If you want to check the battery voltage at rest, so no engine running and everything turned off, you should do it only when the vehicle has not been used for at least a couple of hours but ideally overnight. If you measure the voltage soon after the ignition is turned off then it will show a reading that can be higher than expected.
A perfectly good car battery should be at 12.6 volts or above at rest. The air temperature is important as the battery voltage will decrease as it gets colder so bear that in mind when measuring the voltage in the middle of winter.
With the engine running at a fast idle or above the voltage should increase to between 13.5 and 14.7 volts.
Unless you are using a calibrated instrument, and using it at its calibrated temperature, I would not expect to get highly accurate results. Most cheap multi-meters can be 0.1 or 0.2 volts out so your reading of 12.84 may actually be 12.64 anyway.
As others have pointed out, at 12.84 volts it shouldn't be causing any problems as the system voltage will more than 1 volt higher once the engine is running. A lot of electrical problems stem from bad connections so a good place to start is to look for those.
Corrosion from water ingress is another common cause of electrical faults. Just don't ask me how I know that...
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