balance charging current for batteries connected in series

Question

I've been using this simple Li-Ion charger for quite a while.
It consists of 4x 18650 Panasonic NCR Li-Ion "unprotected" batteries (3.1Ah), connected in series (2+2). Charging was going well with cell voltage difference of approx. 0.05V.

After the battery has experienced short-circuit condition for ~1sec, cells got unbalanced.

^{simulate this circuit – Schematic created using CircuitLab}

Questions:

1. How can I balance the cells back? I've disassembled the battery, I've tried discharging strongest cells by monitoring the voltage and internal resistance to match the week cells, but immediately after load is disconnected these values start deviating due to chemical processes. I've come to conclusion doing this manually is quite a challenge.
2. How can I improve my circuit to not to terminate the charging after i.e. B2/B3 "cell" gets saturated? What are the standard, most common techniques used?
3. Appending the 2nd question - can such a "balancing" solution be also energy efficient? I could easily add i.e. a MOSFET in parallel to each cell and control it via uC. That way I could bypass certain amount of current for each cell. Week would be charged faster. However, placing MOSFET into the linear region would be very inefficient solution. Also, i could bypass the charged cell completely (like, i.e., battery protection circuits do). However, that way I would stress a weaker cell by sudden boost of the voltage (thus current), as I have no opportunity to control the voltage of SMPS.

Answer 1

All you ever wanted to know about active battery cell balancing is covered in this appnote: from freescale

You can use specifically designed ICs or you can use a discrete solution with parallel transistors