Auto Express has highlighted a variation in battery replacement charges within franchised dealer networks.
The publication mystery shopped three dealerships for each car brand to ask for quotes to supply and fit replacement batteries for popular cars.
It said some dealerships quoted three times the price for the same job as the lowest quote from the franchise.
One Mercedes-Benz dealer quoted £1,248 to complete the job on a Mercedes C-Class, compared with the lowest quote of £315.
News editor Jonathan Burn, who led the mystery shop, said: “The problem for motorists is that the retail price of car parts is unregulated and therefore there is little transparency or consistency in parts pricing, even within the same dealer networks.
“When we challenged Mercedes on the staggering charge of £1,248 quoted by one of their dealers, it told us it could only recommend a price, which means there can be variation between dealers.”
Auto Express highlighted that the three Toyota dealerships contacted all quoted the same, £105, to replace the battery on a Yaris.
Vauxhall results were mixed, with a variance of just £1.60 between the highest and lowest quotes for an Astra battery, but a gap of £164.34 between quotes for a Mokka X.
The publication has urged motorists to get several quotes and to consider independents such as Halfords.