I believe you are fine as is. The specs (as stated by Humminbird) is 10 to 20 volts dc. 18 gage wire can easily handle the current required to operate the 997. I have 797, but they both have same power specifications. Be sure not to run it on the TM battery.
Internally, the units microprocessor reads the dc voltages via a a/d converter and does voltage conversions, similiar to your digital volt/ohm meter. It is probably not as accurate as your VOM, as it is just a ball park reading and not system critical. I am guessing at all this, but I'll put money on it that I am guessing good here. ha