General rule of thumb for any batteries; when capacity drops to %80 do not use them. I've got some aging HG2 batteries that I no longer use for high wattage because they are reaching 300 cycles. MXJO cells are decent (I've got one in the M class right now) they just can't be pushed hard. General rule of thumb; when in doubt replace.
I wish this was easy. I have no idea how many cycles my batteries have been through. I have some button tops for Provaris that are less than a year old so no problem. Around 8 flat tops for various mods. The oldest are 2 1/2 years old. No idea where the capacity is at. I currently have 5 mods that use flat tops. I use them all at random with different tanks and resistances so it's unlikely I would notice if a particular battery was running down fast, at least without some dedicated effort.
I've disposed of half a dozen batteries over the last 2 1/2 years, five of them were eaten by Sigelei mods which have a habit of occasionally keeping the display on at the dim setting until the battery gets run down to 2 volts. I heard they shouldn't be used again if they get down that low. The only battery I ever had go off on me was in an old Sigelei 20 that short circuited. It started pumping smoke out of the hole at the bottom when I put a new battery in. I was able to get the bottom cap off before it got too hot to do much damage to my fingers.
I've asked before if there was a way of checking the health of a battery but apparently there isn't, other than watching their performance.
I don't worry too much because I never go over around 12W. And I use Mooch-approved HG batteries from Illumination Supply. Guess it's a bigger deal if you use high power or mechs.
I was thinking about a low tech solution, like retiring them when they get to 2 years old. When I first got on ECF someone suggested they should be retired after 1 year, but other folks replied that 1 year was too soon. But like people have said in this thread they are cheap enough that it's not a big deal.
After writing this post I've decided to go through my batteries and retire any that are over 2 years old. Answered my own question I guess.
