I don't know anything about submarines, but it doesn't seem crazy. Maybe they figure the battery has a 1 in 100 chance of failure and the sub can only make 50 trips in it's lifetime and it wasn't worth it considering the other systems like the weights and the hydrolic something or other that they actually used to get to the surface. Everything is a trade off and the extra battery would have cost money, take up space, could catch on fire, etc.