I have been thinking of building my own large charger or buying a pre-made one, then building a mixture of a smart charger/regulator on each battery to charge each battery independently. I know I could easily spend 1000 for a charger and $40 on each of the 12 charger/regulators, plus my time involved in making 12 circuits.
Recently I found the Minn Kota MK-440 charger, it is a 4 bank charger that will charge 4 12v batteries independently while they are hooked in series. The charger is a reasonable price at a little over $300 each, which puts the price in range of normal EV chargers. I would need 3 for my 144 volt pack and I would be able to add another charger if I decide to upgrade to a 192 volt pack. I have been thinking about upgrading and would like to have that option instead of needing to buy another expensive charger to charge the full 192 volt pack. I am surprised that other EV’ers have not used this charger since it can be left on-board the vehicle connected at all times. Each of the 4 banks can charge at 10 amps and uses the 3 stage charging that will help extend the life of the AGM batteries. I guess I may be one of the first to use this type of charger on an EV, so we will see how it works.
Minn Kota MK-440 information link: http://www.minnkotamotors.com/products/chargers/detail.asp?pg=mk440