You really want to reinstall an amperage gauge. NAPA has numerous varieties.
Batteries are charged by current flowing into them to move the chemicals from solution to the plates. You can easily watch the rate of charge from say 20 amperes to just a trickle, as you can watch the rate of discharge.
Before the advent of digital voltmeters, voltmeters were more or less useless as a volt or a few tenths would mean the difference between charged and needing it and the resolution on analog meters (those with needles), especially the lower priced ones, couldn't show you that difference.
Trust me, reinstall an ammeter. Plus side is you have no rewiring to do; just get one with adequate ranges, undo the wires from the old and connect to the new.