(% full)
A tester is more like a testing board like a zb2l3.
A zb2l3 is a testing board to tell you how much of life is remaining in a cell after a full charge/discharge cycle.
By knowing the original (new) mah and knowing the mah what the cell has left after the discharge cycle.
This is the State Of Health.
For the use in a loco you can use 100% aka 2.8v to 4.2v, my sincere advice would be 3v to 4v range, 80%, speaking at cell level.
If having a lipo/li ion pack in 4s and its almost empty you have 12v it could be mistaking for a lead acid at 40-50% full.
Iwould say just keep the charge up after every run to the 4v per cell, and please keep the temperature of those cells above freezing point.
Or your next run will be very short due to empty battery.
For playing/charging below freezing point just buy (when you must replace) sodium ion cells.
When intending to leave them alone for more than 3 months, let your cells sit at 3.3v-3.5v
With best regards Igor