If you charge either fixed voltage or fixed amperage into a lead acid battery which has just been discharged then within a very short time it is back to fully charged, however leave some time between the discharging and recharging and there is a completely different story.
So typical narrow boat, 4 x 180 Ah batteries, 1 for engine start and 3 for domestic power, at around 6 pm the engine is stopped, and it is not started until 10 am next morning, and even with stage charging by 6 pm next day the 3 domestic batteries have not recovered fully, maybe 90% or more, but not full, so what happens is each day the domestic battery gets lower and lower, so the owner looks for a place where he can get shore power, even once a fortnight is enough, the extra 16 hours charge is enough to recharge the domestic batteries.
If you monitor the power in and out of a battery you can calculate how much charge it needs to replenish the battery, this is what the engine management does, but if you only measure the power put into the battery then you have no way of knowing the state of charge, all you can measure is amps and volts, there is no hydrometer connected to the charger.
So typical smart charger, in my case Lidi, it has 5 levels of charge, 3.8A, 3A, 0.8A 0.1A and zero, and it selects the current depending on the battery voltage, so it starts off at 3.8A, at 12.8 volt it drops to next stage, so now 3A charge, at 14.1 volts it then goes to next stage so now 0.8A, this continues until it hits 14.4 volt, at which point it drops to 0.1A charge rate, with the larger batteries this is often not enough to maintain the voltage, so the volts will drop until it hits 12.8 volt at which point it will return to 0.8A charge rate.
The main point of charger is connect and forget, it is not designed for a fast charge, however in the last stage the mark/space ratio changes over time, so to start with around 1.5 hours at 0.1A and then 1.5 hours at 0.8A, and as the sulphur turns slowly back into acid the time changes so day latter you have 2 minutes at 0.8A and 8 minutes at 0.1A and slowly the pulses get shorter and shorter, and watching the volt meter most of the time it shows 12.8 volt, as it goes to 0.8A it quickly raises to 14.4 then drops again, after a few days it will start to rise above the 12.8 volt, and the 90 Ah AGM battery I have sits at 13.8 volt at 0.1A. The smaller batteries for example the 12 Ah in the jump start box AGM will hit 14.4 volt at 0.1A and start cycling on/off, again sitting most of the time at 12.8 volt.
But to fully charge we are looking at a week, the battery simply can't absorb the charge any faster, it is not the size of the charger that limits to charge rate but the ability of the battery to accept the charge. I had a battery off a chair lift sit for a week taking no charge, then like flicking a switch, it took charge and fully recharged the battery. Before being able to monitor the charger and record the charge rate on my PC, I had no idea of this time taken by the battery being so long. So at moment my Kia Sorento battery is taking 6 pulses per hour of less than 2 minutes each.
So even with the battery to battery charger made by Sterling or Ring, they both make them, or the alternators to battery charger which pulse charges the battery using the voltage decay time to work out charge level, a battery which has been used for 8 hours still takes 10 hours to recharge, often longer, the fork lifts would gas the battery to try and get the recharge time down to below 8 hours.
Now the push bike battery (Ni-Mh) is very different, that will assist me for a 80 mile bike ride of likely with me being not that fit of 5 to 6 hours riding, but can recharge in 2 hours. If I have a bigger charger it could be less, but lead acid batteries take time to charge.
