Ever heard your sailboat batteries start to boil? Well I did! I have two 26 Group Batteries on my boat. Both were hooked up to what my friend likes to call a ‘Battery Maintainer’. It is a solar panel that has a slight trickle recharge to my batteries throughout the day. I have been living aboard the boat on and off the last month and have now unintentionally drained my batteries. I found this out after trying to start my engine with no success. Apparently you are supposed to have one battery for your engine and then your house batteries separate but this was not how my boat was set up. So the ‘battery maintainer’ wasn’t charging my batteries well enough. Even though I was only using it for my cabin lights (cabin lights aren’t LEDs yet).
So after realizing that they weren’t charging I went and bought a battery charger or battery converter. Something I didn’t realize or understand is that even though you are plugged into the dock your dock power comes in 110 volts and your batteries are 12 volts. You battery cant use that energy unless you get a battery converter. Battery converter is needed if you want to make 110v into 12v. Not to be confused with a battery inverter which does the opposite. It turns 12v into 110v. All this has come from lots of reading and asking questions. 
After buying the battery converter I then wired it to both my batteries and plugged it into my 110 outlet which is where I would get the 110v to charge my batteries. It lasted about a 20 minutes, my cabin lights were incredibly bright, and everything was working well except for a boiling sound coming from my batteries. That was my ‘oh sh*t’ moment. Obviously boiling batteries are not ideal and even my untrained mind knew that was a bad sign.
I unplugged the charger and the cabin lights worked for a small period of time and then went out. The power wasn’t sticking! Now I take a second glance at my batteries… They were from Costco and were car batteries…Ugh so next step for me in this process was new MARINE batteries. I bought two new batteries from Fisheries Supply in Ballard, rewired the batteries to the charger and turned the converter on. Success! To be honest it was worth it to just have new batteries anyways I don’t know the last time they were replaced and now I know they are good quality. I have learned a lot from the experience and hope if anyone else has similar problems this will help!
dreamsrus.me 