I have looked at a bunch of threads about this. Was hoping somebody could chime in. I have a battery failsafe triggering at voltages ranging from 12v to 16v on my FC. This started happening after I did a guided parameter setting.
I suppose I just need to set the failsafe levels manually to whatever voltage I need. Would that be BATT_LOW_VOLT and BATT_LOW_MAH?
You need to set
BATT_LOW_VOLT
BATT_LOW_MAH
You also need to set the action you want it to take when it failsafes.
BATT_FS_LOW_ACT
BATT_FS_CRT_ACT
You need to set the arming voltage and current.
BATT_ARM_VOLT
BATT_ARM_MAH
Also this one is important. It determines if the battery voltage sag is taken into account for the failsafe
BATT_FS_VOLTSRC
If you used the MissionPlanner Alt A plugin for the battery settings then voltage settings should be correct unless you picked the wrong chemistry type → Lipo/Lion/LiHV. I wouldnt advise changing those calculated values unless you’ve got a special battery with well known limits.
The biggest problem is the voltage sensor is wrong, more so as the battery voltage drops, and sometimes the battery packs are not as good as you expect.
If you have an analogue current/voltage sensor, a typical power brick, then calibrate the voltage manually using a good multimeter and the lowest voltage that you would ever experience. You’ll need either a stable power supply or another battery that has less cells or is already faulty.
For example if using a 4cell flight battery, calibrate the voltage sensor with a low 3cell pack.
The highest voltage is not as important as the lowest voltage.
https://ardupilot.org/copter/docs/common-power-module-configuration-in-mission-planner.html#calibration
If your battery pack has a relatively low C rating then set BATT_FS_VOLTSRC = 1 as Ricky points out. The current sensing has to be quite accurate for this to work properly though.
You can also play with BATT_LOW_TIMER but I tend to shorten it to 5 instead of the default 10 seconds. Better to know about a low battery sooner rather than later!
I find the mAh settings are OK if you always start with a fully charged battery, but if you have to reboot the FC (or disconnect the battery) then the mAh count is lost.
Ideally we’d all have smart batteries with a CAN connection reporting their actual usage and capacity.
Otherwise set all mAh settings to 0 to disable them, work from voltage level only and don’t push the limits - we would never do that 
You can always run the MissionPlanner Alt A plugin again to check what you’ve got set against calculated values.
Once again I highly recommend using the values from the plug in, and if you get unexpected failsafes then something else is wrong.
Let us know if you need help with current calibration too. It is handy to have.