I'm using a friend's car battery charger on my 2016 Mazda 3, and the amperes meter alternates between max (8) and zero every 5 or 6 seconds. Normally it goes to about half way (3ish) and slowly decreases to zero. I disconnected the leads on the Mazda's battery from the car and the same thing happens. Why is this happening?
A dead (but not necessarily defective) battery will draw a lot of current from a charger. The charger may not be able to supply that current for very long (a few seconds) before it will go into a self-protection mode (will stop charging the load by some mechanism). The charger will then "recover" and once again start charging. The visible result of this is the ammeter going from full-scale to zero over and over. Normally, once the battery recovers some internal charge, the charger will settle down.