I want to power some accessories in my car using illuminated switches on the dashboard. These switches will be wired to the dash illumination circuit, which is normally 12 V but can drop as low as 6 V when fully dimmed. Can I use a 6 V relay here to give 12 V power to the accessories? According to this datasheet, its max. switching voltage is 16 V, so I think this should be safe.
The coil and contacts of a relay are electrically isolated from each other. Powering the coil with mechanically actuate the contacts. The voltages can differ between the two (e.g. 24VDC coil with 230VAC contacts).
The coil requires a specific voltage to turn the relay on and off. This voltage is selected on page 4 of the document you've provided. Looks like your options are 6, 9, 10, and 12 VDC. EDIT: It would be in the best interest that this coil voltage is not fluctuating, and instead, a constant voltage source. Powering a 6 VDC coil from a 12 VDC source will cause the coil to become quite hot.
The contacts are what will be providing power to your accessories. They have a max rating of 16VDC, 35A (page 3). So, 6 to 12 VDC on your contacts will not be a problem, so long as you have a constant voltage on your coil.
According to the datasheet, there are two series of relays FBR51 and FBR52. Both are available in 6, 9, 10 and 12 volt versions.
For the FBR51 series, the coils are rated at 0.6 watts
6V^2/60Ω = 0.6W
12V^2/240Ω = 0.6W
For the FBR2 series it's 0.8 watts
6V^2/45Ω = 0.8W
12V^2/180Ω = 0.8W
See page 4 for the voltage and coil resistances.
What happens if you run the 6V version at 12V?
FBR51 series: 12V^2/60Ω = 2.4W
FBR52 series: 12V^2/45Ω = 3.2W
Since the wattage is related to the square of the voltage you see that doubling the voltage quadruples the wattage.
In layman's terms, they're probably gonna go up in smoke.
The relay DC coil voltage rating on most relays is continious .Also the relay manufacturers have made allowances for practical lead Acid battery systems .For example if you put a relay with a coil nameplate rating of 12 VDC would be fine in a car that is driving along with a charging voltage of say 14 VDC .Normal relay coils have lots of turns of fine copper wire with significant thermal mass unlike certain fragile semiconductors .This means that they are reasonably idiot proof .If you overload a 6V relay with 12V it will take minutes to cause thermal problems . Long before the enamel insulation on the copper wire flakes out the plastic housing will distort which mucks up contact alignment causing fail .So in summary you can overvolt the coil for seconds without punishment. In other words a 24V relay is not going to explode if pulses of 48 V are applied .This means that you could use 6V relays in your 12V system with some RC scheme or some PWM scheme that allows tempory 12VDC but enforces steady state 6VDC.