Has anyone had any problems with Energizer Ultimate Lithium Batteries ?
Brand new out a packet they won't power my Hope Vision light but do work OK in
my Garmin Dakota

Have you got a battery tester? Only ever used tgrn fresh in the garmin and then run them flat in a cheapo led torch. Contacts clean in the Hope, do Other AAs work in it?

The Hope works with "normal" batteries and I have tried two different packs of the new lithium batteries purchased off flebay and neither lot worked, the lithium batteries do seem very light.

the lithium batteries do seem very light.

Maybe you bought the helium batteries by mistake?

I believe the lithium batteries have a higher voltage than other types of rechargeable or alkaline batteries. I think I've read that they're around 1.6V as opposed to 1.4V. If using four in parallel that could be 6.4V instead of under 6V, and perhaps the Hope light has over-voltage protection. When I've used them in one of my front lights the light has reacted a bit weirdly - it doesn't show a battery status monitor, for example. They are nice and light, which gives me a psychological boost on long or hilly rides! They also give great run-times, especially in things like GPS units etc.

I found online the little gem about four batteries in line etc. perhaps that is the problem.
Thanks.