I see some people are aware of this, yet others are completely unaware of the possible dangers. Just because you are using a regulated mod it's not perfectly safe! You need to keep these batteries in matched or "married" sets to protect yourself.
When we say Married, we mean buy them brand new, just for this particular mod, two sets, three sets, one set, whatever you think you will need, and label them. It doesn't matter how you label them, you can use a magic marker, a label maker, fingernail polish, whatever you have that will last! It also doesn't matter how you label them, as long as YOU know what it means! You can go 1, 1, 1, 2, 2, 2, 1, 2, 3, 4, 5, 6, whatever works for you, just so long as YOU know which battery goes with which.
Now your asking, "But I've got a regulated mod, it's got all these fancy safety features to protect me, why does this matter?" That's a great question, and I'm glad you asked it!
It matters because as you use batteries they build up more internal resistance, so they don't hold as much power as they used to, they don't charge up to the same voltage they used to, and they just don't have the amperage they used to. So you take the battery out of your old Hanna mod and throw it in your new rx200 with two brand new batteries. What's the weak link? It's not the protection circuitry, it's the old battery. The other two batteries are expecting the third to be as strong as they are so they are going to give it all they have and suck the weaker battery into thermal overload. OR, one, or both of the new batteries are going to have to work even harder to cover for the weaker one and go into thermal overload. Either way, it's on before the circuitry can do it's magic! Then you get the BOOM!!
Don't think it can happen? Go check out some of the threads on Candlepower forums sometime, I once saw a night watchmen blow up a newish Surefire 9P, using out of the box Surefire CR123A's. Burnt his hand, shot through the sheetrock, came out the siding and finally stopped 50' away after hitting a brick wall! Not a rechargeable battery, not an 18650, a new, out of the box 123A. What the conclusion was there was one bad cell in the box and one of the others in line was working so hard to compensate it went thermal and vented.
I've since gotten rid of all my multiple battery flashlights after a few of these incidents. Now that's a flashlight, pulling maybe 3 amps? Imagine that at 17amps!
So when I bust on people about marrying batteries, it's not just on a whim! There's also a reason I only use single battery mods, and I curse the revival of multiple battery mods, mechs in particular! So know your mod, know your battery, keep them married, keep them happy, please be safe out there!
When we say Married, we mean buy them brand new, just for this particular mod, two sets, three sets, one set, whatever you think you will need, and label them. It doesn't matter how you label them, you can use a magic marker, a label maker, fingernail polish, whatever you have that will last! It also doesn't matter how you label them, as long as YOU know what it means! You can go 1, 1, 1, 2, 2, 2, 1, 2, 3, 4, 5, 6, whatever works for you, just so long as YOU know which battery goes with which.
Now your asking, "But I've got a regulated mod, it's got all these fancy safety features to protect me, why does this matter?" That's a great question, and I'm glad you asked it!
It matters because as you use batteries they build up more internal resistance, so they don't hold as much power as they used to, they don't charge up to the same voltage they used to, and they just don't have the amperage they used to. So you take the battery out of your old Hanna mod and throw it in your new rx200 with two brand new batteries. What's the weak link? It's not the protection circuitry, it's the old battery. The other two batteries are expecting the third to be as strong as they are so they are going to give it all they have and suck the weaker battery into thermal overload. OR, one, or both of the new batteries are going to have to work even harder to cover for the weaker one and go into thermal overload. Either way, it's on before the circuitry can do it's magic! Then you get the BOOM!!
Don't think it can happen? Go check out some of the threads on Candlepower forums sometime, I once saw a night watchmen blow up a newish Surefire 9P, using out of the box Surefire CR123A's. Burnt his hand, shot through the sheetrock, came out the siding and finally stopped 50' away after hitting a brick wall! Not a rechargeable battery, not an 18650, a new, out of the box 123A. What the conclusion was there was one bad cell in the box and one of the others in line was working so hard to compensate it went thermal and vented.
I've since gotten rid of all my multiple battery flashlights after a few of these incidents. Now that's a flashlight, pulling maybe 3 amps? Imagine that at 17amps!
So when I bust on people about marrying batteries, it's not just on a whim! There's also a reason I only use single battery mods, and I curse the revival of multiple battery mods, mechs in particular! So know your mod, know your battery, keep them married, keep them happy, please be safe out there!
before I post. 
