A lead acid battery does not like being discharged, the sulphur hardens over time, and when I found a battery which had been left discharged for some months, putting it on charge it seemed to accept no charge for around 12 days, then as if some one had flicked a switch, it charged up again.
So if a battery is not fully recharged, slowly over time more and more sulphur builds up on the plates, a smart charger which charges to 14.4 volts then allows the voltage to drop to 12.8 volts then turns on again over a week or two can often rejuvenate, a steady 13.8 volts will to some extent recharge the battery, but the cells become unequal, so the smart charger works better.
For a flooded battery (one where the water can be topped up) often we use 14.8 volt but we also monitor the amps, so depending on the amp/hour of the battery a current is selected, likely around 6 amp for a 110 AH battery, and we use a stage charger, normally three stages.
1) Charge at max output of charger until a voltage of 14.8 volts.
2) Maintain at 14.8 volts until current drops to around 6 amp, which should mean battery around 80% charged.
3) Float charge at around 13.4 volt until removed from the charger.
This is how milk float and fork lift batteries were charged.
However with narrow boats the engine did not run long enough, and they use the battery while being charged, so they wanted to charge faster, so they use a pulse charger, this charges the battery then stops and measures the rate of voltage decay, and adjusts the pulses to match the decay rate, Sterling marine make these which can combine the output of two 70 amp alternators to recharge the battery bank often 3 x 160 AH for domestic and one 160 AH for engine starting the latter is just float charged.
Even so they have to try to get a shore hook up one a month to fully de-sulphur the plates.
Best is to move away from lead acid, we use to use nickel iron batteries, these don't mind being left in a discharged state, today it is the Li-ion battery, but the 3.2 kWh (50 volt) one I have will cost £1,100 I am told when I enquired about fitting a second one. These can be charged and discharged faster than the lead acid, but they must not be over charged, so they come as a package with a built in computer which controls the charge and discharge rate, these came in with lap tops, phones, and electric vehicles, I also have two for my e-bikes mine is 12 Ah at 48 volt, and wife's rated at 350 Wh, but the chargers although faster are also a lot more expensive, also use four 20 volt 2 Ah for my garden tools, I used them in pairs, and it takes around an hour before they are discharged, and an hour to recharge so just swap each hour.
However no question lead acid are a lot cheaper.
I have debated the idea of two batteries for narrow boats so one is only charged for a week, and other cycled the swap so other is only charged for a week, this may return all the sulphur back into acid, but before we set it up, my son sold his narrow boat, so theroy never tested.
A 30 amp stage charger will likely cost around £80, one of these on the generator output would speed up the recharging of the battery, however the smart charger I use which is only 3.8 amp on a 80 Ah battery only charges at 3.8 amp for around 4 hours, by that point the voltage has risen and charge rate drops to 3 amp, it will then take 24 hours at the lower rate, but my point is high output chargers will not help at 14.4 volt there will just reduce output, it needs a special charger using 14.8 volt to speed up the recharging, but unless it reduces at 80% charge it will buckle the plates, and if set to drop to float at 6 amp, and you are using 6 amp it will not reduce the charge rate early enough, so with that type of charger you can use the battery and charge the battery at the same time.
So you have to decide how to maintain the batteries, you need once a month to be able to charge for 24 hours, as the narrow boats do.
I would go onto a narrow boat forum and ask on there for ideas, as it seems you are doing nearly the same thing.
