The first set of 18650s are a bit over 6 months old now. My mod read a battery as 3.1 upon discharge. I've always gone by the rules "4.2 charged, 3.2 done but better to discharge around 3.6" If it has under-powered to 3.1 is it time to retire the battery? Sorry if this is a repeat question, used search function, found weak results at best.