I don't think e-clips, or lock washers, or lock nuts, or even Loctite ® is an answer. All of these tend to make the nut not be albe to turn on the bolt. If your problem was vibration loosening the clamp assembly, then any of these would be of some benefit.
But the
problem is not vibration causing loosening, but actual
creep in the metal, where the bolt heads and nuts slowly deform into the soft metal of the battery clamp. The clamp needs to be of soft metal so that it can be fairly easily tightened. In some places this metal creep is called "cold flow." Cold flow or creep is actually a very slow oozing in the solid metal away from pressure.
Think of the "creep" in the metal of the clamp as a
deformation of the metal over time. If the metal deforms a little, a simple tightening will be the best answer. Over the course of a lifetime of a battery clamp, the face that the bolt head works on and the face that the nut works on become significantly closer together.
At regular service of the batteries, and with your regular tightening, one
should always inspect each clamp. After some time, the deformation becomes excessive. If the
two faces that the bolt and nut press on are no longer parallel, the battery clamp should be replaced, or at least temporarily reworked. If the bolt through the clamp is already bent, you needed a new clamp yesterday.
Your scroll saw bolts obviously loosen by vibration, and not metal deformation. Save the e-clips and lock nuts for the scroll saws of the world.
It is a reasonable idea to use a correctly sized special washer to attempt to keep tension on the clamp. Belleville Washers (a.k.a. cupped spring washers) could be used for this. See:
http://en.wikipedia.org/wiki/Belleville_washer Occasionally I have used a split lock washer (for washer types see
http://www.boltdepot.com/Fastener-Information/Type-Chart.aspx and scroll down to washers) but
you should not put a split lock washer against the soft metal of the clamp.
The split lock washer should be sandwiched between a flat washer and the regular nut.
