I found my converter mostly useless for quick charging.
When I have discharged battery and start the generator, the converter will go into overheat switch off after couple of minutes and stay off for long minutes before it switches back on for couple of minutes.
So roughly 1 hr of running the converter gives me 10 minutes of charging.
Get good charger and go from there.
I have not that new Schumacher charger going up to 25 amp and it will show me voltage as well as % of charge.
Not very precise, but gives you idea where you stand.
One thing to watch with computerized chargers is that they might have automatic desulfation mode, who will take long time to charge.