solar panel and battery charger

Plenty of good marine inverter-chargers out there meant to coordinate AC, DC, and Solar for you.

There are solar controllers that have a smart setup for external chargers. If yours does not, no woo.
Does your inverter have a built in charger?
The swamp logic is the solar stuff has diode protection in it and should turn it's charge rate down or off.
A better (auto) charger straight on the battery should work as it would turn it's charge rate down when the battery charges.
I would think the real charger would maintain the float charge level better also.

Out of curiosity, what are you going to run on a 100W panel?
On it's best moment of the day, only making 8Amps.

If just your fridge is on the inverter, on a boat, A normal modern small/medium fridge will consume 70 - 150 watts of power / hour.
I don't care what in on the labels, on a boat, poor air flow, usually warmer than a house, fridges on a boat work harder. More when you keep reaching in for another beer.
This is a gross number but depends on how many times you reach in for a beer or just to stare at what is/is-not there. Then more when the ice maker cycles and cools a warm tray of water.
Then, the in-efficiencies (Peukert’s formula) of the inverter (and other devices), throw in a cloud or bird or poor angle of the sun,, I see the 100W panel helping but not going to keep the inverter/fridge running all night, especially if you keep reaching in there for another beer.
The hardware will help, but your numbers so far, will not last long.
IMO, A fully charged 100AH battery will last 12 hours just with your fridge. Crude and ugly numbers but you are welcome to prove me wrong with your real experiences.
Your solar panel will help a bit, but that battery will be dead in the morning.

Been here, done it, and all good because, the coffee maker needs that gen-set. Really after all that beer the night before..
I would recommend a taller battery charger to get that battery back ASAP to enjoy the rest of the morning quietly with the coffee.

The problem with solar, and I believe it is part of the con, is that the pushers are using watts instead of AH. 100 watts sounds great but it a meaningless since batteries are usually labeled in amps… or at least they were. 100 amps will only output about 8AH during peak sun IF the solar panel is facing the sun. So in a day that 100AH panel will only send 30 Amps into a 220 amp 8D. Assuming the battery was at 50% it will take 4 days to charge it. IF you aren’t using any power.

the math just doesn’t add up

I thought you were understanding what we tried to express.
This latest idea does not make any sense.
It was suggested or a joke.

Your charger should have separate leads to each battery bank (port & stb).
I would prefer the gen-set battery not tied to the stb main and on the third (separate) charger lead.
If the gen-set battery needs a boost, a battery switch could be used to boost from the stb battery.

What battery does the house use?
We may need a new thread to discuss this topic more if needed..

Increasing the battery capacity will help your inverter run longer. This also increases weight.
The batteries you described are in parallel, so in reality, it's still just one single battery, just larger.
Then, your still pouring only 20 amps back into it.
Last longer, more time required to properly* charge.
Watts come out, Watts must still go back in.

You want faster re-charge time, bigger charger, either solar or gen-set generated.
, Now more weight.

Properly charging batteries is a multi step process. The final Float charge must be reached, does not have to stay there long but obtainable.
The sun will long be down before your solar panel can properly charge the batteries up.
Battery mortality just went down quickly.

no that’s pretty common on small boats or low end production boats.

personally I m a big fan of not having all my eggs in the same basket. One bank per engine and gen plus a house bank. If an inverter is added I want it on its own bank so it can never take down the house banks. And bilge pumps.

Yes it can be a bit of a challenge on a smaller boat but worth it