IMHO, 8V is not really a good number, even if that is across one battery, unless you have a 3 stage charger with equalize and it is equalizing.
Applying 8V across one battery constantly (16V total) will boil the electrolyte out of the battery. If the 8V is across both batteries, they likely have bad cells.
What kind of charger/converter do you have?
Usually 3 stage chargers set up for vented lead acid batteries charge at about 14.4V for bulk and acceptance charging and about 13.6V for float charging, only going to 16V for equalizing. Note these voltages are measured across both 6V batteries, across one, you should read 1/2 the values.
Single stage chargers can be all over the map, depending on what design they use.
Note these are general numbers, your charger may be 0.1 or 0.2V different.
I'd check the condition of your batteries and charging system. If your batteries are vented (I assume you have 6V golf cart batteries, they will be vented), you can check the specific gravity with a hydrometer and know exactly how good the batteries are. Check them when you think they are charged.
"The 12V side of life" website has a great tutorial on batteries and charging systems, about 2/3 of the way down this page is the specific gravity chart: