Replacing a battery that keeps going flat can help for a while, but if the cause is the charging system or something drawing power while the car is parked, the problem can come back.
This guide explains the main causes and how they are tested. Any figures below are typical examples for a conventional 12 volt lead-acid battery. Battery type (flooded, AGM, EFB or lithium) and the vehicle change them, so check the vehicle manual and the battery manufacturer’s information.
Cause one: the battery
As a typical example, a fully charged conventional lead-acid battery that has rested for several hours with nothing switched on reads around 12.6 volts or a little higher, and a lower resting voltage suggests a lower state of charge. AGM, EFB and lithium batteries have different figures, and a reading taken soon after driving or charging can be misleading.
Resting voltage alone does not show whether a battery is healthy. A battery can show a reasonable voltage and still struggle under load. Load or conductance testing gives more information, but the result depends on the state of charge, temperature and test conditions, and a low reading can also come from other faults such as a poor connection or a starter motor problem. One reading on its own is not enough to condemn a battery.
Battery life varies with the battery type, the climate and how the vehicle is used. Short trips and long periods parked can mean the battery is not fully recharged between starts.
Cause two: the charging system
On a vehicle with a conventional charging system, the voltage at the battery with the engine running is typically in the high 13s to mid 14s. Many newer vehicles have variable-voltage (smart) alternators that deliberately change the charging voltage depending on the battery’s state, the electrical load and driving conditions, so a lower or changing reading can be normal on those vehicles.
For that reason a single voltage reading is not a reliable pass or fail test. Charging system testing looks at what the vehicle is designed to do, including any battery monitoring sensor, and can include checks such as alternator output under load and AC ripple from a failing diode.
Cause three: something drawing power while parked
Vehicles draw a small amount of current when parked to keep the alarm, keyless entry, clock and various modules alive. After the vehicle is locked, modules go to sleep over a period that varies between vehicles, and the acceptable draw also varies by vehicle.
When the draw is higher than it should be, something is staying awake or has a fault. Possible causes include aftermarket accessories such as dash cams, alarms, trackers or amplifiers wired to a constant supply, a light that stays on, or a module that does not go to sleep.
Parasitic-drain testing needs a vehicle-specific procedure. Some modules take time to go to sleep, and disconnecting the battery or opening a circuit can reset systems or wake modules up, which can hide the fault or cause other problems. If you suspect a drain, it is worth having it tested.
Connections and earths
Corroded battery terminals, a loose clamp or a poor earth strap add resistance. Symptoms can include slow cranking, dim lights, warning lights and a charging voltage that looks fine at the alternator but not at the battery. These are checked with voltage-drop testing under load as part of the diagnosis.
Having the battery and charging system tested
Wired Right tests the battery, charging system and parked current draw to work out why a battery keeps going flat. Diagnosis is charged by the hour. During the first hour you are told the findings, what has occurred and what has been identified. If the fault has not been found within that hour, you are told the progress and can choose to continue diagnosing at the hourly rate, or get a quote for the recommended repair once the fault has been identified.
Based in Albion Park Rail, by appointment only.