I have two marine batteries I need to charge...I tried a trickle charger for 12-15 hours....my minn kota meter shows both of them fully charged but when I test them with a hydrometer no cells have 4 balls floating (which indic full charge), several have 3 but about 1/3 of the cells have only 2.

So what gives?

Is the cheap...but new walmart hydrometer not accurate?

Should I use a 2 amp/hr charger?

Or are both batteries going south?