UKH

Can anybody advise if using a Petzl Core battery torch in British winter temperatures adversely effects the time it can be effectively used for? (Will I get significantly less light / working time if it is cold?) 

You might have seen this already, but Dan Bailey wrote the following within last year's Compact Headtorch Group Test:

The ACTIK CORE's operation was untroubled by its night at -20, and continued to function as expected. Initially the CORE battery's burn time didn't seem to be seriously affected by the freezer, keeping up its full power output for the first two or so hours, before dropping to medium. By 3:30 the light was visibly fading - still enough to get you out of trouble on the hill, at a pinch, but well short of its usual brightness. Overall I think this torch is the best in the review for winter use, being less compromised by cold than the rest.

Here's a link to the rest of the review, just in case it's of interest: https://www.ukclimbing.com/gear/camping/lighting/compact_headtorches-12111

Many thanks Rob. I hadn't seen it and it is exactly what I need to know. 

I use mine regularly for splitboarding/running in the dark in Austria in winter. No issues with battery life.  

Used Scottish winter climbing no problems 

Depends on the setting - constant and brightest will deplete the battery to at least amber and possibly red after a couple of hours in winter, especially in the Alps where it's generally colder. Reactive is better. I've exhausted a fair few Core batteries across a number of different torches over the years and so always have a second 'full' torch, as well as a third much weaker torch to use in a hut / bivvy shack. I suspect performance declines with battery age as well.

Hard to tell about a specific torch like the Petzl without actual measurements. Sure, you will lose some capacity when it's very cold. While Li-ion batteries are said to work down to -20C, useful capacity decreases just as with any battery. One figure floating around the internet is "50% capacity at -20C", but it really depends a lot on the actual chemistry (newer is better) and even torch usage and design. When the torch is on (especially in higher modes), it produces waste heat that might keep the battery "toasty" enough. For example, the Actik Core (plus Tikka and similar models) is plastic, the LEDs have no heatsink and are very close to the battery = more heat stays inside. Usually not good, but might help quite a bit in severe cold. So I'm just guessing, but I'd say something like 50-80% usable capacity at -20C?

Of course you are still much better with Li-ion than NiMH or (ouch!) alkaline for winter

One thing you should never do is charge it anywhere near or below zero. You could seriously and permanently damage the battery that way.

Or you could consider non-rechargeable Lithium AAAs, if it works with them. Usable down to -40C, same capacity as the Core but somewhat costly (around 12-14 AAAs for the price of a spare Core battery). I use them in winter for gear I really want working (like backup light and avalanche beacon - if compatible). The downside is their low resistance and almost flat voltage right up to the end, unlike alkalines or NiMH. When used in a "dumb" torch like an old Tikka it might shine constantly for long then die quickly without giving you much warning. 

I take it the "freezer test" wasn't actually done in the freezer, right?

To get a reasonably useful comparison of their performance in the cold I left all five models, fully charged, in the freezer overnight, at a cool -20. I then [removed them from the freezer]* turned them on at full power and set the clock going...

* - added by me, but it's strongly implied from "emerged from the freezer", "came out of freezer" and such further in the review.

I don't think that's the best methodology, it says more or less just how fast each battery+torch combo warms up to house temperature and not much how it works in the cold. Perhaps a frozen webcam on manual exposure recording the torches inside could work well enough? You could even stream it...

Of course, I get it, not even UKC has some industrial freezer facility to run full tests in with. Although it would be quite a sight - reviewer stomping around in a big freezer warehouse, holding a stopwatch with cold hands while looking at torches. He could even test sleeping bags or belay jackets at the same time! You should definitely consider it

But the rest of the review is good, with extensive usage outside (like always here) that tells a lot!

Dan has just gone on holiday so I can't ask him about the exact details of methodology, but given that I've seen his freezer - and it's not that big - I'm going to assume that he didn't lock himself in there for a few hours

No need for asking him - the review itself is good, extensive and with actual use in the field, even though I had doubts about the usefulness of the "freezer test", I enjoyed the rest of it!

Once I tried to (half in jest) to approach one company that makes frozen veggies, asking if I could sleep in their big freezer warehouse to test some winter gear. Unfortunately - but understandably - that didn't exactly work out (I'm just a random guy after all).

Although them mentioning "standards we need to uphold" had me slightly insulted

Frank R. beat me to it. I'd suggest lithium AAA cells for extreme cold, though I think in most conditions the Li ION Core battery would be fine.  The Actik Core headlamp is compatible with lithium cells (always check with other gear).

I take lithium cells as spares for longer trips; they are very light weight, contain a huge amount of energy, and have a shelf life measured in decades.

I had a job designing hydraulic starting systems on gensets for the British Antarctic Survey, we used to dump them in the local intervention meat store for a week at -a lot for a week then go and try to start them, the apprentices were the victims that pumped until they dropped. The 16ltr Volvos were the challenge!