The best way to make your batteries last is to run at the slowest speed setting until you find it necessary to run fast or surface and run faster. You can also tell your CE to not charge batteries in order to get every possible knot of speed. 25% is generally the lowest I will let them run down. Running at lowest speed I eventually run out of air but still have at least 75% battery left. Plot a few sound bearings to get a general idea of the target's course, then surface and make a high speed dash, submerge to pick up the trail again and repeat until you intercept.
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