A friend had a 2310 that was down at level 4 cause it sat in a drawer for a long time. He gave it to me to charge. Well knowing that this model has the swimming pool sized battery, walking from the house to the drive way in the sun wasn't going to cut it. I wasn't content to let it sit in a window sill in indirect light for days/weeks. There had to be a better way. A method that would allow the watch to be in the sunlight for hours and not get too hot. Here it is.
Take a drinking glass. Insert the watch into the glass with the face close to the top. If you choose the right sized glass the band will naturally flex against the sides and hold the watch in place nicely. Fill with mildly cool water. Fill until water is half way up the head of the watch, but not spilling over onto the crystal. Set it out in the sun. Check it after 2 or 3 hours. The water may be slightly warm. Exchange for cooler water. Repeat if necessary. I made it from level 4 to level 2 today with this method.
Sjors your chemistry background may help, or if there is someone else here with the info. What would be the optimal temp? I know you wouldn't want it too cold nor too hot for charging the battery.