I've been working on this for quite awhile now, with the biggest problem being too much current draw from a low cell. With 96 of these chargers forced to share a ~1500W plug it would blow a breaker in no time. Some people use resistors or long wires that act like resistors to limit the current. But in the end that limits the charge current through the entire charge cycle and as soon as the voltage comes up on the cell a little bit to get over that brief high current period then they are charging at 1/2 current or less. I wanted to turn each of these dc/dc converters into a true CC-CV charger, I went through various feedback methods and thought I had it solved when I used a feedback loop that would vary the output from 3.3-3.6v, but then I plugged in a low cell and 3.3v wasn't low enough to limit the current to the preset level. DAMN... back to the drawing board. What I needed was the full swing to adjust the trim voltage below the 3.3v nominal output of the converter. This involves bringing the trim pin from full positive to close to or full negative to get the output voltage in check. Now I have a system that is easy to adjust the CV voltage, then you can adjust the CC current independent of the CV voltage.
There is a 0.001 ohm shunt attached to the negative terminal of the charger, the whole circuit is referenced to the charger negative. When current flows it creates a positive voltage across the shunt which is fed into an op-amp to bring it up to a usable voltage (adjusting the gain is also the method of adjusting the CC current). The output voltage is then stabilized by an R/C network (without the cap the loop would oscillate) this feeds directly into a Pic microcontroller that turns the input voltage into serial data that controls a digital pot. The pot is attached to the +/- and trim terminals, however between the pot and + terminal is a normal trim pot that sets the CV voltage.
So basically it measures the current (by measuring the voltage across the shunt) The output voltage of the op-amp indirectly sets the trim voltage on the dc/dc converter, so as the current rises the voltage is trimmed down to keep it under control, and as the current drops the voltage goes up until it reaches the CV set point.
I still have to tweak a few things but it's working very nicely now. I've been charging an A123 20ah cell from almost completely dead at 3A, something that was not otherwise possible because of the low internal impedance of the cell, it would try to draw as much current as possible.
The next step is to decide if I want to make the current adjustment remote so that a single pot near the charging port could be used to adjust the charging current of all the chargers on the fly when you plug in making it easy to adjust for different voltage inputs allowing higher charge current, or adjust for shared plugs much like the Manzanita chargers can do. In this case the actual charge current at the cells doesn't matter, but an A/C current meter would allow easy adjustment of the charge current to make use of all the current available from a plug and reduce the total charge time.
Total cost of an 8kw capable charger (only 3kw for the moment based on the 48v power supplies I'm using) is about $1300. I have a source for some larger supplies that would let me charge at 8kw, but I would want them mounted in my garage, and not on the car due to weight/size. I would then plug in a high current 48v cable instead of an A/C cable. Cost for those is around $550 (surplus) Add in an anderson connector and some meters and it's probably 2 grand for an 8kw charger that can't overcharge a cell, and is completely adjustable for CC and CV. (LVC still needs to be handled)
Unfortunately retail price on something like this would be $10,000+ using new parts and covering the design and prototyping costs.