Garmin GPSMAP 178 (greyscale) expected battery life?

[technoyak]

You read it folks . . . I am curious to know what should I expect from my recently installed GPSMAP 178 in terms of battery life. I am using a 12V that came from a electric scooter. Since it is bigger than my 7ah (one I have used for many things over the years) I suppose it is an 8ah. Here's the kicker though . . . I am also running LED nav lights or to be exact, a lighted (LED) switch to turn the nav lights on and off. I went fishing yesterday and didn't use the lights at all during daytime (about 4-5 hours in the morning, NO LIGHTS, just the LED switch). Then I went out again in the afternoon and lo and behold . . . my sounder stopped working less than ten minutes after hitting the water. Water temperature was reading, just no sounder. GPS was fine too. I though it was that the (Gladware) water pool for the transducer may have developed a leak, but no, it was holding fine. I suspect either the battery may be faulty, the transducer takes more energy than anticipated (200/50 model), the GPS may be taking more energy (not likely) due to my "soon to be patented" (LOL) wire in the BNC connector antenna, the LED switch may be taking more than expected. The voltage read 8 point something volts on the GPS last I saw it, yet when I checked with the meter, it read 12V solid. One more thing I would like to know (don't worry this is simple). Is the sounder the first portion to die due to low voltage?

Any help?