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You should obtain an inexpensive digital multimeter (about $10) and follow this info I pasted some time ago.
The following checks may be helpful since you had or may still have dimming lights (including the warning lights). The battery voltage with no load (ignition and everything else off) should be 12.6 V - if at all lower, the battery may have a bad cell or an internal short.
With a load such as headlights only turned on (ignition off) the battery voltage should drop very little, e.g. to 12.2 to 12.4 V, unless the lights are left on for a half hour or more. If drop is more than this then either the battery was not fully charged at the start of the test or is becoming weak or is not holding its charge.
With the engine running there should be an overvoltage, around 13.8 to 14.2 volts (measured across the battery terminals) depending on ambient temperature (higher voltage at lower temperature), engine speed, and what electrical loads are operating. A dead charging system will not produce an overvoltage (you might read only 11.5 volts or even less) and a weak one will drop to smaller and smaller voltage, say 13 or 13.2, as various additional loads (lights, wipers, defroster) are turned on. If the charging system appears weak, it is important to check that all connections, particularly grounds, are intact and not corroded. Sometimes due to a bad connection the charging voltage will appear ok at the battery but will be low at some other point. Then the accessory connected at that point might not work well. Sometimes on fuel injected cars the engine will not run well and/or you will get a 'check engine' light due to a poor connection. If with the car running you get 14.8 or 15V at the battery then the system is overcharging and you may have a bad voltage regulator. Various lights on the car may burn out prematurely.
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