Heated Gloves

On that ride yesterday I used my heated gloves for the first time in anger!
They performed very well, I was really pleased with them. 
When we departed I switched them on and set them on Medium heat. the batteries are Lithium-ion and there are three settings;
Low >40 degC
Med >50 degC
High >60 degC
Considering the temperature was 5 degC, and we had a windchill factor when we were doing 70mph on the motorways, they kept my hands quite comfortable.
However, when we set off to go home I switched them on but did not put them to Medium, they defaulted to the High setting. So after about 2 hours, with an hour to go, I started to notice my fingers getting cold, and then colder! The batteries had run out of juice but that was my own fault, they performed within the specification.
But I do some long rides, e.g. up to South Shields is nearly 300 miles so about 5 hours riding. I also go to the NMA 3 times a year and have some longish ride-outs. So I've ordered a spare pair of batteries that I can charge and keep in my bike top box to either change en-route or change over the batteries before departing to ride home.