Originally Posted by
canebreaker
Turn the trolling motor off and see what voltage reads on the Garmin. Most likely it will read higher.
Here's somethings to check:
When the battery is finished charging turn the Garmin on to get a new voltage reading. When you get to the launch turn the Garmin on and get a reading. The readings should be the same. As batteries age they can lose some volts but it won't be much, like 12.4 at home and 12.2 at the ramp. If the battery is losing more than .4 volts during an hour trip then there is a short somewhere that's draining the battery.
I have 2 trolling batteries on my boat, 2 years difference in age. The new one would read the same voltage at home and the launch. The older would lose voltage. The only difference in the wiring was they went through a sector switch. I search the boat over 3 times for a short. 3 months later I was talking to an old coworker. Is the older battery damp/wet? Yep, soaped and brushed, rinsed that battery and the voltage lose stopped.