2002 Honda Odyssey battery testing

Electrical problem
2002 Honda Odyssey 6 cyl Front Wheel Drive Automatic 100k miles

My 3 yr old battery loses all charge overnight.
Autozone tested the battery and alternator as good.
I started troubleshooting for parasitic loads by disconnecting the charged battery overnight.
Next morning the battery was very low, but managed to start the car. That night, I remembered an old tool I acquired 35 years ago for around $4.
With my battery hydrometer, I measured the specific gravity of the electrolyte in each of the 6 battery cells.
I found 5 of the cells at full charge, and 1 cell at 1/4 charge. Tomorrow I will be replacing that "good" battery.
You might consider this simple tool and measurement for inclusion in your guide.

Best Regards