IPhone 6s shuts down randomly appears to be too cold at 60f

My iPhone 6s shuts down randomly at warm temps and will only start up again if the phone is gently heated to where it feels warm in hand. This started about a month after I got the phone from a family member to replace my iPod as its battery is done and will deplete in 5 minutes so has to be plugged in at all times. Same thing with the phone, but used to be good for 45 minutes, now it will shut down in a couple of minutes unless plugged in. The battery could be 100%,but will shut down and require it to be plugged in and warmed to restart and then usually only shows 10%. Only shows 10% about three quarters of the time on restarting the rest of the time it will show 100%. The battery life started out 94% and in the last 6 months has dropped to under 80% and says it needs service once it dropped below 80%. I am pretty sick of buying iPods and iPhones that the batteries last only a couple of years. These are expensive and I am on a fixed income.