If your car's instrument panel includes an ammeter, it will tell you how well the charging system is working - the difference between the charge going into the battery and the power being used from it.
Alternators are powerful enough to cope with the demand for current made by
a modern car's electrics, but dynamos often are not. So if your car has a
dynamo, and its output is inadequate, it makes sense to swap a dynamo for an
When you have tested the charging system of a car fitted with an alternator, and the checks in How to test a car battery point to a fault in the voltage regulator, make sure that you need to replace it. The fault may be elsewhere.