question

Charging individual batteries rather than the entire array will make it difficult to monitor state of charge. A Victron battery monitor needs to see all charging and discharging currents in order to calculate state of charge. If you charge individual batteries, then the battery monitor won't see that charging current and therefore not update your SOC number.

The only solution I can see to charge a 48 volt battery bank is to use the Skyla TG with a transformer at the input to accommodate 120 volt shore power. The Victron 3600 VA isolation transformer should be sufficient for a single Skyla TG 48/50 and supports automatic 120/230 volt input selection. You should also be able to use 120/240 split phase power to feed the Skyla TG 40/50 directly if that power is available. The isolation transformer also provides galvanic isolation for your houseboat.

Also note that connecting batteries in series does not increase the AH value. With 2 24 volt 150 AH batteries in series you have 150 AH at 48 volts then two banks of this makes 300 AH at 48 volts. Your watt-hour values do double with series connected batteries so a single 150 AH 24 volt battery is 3,600 WH so your 4 batteries will be 14.4 KWH.

Thanks Kevin, good points and thanks for correcting me on total Ah of the system running at 48v.

As for SOC, one question the boatbuilder and I now have is whether the individual batteries in each bank would be likely to become out of sync with one another over time as they charge. It seems (to my lay mind) that they should have the same SOC when I start charging them with shore power, since they have been functioning as a single 48v battery up to then. And if they have the same charging input, I would think they would generally stay in sync with one another through the charging process. Each battery does have bluetooth communication with an app that would show me (but not the charger, obviously) what the SOC is, and if I monitor that I would think I could prevent a mishap from connecting them in series?

Another question—whether I would even have to separate the batteries to charge them independently. The builder I am working with did a boat for another client that had a bank of four 12v batteries connected in series-parallel, and they were charged simultaneously with four 12v chargers without separating the batteries at all. I had assumed earlier that I would need to disconnect the series in order to charge each battery at 24v, but maybe this is not true.

Thanks for the input, everyone!

LFP batteries typically "top balance" during the absorption phase which equalizes charge (and voltage) of all cells inside the battery. To achieve what you want, each battery would need to be charged long enough to finish the top balance. Series charging may require an external balancer but some batteries with internal BMS will balance across the batteries also during series charging.

I'm not sure how well you could achieve equal charging with independent chargers at the same time the battery bank is also supplying loads. Certainly a single charger across one battery might find other paths to other batteries so may not charge the battery is is connected across in a predictable manner.

Adding to the complexity is the charging from the solar array. This presumably would charge the 48 volt series-parallel battery bank in place.

It sounds like your batteries each contain a BMS that includes it's own SOC indication. But you won't have a total SOC for the entire battery bank. If you are willing to shut down the system and monitor individual SOC and adjust charging accordingly you will probably be OK, but I wouldn't expect this to happen automatically. I'd want the system to manage itself without human interaction and would look for a charging solution that I could set up once and forget about.

If not too late, might be better to go with 48V batteries.

Yeah, I’ve wondered if I should have gotten 48v batteries. It is too late. But—at least with Victron equipment, shore charging them would have required a 240 hookup which I don’t expect to always have access to when I need shore power. Hopefully I’ll be nicely topped off by the sun in most instances and someday I’ll feel like finding shore charging solutions was much ado about nothing, but I’m trying to prepare for worst-case scenarios and create some redundancy in how I can charge my batteries!

Or, as Kevin mentioned above, adding a transformer to raise 120 to 240. I am quickly learning about all the tradeoffs among different options—simpler in one way, more complicated in another!