I question the choice of start versus deep cycle batteries and maybe the charge rate. My thoughts:
I agree with the idea of a start battery for the bow thruster. A bow thruster is generally used for seconds with high amperage draw (like an engine starter). If you go to the trouble to calculate the momentary usage of the thuster in watts, you will find your refrigerator uses more watts per hour.
Now if you think about the amperage draw of an anchor windlass, possibly running for minutes, that's deep cycle duration consumption (all be it at much lower amperage). While it may not be practical to run the windlass off the house deep cycle bank or the engine battery (assuming the engine alternator can handle the windlass load), those would be better choices than the start battery you are using.
If I had to guess, the windlass batteries may have reached the end of their life or lost a cell. The charger kept charging at full output (never was able to bring the voltage up to reducie charging rate) until the fuse popped.
I'm tired of fast moves, I've got a slow groove, on my mind.....
I want to spend some time, Not come and go in a heated rush.....
by The Pointer Sisters