I followed the advice of this excellent guide to series/parallel switching.
http://visforvoltage.org/forum/3166-...t-2-simplified
And installed a bank of 12 volts chargers under the seat.
To drive, I plug in the series connector that connects the 60v pack to the controller.
To charge, I plug in the parallel connector that connects each 12v battery to its individual charger.
I will no longer have any issues with one bad cell in my pack destroying the rest of them. And if I upgrade to a higher voltage later, it won't involve as much rewiring.