Help a Newbie With Some Battery Problems (+LiFePO)

[Toubab]

Hello everyone!

I have a somewhat selfish request to make... I'm basically a complete newbie to all things electrical and mechanical but I moved on to a sailboat a few months ago so I'm trying my best to learn as quick as possible. My problem right now is I was counting on the dated equipment on board to last me long enough for me to learn what I needed in order to replace and upgrade things but that's not working out. Several systems are having major problems and my 12.5 hour night shifts make doing the research tough and right now my head is spinning trying to figure out what to do with my batteries. I was hoping to throw myself on the mercy of the forums and see if someone could maybe just spell a few things out for me.

First thing... my house battery is shot... done. It's so dead that even plugged in to shore power it couldn't even run a few lights. I jerry-rigged my engine start battery to do double duty and that worked for a little while but now that battery is starting to do the same thing. Plugged in to shore power, I tried to turn on my water pump and everything on board went dark. No tripped circuits or anything, just as if someone pulled the plug. I just bought 2 Group 31 AGMs that I planned on using in parallel as a starting bank with a big reserve capacity. My thought was this would give me a good back up if my house bank dies while out on the water (as it did last week) and I need to do a quick ghetto rig to run lights and radio (as I did last week). Would there be any problem, theoretically, with using these to pull double duty as house and starting batteries until I get my actual house bank figured out?

I'm worried that there could be something wrong with my battery charger since the battery I've been using is now also starting to die, but that battery is also really old so I think it's pretty likely that that's the real issue. However, my boat did flood a good bit a few weeks ago (a story for another time) so it's possible that the charger got damaged then. I have a Sentry G120-3NL. I tried googleing to find a user's manual or something for it but was unsuccessful (I am stuck using my phone though...) Is there some way to check the charger and make sure it's working properly?

Next issue... I'm planning on replacing my house batteries with LiFePO and I've been reading all the info I can find on here about those and I think I'm just starting to get a handle on the fundamentals there. I'm trying now to figure out how much capacity I can fit and where I can fit it so I can start to price that out. I didn't get a chance to do a survey of my average power usage since I hadn't even gotten everything working before my house battery died. My goal is to eventually be able to live off-grid, so does anyone know of a good way to get at least a rough estimate of the capacity I'll need?

Then I obviously have some more research and work to do on how I'm gunna set up my LiFePO bank but I think there are a few fundamental things I need to figure out ASAP.

I don't think my current charger will be suitable for the LiFePOs so I've been thinking about how to replace it. My current best idea is to have 2 separate chargers, one for the LiFePO house bank and one for the AGM engine bank, and have shore power and the generator run both of them. That way, I can easily set up the charging regimes for the different chemistries, have some redundancy in case of emergency, and run the generator off the AGMs without worrying about running them down. (The generator was wired to run off the house batt before, which seemed silly to me...) Then I'd have the engine alternator also hooked up to the AGMs. Is there anything wrong/stupid with this idea? Would it cause a problem when I want to add solar/wind/whatever?

And then just something that's in my mind is that I'll also need to add an inverter at some point but I'm not sure if this is really an important consideration right now or not.

Sorry for the long post and especially for requesting the bit of spoon-feeding but it's been so hard to find info for complete beginners. The info I've found has been great but it really makes my head spin and for every question it answers I have 2-3 new ones!

Thanks so much!