I do a lot of parallel charging. The basic rule of parallel charging is you can charge together any packs (different c rating, different mAh capacity) as long as they are the same cell count and approximately the same voltage when you hook them together.
The 'rule' about being the same voltage can be a pain at times because it means in theory you cant hook together a half charged and a depleted pack due to the fear of high amps flowing between the two batteries. The rule of thumb I'd heard was that the packs needed to be within 0.2v of each other.
I got to thinking if this 'rule' was overly conservative. From flying yesterday i has several 3s 2200mAh packs, most discharged down to 11.1v and one only discharged to 11.5V. The 'rule' says that I cant couple these together for charging due to the voltage difference being too great. I figured that I could hook them together through a wattmeter with the high charged battery on the input and the low one on the output and see what current flowed.
So to cut a long story short I hooked the 11.5 and the 11.1V battery together and momentarily got 6A but this settled to 3.5A within a minute. These batteries are 2200mAh 5c charge rated which works out at 11A, so no problem at all for the battery.
conclusion is that about 0.5V difference between batteries shouldnt be an issue for modern LiPos with 3c or greater charge rates, so can stop checking all my batteries before I hook them together
Now i got brave.. I was wondering what would be the worst that could happen if i connected a fully charged 12.6v Lipo up to a discharged 11.1v lipo, thats 1.5v difference
And the answer was... Briefly 23A flowed, but this dropped very quickly and within 30 seconds the current was within the 11A charge rate. I left the batteries connected for several minutes until their voltages balanced. At no time did either battery get the slightest bit warm or did any 'puffing' occur.
While I'd not recommend connecting fully charged and fully depleted batteries together based on my experiment nothing dangerous occurs and no short term battery damage is evident.
One thing you really dont want to do though is connect batteries of different cell counts. I've done this twice by mistake and it's not pretty