How di do RHJ,
not that simple I am afaid (as you guessd) but for most part, you do not need to know the rest.
The charging of any Battery and Capacitor is based on a timing constant
T(time) constant of charge T(c) = Capacitance x Resistance or T(c) = CxR where R = Resistance of circuit + dynamic resistance of battery (r)
Thus T=C(farads) x (R+r) Ohms answer in seconds.
This is to the best of my ability from years ago.
You nev mentioned teh condition of the battery, this is of huge importance.
Plates get build up of crap and bend which can short out cells etc.
If two cells short plates due to build up of Electrolytic deposit or indeed with Heat they buckle and come in contact, the battery will appear to charge up very quickly but it will have a lower voltage and heat up in the proess but it is useless at this point. The Internal resistance can only be measured using Shunt meter which I doubt you will have and the voltage meter is useless unless really shagged with the plates in full contact where it will not read 12+ volts (up to 13 in charge mode). PS unless using high Impediance (Z) mode of meter, you will have very little of it left by now!!!!!
If a Battery (lead acid) has been left go flat, it will not recover from this fully and its life is seriously shortened.
There are a few batteries that use a GEL Acid type and do not spill. They are high TORQUE which really means high heat tolerance as if you depleate them quickly the heat up hotter than when charge (given you are drawing up to 80 Amps min with Starter motor. Finally and sory for the info but a battery that says 80 AMPere hours as in AMPx Hours does not actually mean 80 amps for one hour or 1 amp for 80 hours, that is more of a theory. Again the Internal Resistance and Heat of the battery will decide this more as in condition of battery and ambient conditions of use TEMP etc.
I will have to check on previous threads as to which one were recommended.
Again ask Local Mechanic on RNLI lifeboat.
There is much written on this and there are many who know more than I do.
Get the battery checked each season in Garage having charged for 24 hours,,,