LED Meters

Does this mean that I need to buy 2 more 12V batteries just to fire up the LEDs? Why? Help!

Fortunately no.

The main reason is quite simply that analog (moving coil) meters use the signal they are measuring to display a representation of the value over the whole scale (eg- say 0-20V). LED meters do the same thing but electronically, and the LED power supply has a limited voltage tolerance that it can work on (for example 10-14V DC).

The reason the manufacturer suggests a separate power supply is so that the voltage powering the LED electronics is maintained within the tolerance range of the LED meter, too high and the LED meter could be damaged, too low and the LED meter won't work properly.

The easiest way to check if the LED meter will work is to check the voltage tolerance in the manual, if it is between say 10V to say 15V DC you should be ok as this is the common voltage range you would expect to see on a 12V battery (although motor starting may potentially cause this to drop momentarily below 10V in some cases).

If the voltage is outside this range (or you are unsure and want to follow the manufacturer's specifications) the easiest way is to add a separate 12V DC to DC converter that can take the wide voltage range swings of your 12V system and provide a constant 12V out for powering the LED meter electronics. It doesn't need to have a a high current output (ie- be big and expensive) as the LED meter electronics only require mA of current (see LED manual for current and connection details). You should be able to find suitable DC to DC converters at your local boating shop or electronic stores.