not too difficult, if You get a suitable primary circuit
f.e.:
http://www.praktiker.at/prjecs01.htm
http://www.praktiker.at/download/lepse803.pdf
once there was a circuit available based on that links above. Used that ECS-pulsating method.
That circuit (the green one) was named
"ECS-Herzprint".
Originally I used it with a three system (Ni-Cad, Ni-Mh, Pb) setup and lower charging current.
Just changed that 1st setup to more power - up to 3 A for 14.4 V Li-Ion - and a 4th system --> Li-Ion
(costed me the battery cell charging tube that was mounted at back and right - the transformator now is too bulky)
... still have to mount all the other connectors I want to incorporate - for years
large rotary switch: number of cells (up to 10)
small rotary switch: System (offers only three, but the additional circuit with the 2 lying potis + the yellow wires simulate a 4 th one)
small silver knob: current
switch below: suitable voltage simulator (when cells are too depleted to have the driver start charging)
potis inside: set end voltage for Ni-Cad, Ni-Mh, Pb, Li-Ion + set temp of temp feeler
.. a nice project but now I would be helpless without
dunno if there is something still available...