[oil pan 4]

It almost sounds like the battery is never getting a full charge.
Your typical 40lb lead acid battery holds up to 2 kwh from 0 to a full charge and about 1kwh of useable power between 12.8v to 10.8v volts.

I charge a lot of batteries at work and that sounds pretty typical, where the voltage starts high and then drops once the battery starts to take on power.

I had a 1.5 amp charger for a long time, seemed to work ok for my car, but was useless for propery charging the large battery in my truck.
The problem is large batteries soak up 1 whole amp just to maintain when fully charged so at 14v realisticly you only have about 0.5 amps of useable charging power through out a good portion of the charging cycle.
To make sure your large battery is fully charged you want to get the voltage up to between 14.7 and 15 volts and make sure the battery is drawing no more than 2 amps, depending on size.
For charging real batteries you need a real charger.

I have some salvaged solar pannels and I am going to configure them for 22.5 volts for float charging the 12 volt battery in my truck. When I configured the panels for 15.7 OCV they could not push hardly any amperage through my half dead test battery.
From what I have found solar panels that put out any less than 17 or 18 OCV are useless for charging 12 volt batteries, they look cool but don't really do anything.