Backup light

[AlexSheppard]

Fenix E12 - looked at those.  Could well be the best contender.  Just might need two to balance things up.

I use a HL60R as my main light and an E12 permanently zip-tied to the side as my backup. In my entirely unscientific field-testing I've found the E12 roughly similar to a Pixa 2 used on mid brightness; not great but entirely good-enough for a secondary light, would recommend.

The E12 is so small and light I've never noticed the weight on the side from it or felt the need to get a second one just to balance the helmet, so no worries there! Also a fan of how simple it makes spare battery arrangement; spare 18650 and spare AA will fit nice and snug together in a little waterproof case, alongside a wad of paper towel to stop any rattling and soak up any errant moisture.

I've got a hankering to pick myself up one of the little Petzl E+Lite s and stash it inside my tacklesack alongside the spare glasses and emergency chocolate bar, and that'll about do me for lights (I claim for now, but the lure of expensive shiny things is always strong).

 

[Ian Ball]

One of the benefits of a detachable 2nd light is that you can lend it someone whose light has failed and doesn't have their own spare.

 

[Cantclimbtom]

FENIX Wf05e is a tiny 2 x AAA penlight (plastic body) torch that claims 85 lumen for 11 hours.

I have it as a side torch in addition to a custom duo,  38g empty and about another 25g for the pair of batteries (plastic is lighter but less robust, so depends on where you're going)
So another vote for the Fenix side mounted pen torches!

 

[HeathJ]

I used a Fenix E12 for a number of years and used it twice when my main lamp died, including the majority of the trip from County pot to Cow pot. I found it to be perfectly adequate when used on the middle setting (50lumen?) especially if caving with a few other people.

Worth investing in some reusable cable ties for attachment, as these make it not particularly inconvenient to lend the backup to another caver, as most helmets have enough vent holes to make some sort of attachment possible.

 

[AlexR]

Another vote for the E12's, I have two E11's attached to my helmet with cable ties. They occasionally get used for additional light when taking pictures.
For expedition use I carry an additional Fenix HL55 on me which I also use when the helmet comes off in camp.

I used to have only one E11, but had to cut it off to lend to a mate whose light failed in Colostomy, Peak.  Attached it to her helmet with duct tape and cable tie remnants, I liked the tip above about reusable cable ties. No longer having a backup made me nervous, so got a second to calm my paranoia.
They are unnoticeable in terms of weight (29g), though add a smidgen of width to your helmet. The only downside to me is that there is no way to lock the switch, that being said I've only ever drained the batteries by accidentally leaving it on after photo use. Just goes to show, human error remains the biggest danger (certainly in my case).
The E11s have been in use for 6 years or so, I check them periodically and it's quite impressive what they've put up with.

If you chose to carry all your backups on your helmet I'd consider using a capture cord. My helmet is attached to my chest harness with a short length of shock cord and small carabiner.

100% with you on the strong dislike of carrying things around your neck, the only thing that I put there is a knife, and that only grudgingly. This is also on a pull-cord, so if it gets caught badly on something it won't strangle me.