Can you use a 5v adapter for 6V

No, a 5v adapter is never recommended to power a 6v device. Because we need a voltage higher than that of the battery before it will charge properly. As a device is required to run from the voltage rating it is given, hence if we use a 5v adapter for a 6v rated device, the device won't start at all.

Can I use 12V adapter for 6V

This will run your battery dry and create a thermal runaway scenario, which is very dangerous. Again, some people use a 12V charger to get their 6V battery jump started, but it is never advisable to use a 12V charger to try to fully charge your 6V battery.

Is 6V too much for 5v

yes, 6V is enough to trigger a circuit inside the PI that will short the 5V input, in order to save the 5V logic, which will otherwise be damaged. the shorting will then blow the PI's fuse!

Can I use 5v for 4.5 V

I've run many 4.5v devices off a 5v USB power supply without problem. Most devices can tolerate it fine, because a brand new AA battery can deliver in excess of 1.6 volts, so the device is designed with some leeway.

Can I run a 6V motor on 5V

At some point the supply voltage could be too low for the motor to start up, but powering a 6V motor with 5V should not be a problem. If you want to increase the speed and torque of your motor then you could do that using an appropriate step-up voltage regulator.

What happens if I use a 5V adapter on a 12v device

That said, I'd suggest against using a 5 volt supply for a 12 volt device. For a start, if it really requires 2 volts (or close to it), you will have a problem because 12 x 2 = 24 watts, while 5 x 2 = 10 watts, so the power supply is under-specced to do the job at maximum load.

What happens if I use a 5V adapter on a 12V device

That said, I'd suggest against using a 5 volt supply for a 12 volt device. For a start, if it really requires 2 volts (or close to it), you will have a problem because 12 x 2 = 24 watts, while 5 x 2 = 10 watts, so the power supply is under-specced to do the job at maximum load.

What happens if I use a 6V adapter on a 5V device

Can I use a 6V adapter to power a 5V device If you use 6v adapter for a 5v device than it may damage. I f that device is sensitive then it will damage soon ange if the device is not so sensitive even though damages but it may take time. You can drop the voltage from 6volts to the 5volts.

Can I use 5V adapter for 12V

Most likely not. But as always it depends on what product you are trying to power up with the 5V instead of a 12V source. A fan may work but slow, a heater will work but at 17% power output, a peltier cooler/heater will work but will not remove as much heat.

Can I use a 5V adapter to power a 12V device

That said, I'd suggest against using a 5 volt supply for a 12 volt device. For a start, if it really requires 2 volts (or close to it), you will have a problem because 12 x 2 = 24 watts, while 5 x 2 = 10 watts, so the power supply is under-specced to do the job at maximum load.

Will 6V LED work on 5V

Your lamp will run slightly dimmer at 5V (compared to 6V), but it should be fine.

Can I use 5V adapter for 12V device

That said, I'd suggest against using a 5 volt supply for a 12 volt device. For a start, if it really requires 2 volts (or close to it), you will have a problem because 12 x 2 = 24 watts, while 5 x 2 = 10 watts, so the power supply is under-specced to do the job at maximum load.

Can I use 5V charger to charge 12V battery

No way it'll do 12V or any other chemistry. To charge any battery, you need to give it a voltage higher than what it's going to charge to.

Can you run a 6V motor on 5V

At some point the supply voltage could be too low for the motor to start up, but powering a 6V motor with 5V should not be a problem. If you want to increase the speed and torque of your motor then you could do that using an appropriate step-up voltage regulator.