When I first bought Coconut there were three batteries on board, not connected and hanging out in the shop waiting to be installed. I later learned they were purchased in 2009, so in 2014 they were already old. Because I knew nothing about battery banks, I had an electrician install them. They were 12V led/acid batteries, size D27 and of the Powerstride brand. I thought you only had to water them once a year, but it turns out it’s once a month. Oops. I didn’t learn this until the batteries had been in use for five years and due for replacement anyways. They still were holding a charge just fine during the day with the solar panels, but at night the voltage would drop dramatically. I didn’t want to have to worry about JaLos the autopilot draining the batteries on a night passage. Hell, just being able to leave my cabin lights and radio on after dark would be nice!
Plenty of people told me to look into golf cart batteries. They are only 6 volt but have more amp hours. It sounded great, until I saw how big they were. Because they are 6V you need 2x as many (I think?) than you would if you only went with 12V. I don’t think I could fit even one golf cart battery where the current batteries are, they are too tall. Relocating the battery bank is absolutely not an idea I was interested in entertaining. Another important tidbit I learned was to go with an AGM battery. AGM’s do not require maintenance, so there is need to go on a treasure hunt to find distilled water as most countries outside the US do not carry it. Also, the battery for the engine bank absolutely must be rated a “starting battery” (or at least dual purpose). Mine previously was not.
I went ahead and splurged on Powerstride AGM’s, and got a Lifeline Starting Battery. I made space for two more batteries, which was the perfect amount! I just had to have an electrician come to make new battery connectors to run the now four house bank batteries in parallel. The only thing I would need to check was to make sure the solar charge controller was compatible for AGM’s, as well as the battery charger. I honestly never use the battery charger because I am rarely connected to shore power, but it could definitely ruin the batteries if it’s not compatible.
The one chart that has helped me understand how my batteries are doing is below. Looking at the percentage on the battery monitor wasn’t entirely helping me understand what was going on. For example, it would show the batteries were at 85% but be at 12.20 volts, which as you can see is actually 60%. I am happy to say the voltage so far hasn’t gone below 12.5, and that was when I was charging my laptop after dark (whaaaat!). I think four batteries will be plenty sufficient for my minimal usage, the only appliance I am missing is the fridge/freezer unit. Once the area gets re-insulated, I am curious to see how much power it draws.