Battery on iPhone 6S Plus quickly diminishes and reboots every few seconds after being in cold weather.
you might wanna take this opportunity - Apple have sent a message to their customers offering $29 battery replacements for any and all iPhone 6 / iPhone 6s etc phones that are experiencing crashes/reboots and worse battery life. Hope it helps! The link is at
https://www.apple.com/iphone-battery-and-performance/
Here are the bits that might be of most use to you as information:

To address our customers’ concerns, to recognize their loyalty and to
regain the trust of anyone who may have doubted Apple’s intentions,
we’ve decided to take the following steps:
Apple is reducing the price
of an out-of-warranty iPhone battery replacement by $50 — from $79 to
$29 — for anyone with an iPhone 6 or later whose battery needs to be
replaced, available worldwide through December 2018. Details will be
provided soon on apple.com.
Early in 2018, we will issue an iOS
software update with new features that give users more visibility into
the health of their iPhone’s battery, so they can see for themselves
if its condition is affecting performance.
As always, our team is
working on ways to make the user experience even better, including
improving how we manage performance and avoid unexpected shutdowns as
batteries age.