In order to see if it's the battery or the
mod or the charger, do you have another battery to try in your zmax, or another APV to try with the battery?
I'd suggest the battery is not charging rather than a fault with the zmax. Mine says the same when it's time to charge the battery and I've had no issues with it.
Do you have a way of testing the voltage of the fully charged battery? maybe another APV or a multimeter?
Finally, mine doesn't fire when I put the battery in with the wrong polarity, but I know some
mods do show a low voltage fault when they are installed incorrectly - the nipple end should go in first (not trying to be a smart-... here, I've done it myself)