If you travel underwater at Ahead Slow (speed 1) you won't use a lot of battery power and can easily remain under all day long. You won't get far since you're traveling at 1-2kts, but you can stay under for like a day. Anything faster and your recharging will take forever. Also recharge while going at a slower speed. At "speed 2" with a VIIB you'll go at 10kts, have fuel forever and recharge pretty quickly. Once recharged you'll speed up to 11kts. At faster speeds you'll lose more speed while recharging and recharging will be slower.
Also remember even though your CO2 alarm starts to go off at 10%, nothing will happen till 100%. At which time you'll all die. So remember, save!
