Bro... its normal thing ... there is not any issue regarding battery or dropped voltage.... its a usual thing....
The reason why AC button, defogger button, lcd dims ... whenever you turn on your lights even parking lights it dims automatically, because in night we turn on the lights and there is dark outside and inside cabin, you can see the on lights easily... but in day light we have no need of lights so it didnt dim...
To check the dimming issue .. in Day time turn on your lights these lights will also dim at that time....