What voltage is too high for a car battery?

A healthy twelve-volt charging system holds roughly thirteen and a half to fourteen and a half volts with the engine running, and some smart charging systems briefly go a little above that during regeneration under braking. Sustained running above about fifteen volts is overcharging, and above sixteen it is doing active damage. Measure it across the battery terminals with the engine at a fast idle and a few electrical loads switched on.

Can overcharging damage my car?

Yes, and that is the reason to treat this code as urgent despite the car driving normally. Excess voltage boils the electrolyte in the battery, which shortens its life dramatically and can make the casing swell or vent. It shortens bulb life, so you find yourself replacing several in quick succession. Most expensively, every electronic module in the car is being fed above its design voltage, and control modules do fail from sustained overcharging.

Will P0563 fail an MOT?

Yes. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times in the DVSA data behind this site. There is a second route to failure with this particular code: a battery that has swollen or is leaking electrolyte because of overcharging is a defect the tester will record, and any bulbs that have blown as a result are separate defects too.

Can a new battery cause P0563?

On a car with a smart charging system, yes. These cars vary their charging voltage according to the battery's type, age and state of charge, all of which are stored in a module. If a new battery is fitted without being registered to the car, or a battery of a different type is used, the charging strategy no longer matches the battery in the car and the voltage can end up too high. Ask the fitter whether the battery was coded, because it is a quick job with the right tool.

Powertrain code · standard

P0563System voltage too high

Standard definition: System Voltage High

Last updated 27 July 2026

Quick answer

The car's electrical system is running above the voltage it should, which usually means the alternator's regulation has failed. It is the less common of the voltage codes and the more damaging one, because overcharging cooks batteries and electronics.

How the car works this out

The engine control unit watches its supply voltage and knows the ceiling: on a conventional twelve-volt system, sustained running much above about fifteen or sixteen volts is not normal charging, it is a regulation failure. P0563 is set when the measured voltage stays above that threshold. Unlike a momentary spike as a heavy load switches off, this is a persistent condition, and the control unit flags it because every electronic module in the car is being fed more than it was designed for.

What you might notice

Headlights unusually bright, sometimes visibly brightening as revs rise
Bulbs failing repeatedly, often several in a short period
A smell of rotten eggs or hot acid from the battery
Battery casing bulging, or the battery needing topping up frequently on older types
Multiple unrelated electrical faults and warning lamps
Battery charge warning lamp on despite the battery being charged

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A failed voltage regulator inside the alternatorCommon
How to check: Measure the voltage across the battery terminals with the engine running at a fast idle. Anything sustained above about fifteen volts confirms it.
2A faulty battery monitoring sensor on the negative terminal, on cars with smart chargingCommon
How to check: Read the battery sensor's live data. A sensor reporting a wrong state of charge makes the control unit command a charging voltage the battery does not need.
3A replacement battery of the wrong type or one that has not been registered to the carPossible
How to check: Ask whether the battery was coded when it was fitted. Cars with smart charging change their strategy depending on the battery type stored in the module.
4A poor earth between the alternator, engine and body, which confuses the regulationPossible
5A fault in the alternator control wiring, where the engine control unit commands the alternator's outputPossible
6A faulty control module misreading the system voltage, which is uncommonLess common

What to do about it

1Do not ignore it. Overcharging is one of the few electrical faults that damages other parts as it goes, and it can boil a battery and take expensive control modules with it.
2Measure the charging voltage at the battery terminals. It is a two-minute check that either confirms or eliminates the fault immediately.
3Have the battery inspected as well as the alternator. A battery that has been overcharged is often ruined even after the charging fault is fixed, and a bulging or hot battery should be replaced.
4Mention any recent battery change. A battery not registered to a car with smart charging is a genuine cause and a straightforward one to put right.
5Check the earth straps, because poor earthing upsets voltage regulation as well as causing low-voltage faults.

Can you keep driving?

Only to a garage

Go straight there. Overcharging can boil the electrolyte out of a battery, which produces hydrogen and, on a sealed battery, pressure. It also stresses every electronic module in the car. This is not a fault that leaves you stranded immediately, which is exactly why people leave it, and the damage accumulates the whole time.

Will it fail an MOT?

An illuminated engine management lamp is a major defect with 371,859 recorded failures behind it. This code brings a second, more direct MOT concern: the tester will record a battery that is insecure or leaking, and an overcharged battery that has swollen its casing or is venting electrolyte is exactly that. If overcharging has caused bulb failures, any lamp that is not working is a defect in its own right.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

What voltage is too high for a car battery?: A healthy twelve-volt charging system holds roughly thirteen and a half to fourteen and a half volts with the engine running, and some smart charging systems briefly go a little above that during regeneration under braking. Sustained running above about fifteen volts is overcharging, and above sixteen it is doing active damage. Measure it across the battery terminals with the engine at a fast idle and a few electrical loads switched on.
Can overcharging damage my car?: Yes, and that is the reason to treat this code as urgent despite the car driving normally. Excess voltage boils the electrolyte in the battery, which shortens its life dramatically and can make the casing swell or vent. It shortens bulb life, so you find yourself replacing several in quick succession. Most expensively, every electronic module in the car is being fed above its design voltage, and control modules do fail from sustained overcharging.
Will P0563 fail an MOT?: Yes. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times in the DVSA data behind this site. There is a second route to failure with this particular code: a battery that has swollen or is leaking electrolyte because of overcharging is a defect the tester will record, and any bulbs that have blown as a result are separate defects too.
Can a new battery cause P0563?: On a car with a smart charging system, yes. These cars vary their charging voltage according to the battery's type, age and state of charge, all of which are stored in a module. If a new battery is fitted without being registered to the car, or a battery of a different type is used, the charging strategy no longer matches the battery in the car and the voltage can end up too high. Ask the fitter whether the battery was coded, because it is a quick job with the right tool.

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