My battery charge system light came on wanna know if it's a bad battery or alternator
My car has 196000 miles.
My car has an automatic transmission.
There are two key tasks in a charging system diagnostic initially: one is to measure the output of the alternator, at the battery terminals, and compare that output against the factory specs; and the second is to load test the battery (battery has to be fully charged to load test) to see what the condition of the battery is.
Even if the battery "passes" the load test, and particularly if the pass is marginal, if the battery is more than 4 years old, it might be recommended for replacement. Obviously, if the battery fails the load test, it is replaced as that could account for the problem you are having. It is also possible that the battery and alternator are good but you have, for example, a lot of corrosion on the battery terminals causing high resistance and poor re-charging of the battery.
I recommend having a certified mechanic, such as one from YourMechanic, come to your location to perform an inspection to determine why the battery will not hold a charge and suggest the necessary repairs.
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