Marrying batteries

[BrotherBob]

I have just found that a mod are use GX 350 has one battery that is not all holding charge as well as the others,

Might like to read:
18650 Battery Buying Guide for Vapor Users – Wake and Vape Blog
Index | E-Cigarette Forum
Battery Safety: Will You Marry Me? - Mt Baker Vapor
A Guide to Safe Charging - The Complete Guide to E-Cig Safety
Vaping 101: Battery Safety
7 Best Practices to Keep Your Charge
Mooch's blog | E-Cigarette Forum

I also have found that a number of new mods seem to balance charge for the USB port

Wise choice, I also follow the manufacturer's recommendations until their proved wrong, which I have found, is rarely the case.