dead battery

[Woodie1]

The other day we went to drive our 39 & the battery was dead. I put the charger on it on a quick charge & it started up after a minute or so. I drove 20 miles & didn't think any thing of it. Tried to start it & it was dead again. I did a slow charge of the battery & it has started up fine several times. According to my battery tester & a hydrometer the battery was charged fully. I also had the battery checked at an auto parts store & they said it was good. I had it charged to 6.25 volts & the next day it was down to 6.17 volts. I've tried the parasitic draw test & the meter shows .025 amp. I've done other tests found on Ford Barn & in books I have but nothing gives me a good clue as to the source of the problem. The battery gauge on the dash never gets into the normal range. It's always a little low. The system is a 6 volt & the generator is a single post 2 brush. Any hints would be appreciated.