276c Garmin chart plotter

We have a 276c Garmin chart plotter\gps. The battery went dead and I bought another. It will not charge. We updated the firm ware and it still will not charge. Garmin wants $175.00 to fix it. We are going to use it as a backup system. This would be the second time we would have had to have it repaired.
We have updated the firmware but it still does not see the update battery.
Because we are going to put new electronics on the boat it is best for us to just use the cig. Lighter adapter to power it or we can hook it right to the house batteries.
We are going to sell the battery and the old battery charger and just use the house batteries.
At this time we just use it to get back into the harbor when there is lots of fog.