[penneyma]

My 95' 3500 Express did not have a generator but the electric panel is pre-wired for one. Instead of spending who knows how much for a diesel generator install I opted for a 300 amp hour 12v lithium-iron battery bank and a 3000 watt inverter. It does everything I need it to do. I stress tested the system this summer on a 90* day by first cooling down the cabin while on shore power, then going to anchor out to swim all day. I had my V berth AC running at 75* while at anchor, two refrigerators, water pumps, toilet, and the radio going while on the hook. I had 6 hours of capacity running with those loads (which you can monitor in real time with the battery app).

My set up is three 100ah WattCycle trolling batteries with Bluetooth, a Giandel 3000w pure sine inverter, an 18 amp 12v DC-DC converter (it goes between my alternator and the LFP battery bank), and the various cables, switches, and buss bars. I think I'm about $1100 all in on materials (I did the work myself and had some wiring on hand).

I removed the house Group 31 flooded batteries and reinstalled the LFPs in that portside space. I left one flooded battery as a charging battery as follows: wire the alternator to the flooded battery, then wire the flooded to a Victron 18amp 12v DC-DC converter, which then is wired to the LFP bank. You do not need an isolated converter because the boat has a common ground. I went with 18 amps firstly because I didn't want to tax the alternator, though I could have easily gone to 30 or more amps without much issue. Second, I'm not planning to use the alternator to charge the battery bank from 0-100% since I really do not foresee my use being more than an overnight on the hook and going to a port with shore power afterwards. Third, if I did need that sort of deep charge for an extended period (like going to the Georgian Bay in Lake Huron), I'd invest in a few solar panels and an MPPT charge controller to do the heavy lifting rather than burning diesel on an inefficient charge.

At port the bank is charged with a dedicated 40 amp LFP charger. You do not want to use a charger that doesn't have an LFP setting because they float differently than flooded or gel. You will want to wire the plug for your charger directly to your shore power inlet with an inline fuse or on a line with a dedicated breaker so you can turn it off while on your inverter.

I hooked my batteries together with heavy duty buss bars. Do not cheap out and just try to daisy chain them with battery cables because these batteries are sensitive to path of least resistance. You'll find (like I did) that they'll discharge unevenly if you have the load pulling from battery A while the negative is pulling from Battery C. Make sure to install on-terminal fuses for each battery. Mine are 100 amp.

Once you're hooked together, WattCycle's app lets you control the battery settings through their onboard Bluetooth (don't cheap out and get the kind without Bluetooth built in). Make sure to set all three batteries to parallel mode so they can communicate with one another to manage the draw.

Finally, run #10 or better wire to the generator switch on your panel. It worked like a charm. My inverter came with a remote and a control panel, but I find the remote works great and I have no need to see the state of charge on a panel since the app does a better job anyway.

I gained around twice the usable amp hours at less than half the weight since a group 31 is around 55 pounds and these LFP are only 22 pounds. You can flatten the LFP batteries and it really won't kill your life expectancy on an application like this. 

Note: My V berth AC is a new MarinAire 6k BTU 115v unit that replaced the original when it went out last year. It has built-in soft start and the inverter has zero problems powering it. The whole AC set up pulls around 8 amps running, including my 600 GPH pump.

Hope this helps someone else considering an electric conversion.