There's 5 ways (that I know of) to charge an NiMH battery: trickle, timed, dV/dt, dT/dt and dP/dt (AKA I-C3). Trickle just charges endlessly at a very low rate. Timed charges at a medium rate for a fixed period of time. "dV/dt" refers to "Voltage delta vs. time delta" and basically charges at a medium fixed current and watches for a change in voltage in a short period of time - when the cell is charged, the voltage droops by a small but measureable amount. Then there is dT/dt or "temperature delta vs. time delta" which charges at a fairly high charge current and watches for a sharp increase in temperature in a short period of time. Lastly, there is dP/dt or I-C3 which charges at a very high rate of charge and watches for a sharp increase in pressure in a short period of time.
The Rayovac system has cells that have a pressure sensor in them. The sensor tells the charger to shut off when there's an increase in pressure. The cells are charged in parallel but the charger shuts off each cell individually.
It's hard to adapt dT/dt or dP/dt to a series charging system because you are relying on the ability to disconnect each cell as it reaches a certain temperature or pressure. So you wouldn't charge a 36V battery at 36V, but would charge all 30 cells individually in parallel and shut them each off as they reach full charge. This wouldn't be too hard to do, but it's not as cheap or as easy as the trickle, timed or even dV/dt systems. And it's nowhere near as easy as a basic SLA charging scheme.