Glad to help.
Just a little more information... in commercial lithium battery packs, like those used in laptops, each cell has its own protection integrated circuit and the pack is kept in balance. Radio control hobbyists use special chargers which have an output for each cell in a battery and this too keeps the cells 'balanced'. Effectively, it monitors them all individually and keeps them at exactly the same final charge voltage to within a few mlilivolts.
I'm a flashaholic (a flashlight enthusiast - a side effect of enjoying caving and night-time hiking) so I use a lot of lithium cells for powering high-power flashlights. I always make sure that any cells that will be used in series in a multi-cell flashlight are charged to exactly the same end point. Cells will age differently so, as they vary throughout their life span I will group them into pairs (or triplets, etc) that are within a few tens of mV and can thus be safely used in series*. Any single ones left after that will only be used in single-cell lights.
I've seen the results of LiIon cells exploding from short-circuits or reverse charging due to a failed cell in a pack. They can burst the aluminium wall of a flashlight and spit burning lithium out like a firework.
The same principles apply to all types of cell chemistry, but for most the results are not as spectacular as with lithium. NiCd etc will just die and sometimes burst.
*To check cells for charge closeness, let them rest for an hour after charging to a final voltage, then measure with a precise voltmeter. I write the voltage on a label on the cell, along with a number of charges tally. I tend to check them every ten charges or so but some people check them every time.