Ah ha! Finally, a question I can actually help with - I even have a 510 mega as well.
If you're going through that many batteries it has to be a charging problem, bad charging is the most common way to destroy a battery. Batteries prematurely die for 3 reasons:
1. Consistant over discharge - they're drawn down totally flat, this damages the anode and cathodes and chemically changes the electrolyte so its not available. Genuine Joye 510 batteries have circuitry in them to prevent that (they blink), knockoffs may not have that protection but you still wouldnt do this because you wouldn't have been getting much vapor from them - so this isnt likely.
2. Consistant over charging - the charger applies too much voltage to the battery and destroys the electrolyte, anodes and cathodes. Mainly its the electrolyte that goes on this scenario.
If this was the case when a battery is new you'd get really good vapor from it for the first week or so before the battery cooked. From what you describe this isnt likely either.
3. Consistant under charging - the charger is either not supplying enough voltage to the battery to charge, or its cutting off the charge cycle way too soon. The battery just never gets a good charge and soon (yes even lithium-ion) it develops a memory and it CANT get a good charge, regardless of putting it on another charger.
USB chargers are notorious for this, and they dont produce much amperage either. I've noticed that I dont get nearly as good battery life if I've been using one particular USB charger of the two I've got. Also, the usb charger should be warm (not hot) to the touch at the completion of a charge cycle. In general wall chargers do a MUCH better job of charging - unless you just happened to get a bum charger (it happens).
In this scenario you wouldnt get very good vapor production, even from a new battery, and the battery would start blinking and shut off within just an hour or so of use if you're lucky. My 510 mega gives me a good 4-6 hours of near constant vaping.
So, your most likely problem is your charger sucks
If you'd damaged the connection you wouldnt be getting any charge at all. Most battery chargers use the batteries internal resistance to tell when the battery is charged up, when the internal resistance goes up the battery is charged. Its possible that you have corrosion problems causing a highly resistive contact, but you said they're clean.
BTW, when you get a good charger you might be able to recover some of your old batteries by leaving them on the charger for a few days, draining them down, and then letting them soak again.
Also, if you're using automatic batteries (dont know if they make 510 mega automatics or not) then the most likely culprit is that you're overfilling the carts and getting juice in the battery and damaging the batteries circuitry. These batteries are remarkably complex, I got curious and took one apart on the electronics bench at work after I dropped it in mudpuddle
