96 caddy. dash can display digital voltage readings. start car, 13.8V. drive a mile and it drops to 11V. come to stop and it might go up to 13.5 again. belt is tight. all battery connections have been cleaned. been doing this for 2-3 weeks. car starts fine. today i drove 5 miles and it went to 11.2, than 10.9, than 9.9 and got low voltage message. got off road and car died. won't crank. left it and will check it tomorrow. now, should i jump it and let donor car charge it for 1/2 hr or so and head for home or take out battery and bring it home and charge it for 3-4 hrs?