Replacing a battery that keeps going flat is the automotive equivalent of taking painkillers for a broken arm. It helps for a while and the problem comes back.
There are only three things it can be, and each has a number attached to it. Here is how an auto electrician separates them, and what you can check yourself before booking anything in.
Cause one: the battery has actually failed
A healthy 12 volt battery rests between 12.6 and 12.8 volts after sitting for a few hours with nothing switched on. At 12.4 volts it is around 75% charged; at 12.0 volts it is roughly 25% and being damaged every time it sits there.
Resting voltage alone is not enough, though. A battery can read 12.6 volts and still collapse under cranking load because its internal plates have sulphated. That is why a proper test measures voltage while the starter is turning. Below about 9.6 volts under crank on a warm engine indicates a battery at the end of its life.
Typical service life in the Illawarra climate is around three to five years. Short-trip driving shortens it considerably, because the alternator never runs long enough to replace what the starter took out.
Cause two: the charging system is not keeping up
With the engine running, a conventional charging system should show between 13.8 and 14.8 volts at the battery terminals. Below 13.8 and the battery is slowly losing ground every time you drive. Above about 15 volts and it is being cooked.
Modern vehicles with variable-voltage smart alternators complicate this. They deliberately reduce output to around 12.3 to 13.0 volts once the battery is full, to save fuel. That is normal on those vehicles, and it is also why a plain voltage reading can be misleading without knowing what the vehicle is supposed to do.
A less obvious charging fault is high AC ripple, caused by a failed diode in the alternator. The battery gets voltage but never a clean charge, and the symptoms look exactly like a bad battery.
Cause three: something is awake while the car is parked
Every modern vehicle draws a small current when parked to keep the alarm, keyless entry receiver and various modules alive. Once all modules have gone to sleep, which can take up to 45 minutes on some European vehicles, total draw should be under about 50 milliamps.
Above that, something is not sleeping. The usual culprits are aftermarket accessories: dash cams without a low-voltage cut-off, alarms and trackers fitted to a permanent live, amplifiers with a remote wire tied to constant power, and boot or glovebox lights whose switch has failed.
The test is an amp clamp or ammeter on the negative lead, then pulling fuses one at a time and watching for the draw to drop. The fuse that changes the reading identifies the circuit.
The one people forget: earth resistance
A corroded earth strap or a battery terminal that looks clean but has corroded under the clamp adds resistance to every circuit in the vehicle. The symptoms are dim lights, slow cranking, odd warning lights and a charging voltage that looks fine at the alternator but not at the battery.
The measurement is voltage drop: with the circuit under load, the difference between the battery negative post and the engine block should be under 0.2 volts. Anything higher is a fault, and it is one of the cheapest faults there is to fix.