Millions of cars across the UK could be at risk of serious issues, prompting experts to issue urgent warnings for motorists.
New data has found that 8.6 million cars on the road could be running low on power as a result of a weakened 12-volt battery.
Analysis from Carly indicates that one in four vehicles is affected, representing 8.62 million of the UK's 34.5 million cars.
Experts warned that drivers could see their vehicles suffer from sudden non-starts, security glitches and unwanted repair bills.
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In the worst-affected cases seen through the research, more than 40 per cent of vehicles were found to have weak 12-volt batteries.
The research, which analysed more than 550,000 vehicles between January 2025 and 2026, found issues from models across most major car brands.
Motorists often see the first sign of an issue being that the car won't start, which can often happen at the most inconvenient time for drivers.
Failing 12-volt batteries remain one of the leading causes of roadside breakdowns, an error that can bring headaches and huge costs for motorists.
Drivers could face huge costs and risk breakdowns if they have issues with their 12-volt batteries
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Low-voltage batteries can affect the reliability of the vehicle, as they can break down more frequently if the issue is firmly established.
It can also "compromise personal safety and disrupt security systems", especially if it leads to a breakdown on a motorway or major A-road.
The 12-volt battery is crucial for the vehicle's electronics, which can impact central locking, alarms and keyless entry.
Emma Brown, UK growth manager of Carly, highlighted the dangers associated with the degradation of batteries.
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Motorists could be more at risk of breaking down if they have an issue with their 12-volt battery
| PAShe said issues with the battery can make the vehicle more vulnerable and can increase the likelihood of further issues being present further down the line.
The expert continued, saying: "Because battery degradation develops gradually, it’s easy for drivers to miss the warning signs until the vehicle suddenly fails to start.
"In modern cars, a weak 12-volt battery can affect far more than just starting the engine, which is why early diagnostics and better visibility of battery health are so important.
"Identifying issues sooner can help drivers avoid more disruptive failures, breakdowns and the knock-on repair costs that often follow."
Motorists could be slapped with repair bills worth hundreds of pounds if their car has battery issues
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Carly's Repair and Maintenance Cost Estimator showed that drivers will face average costs of around £150 and £330 for a replacement 12-volt battery, including fitting.
However, drivers can face much higher costs if the "battery issue" is actually linked to the charging or starting system.
It stated that drivers can expect costs of around £330 for starter motor repairs, or prices as high as £800 for an alternator repair.