How to check the battery and alternator with the app
Three voltage readings answer the question of who is to blame: the battery, the alternator or the wiring.
Let the car stand
An hour at least, a night is better. Straight after a drive the battery shows a surface charge and looks better than it is.
First reading: ignition on, engine off
A healthy battery shows about 12.4–12.7 V. Below 12.2 it is discharged; below 12.0 it is deeply discharged and may not survive it.
Second reading: while cranking
Watch how far the voltage sags while the starter turns. Below 9–10 V the battery is not coping, even if the resting figure looked decent.
Third reading: engine running
Usually 13.8–14.6 V. Lower means the alternator is not charging, or the wiring and earth are to blame. Above 15 V is the voltage regulator, and that is dangerous: overcharging kills a battery in weeks.
Load the electrical system
Switch on the headlights, the heated screen and the blower at full. The voltage will sag but should stay above 13 V. If it drops below, the alternator is short of capacity, and in winter that becomes a morning no-start.
If doubts remain, look for a drain
If the battery goes flat over a couple of days with normal readings, something is draining it. The first suspect is a cheap adapter left in the socket: it can draw current around the clock.