I have been away from the narrow boats for over 25 years so maybe out of touch, as to junk from China maybe, I have a much larger set up, which I did not think appropriate for you, maybe I am wrong?
The point I have been trying to make is the time, what ever charger used, and what ever type of lead acid used, taken to recharge the battery.
Maybe if you said a little more about use, and situation it would be easier to point you towards best route. It was a long time ago when I worked on the Falklands and with true off grid living. The Eberspacher - Webasto type diesel heater is used a lot with narrow boats, but also heritage railways, I know the Tan-y-llyn railway uses them, the one I work at uses steam, I know my trip with them on the cold turkey run where narrow gauge people get together to swap ideas etc. Their heaters stopped working well before the end of the trip, but since we use steam I did not enquire that much about how the charged the batteries up.
We use batteries for Christmas lights, but very little else. Our engines originally ran through the centre of Welshpool, so have steam turbine generators, although most no longer working, as we don't really need them any more as no longer run through the streets, same applies to the bell, whistle is all we use today.
Back in around 1976-7 I was studying auto electrics, and we had what seemed a simple home work, "Why does the voltage of a lead acid battery reduce" but when I came to look it up in the collage library I could find info on other cells, but not lead acid, no internet then, I got 9 out of 10, but it triggered an interest.
However back then we could not monitor batteries, today I have an energy meter which plugs into the mains and reports back to my PC, and being retired I had time to play, I was really surprised to see a battery recover as if some one had flicked a switch 10 days after being put on charge, the sulphur goes hard on the plates, but if the voltage is maintained it will slowly soften and the battery can often recover, but 10 days was well past the time I expected, normally I would have dumped battery well before that.
But off grid living one is hardly going to be able to leave a battery on charge that long, so what happens is each time charged a little bit more sulphur remains on the plates until the battery is useless. So under charging damages the battery and over charging damages it, And getting the balance is not easy.
