Ok.. let me go over the basics.. I think a lot of information has been discussed, but let me put it all in one post.
First, most lithium ion 18650
batteries are fully charged at 4.2 volts. There are a few that are fully charged at 4.35, but I've only seen like one or two out of 1000 that are like that, and they're not high amp
batteries. Sony VTC
batteries should be fully charged at 4.2 volts... and I would assume all the batteries you have should be charged at 4.2 volts.
Dark marks on the negative end of the battery are usually caused by sparking. This is usually done by a poor connection on that end, and sometimes by running the battery at a high amperage. I would check the negative connector on both your mod and your charger. Most of the time, the dark marks are due to usage in the mod, but can be rarely done on the charger. I would clean the connectors on both the positive and negative side of EVERYTHING.. both mod, charger, and lightly clean the contacts of the battery as well. For the mod, make sure the battery fits in snugly. if it's too easy to put the battery in, then you might have a poor connection on the battery which causes some arcing.
As far as the charger, what Nitecore charger do you have? what amperage does it charge with? I have a nitecore I4 that I have been using for over 5 years with no problem. It has a .750 amp per "region" two regions, and four battery slots. Slots 1 and 3 are one region, slots 2 and 4 are the second region. If I were to charge four batteries, each would be charging at .325 amps. if I charge 2 batteries, I can control what amperage I charge at depending on which bays I put it in. If I put two batteries If I put a battery in the first and second slot, then they would each be charging at .750 amp. If I put both batteries in, one in slot 1, and the second in slot 3, then each would be charging at .325 amps, since slot 1 and 3 are sharing the .750 amp limit on that region. For a married set of two batteries, I prefer to charge them on the same region, even though it takes longer. The limitations of the charger come into play when I want to charge one of my married three battery setups. I make sure I have a lone battery that needs to be charged to charge with the three battery setup so all bays charge at .325 amps. Sorry if that was confusing, but take the time to understand that, since marrying your batteries can really prolong the life... charge married batteries at the same time, and at the same amperage, and the batteries will last a long time. Half my batteries are over 2 years old and still hold a good enough charge for me.
Lastly, is the voltage reading on the mod. Batteries output whatever they can... they start at 4.2 volts, and can be discharged down to usually 3.4 to 3.6 volts. Regulated mods use capacitors, which hold a bit of a charge. If your battery is slightly drained, and outputting 4.0 volts, the mod stores some energy in the capacitors and draws from the battery. It pulses whatever voltage you want... So if you want 5 volts to come from your mod, it's not pulling 5 volts from the battery, it's still pulling whatever the battery gives, but it pulses 5 volts to the atomizer. So it shouldn't matter what the voltage says on the mod as far as battery drain is concerned.
If you have a charger that has settings to change the voltage for a full charge, keep it at 4.2 volts. My Nitecore I4 has no setting. If there is a toggle switch or setting, keep it at 4.2 volts.
