I have done that to resurrect a weak agm battery which had trouble fully charging and keeping the charge once charged in the car. I checked it with my electronic tester, and it read as 'bad' while fully charged. I hooked it up in parallel to a good battery. Then I put it on the charger. After a day or so, I'd use a battery load tester to deplete the good battery a bit then put it back on the charger. After about a half dozen cycles like that the agm battery finally took a full charge. I put the agm on my electronic battery tester and it read 'good', 100%. Saved a very expensive 3yo, 70# agm NAPA battery for my 09 charger, that NAPA wouldn't warranty because I didn't have the receipt.
