Turning a hybrid electric vehicle (HEV) into a plug-in hybrid (PHEV) is more than adding cells to a battery pack, reflashing the controller, and installing a charger. The 2013 Ford C-Max and C-Max Energi are examples of what it takes to develop conventional hybrid and plug-in versions of the same vehicle—that is, many specific parts, software, and validation.
Both cars have lithium-ion (nickel-manganese-cobalt oxide) cells, which are chemically very similar. But those in the plug-in C-Max Energi, which has a 21-mi (34-km) EV range, have thicker electrodes and store more energy.
Why wouldn’t the C-Max conventional HEV have the same electrode thickness? Because thinner electrodes have less impedance, so the cells (of which there also are fewer) can deliver electric power faster. HEV batteries are a power source for acceleration assist, only minimal EV operation.

Designing for dual function

PHEV batteries, however, primarily are an energy source and, for EV operation, must be able to discharge deeply and take repeated recharge cycles over many years. But once EV energy is depleted, plug-in cells also must function in HEV mode. So electrodes’ design and other aspects of energy cells are a balancing act.
The prototype Prius PHEV evaluated the idea of a pair of cell packs: one type, larger for plug-in EV operation, the other (smaller) engineered for hybrid mode when the larger one's capacity was depleted. But the production model has one pack doing double-duty and controller software to optimize each function. That's the approach all other makes including Energi also have taken.
The “full” or conventional HEV version of the C-Max has 76 of the power cells wired in series, rated at 1.4 kWh. The plug-in Energi has 84 of the energy type in series, rated at 7.6 kWh. The plug-in uses 6.5 kWh for EV range and allows a residual of 1.1 kWh for HEV operation.
The physical size of the Energi PHEV pack, of course, is much greater. The HEV cells are each 120 x 85 x 13 mm (4.72 x 3.35 x 0.51 in); the Energi's are 148 x 91 x 26 mm (5.83 x 3.58 x 1.02 in). And the Energi pack has higher peak voltage (361 vs. 327, during regenerative braking). There's comparable HEV capacity for such operations as idle stop/restart, Ford engineers said.

Battery temperature controls

Battery temperatures are important factors for PHEV range, cell life, and performance. According to Gilbert Portalatin, Ford's Chief Program Engineer, Electrified Powertrain Programs and Integration, tests are run at extremes of -35ºC (-30ºF) and 82ºC (180ºF).
EVs such as Ford's Focus and the Chevrolet Spark variants, and even the plug-in Chevy Volt (all with much higher-capacity battery packs), have an active electric-pump-driven heating and cooling system using liquid coolant and siamesed in a heat exchanger with the vehicle HVAC. The object is to keep pack temperatures within 0-30°C (32-88°F), a protective range for fast recharging systems.
C-Max and Energi employ only fan-driven air-heating/cooling systems, relying on a sensor to monitor battery compartment temperatures. The Energi employs control strategies to direct cell temperatures to within 0-45ºC/32-113ºF for maximum EV driving range.
If the battery pack temperature is lower, the Energi will operate in EV at reduced power until the cells warm up during normal cycling of electricity between generator and battery pack, from drive operation and regeneration. There also may be heat provided by airflow through ductwork from the cabin if the climate control is in the heating mode.
There certainly will be heated air blown into the battery compartment from the cabin if the Energi's engine is started.  This occurs when the defroster is turned on or if the weather is extremely cold (the 2013 Chevy Volt also may employ a similar engine-start algorithm for battery pack heating in extreme cold).
For hot weather, the C-Max fan draws in what will be cooled cabin air provided by the vehicle A/C. During Energi-recharging, the fan draws in outside air, which even in hot weather is likely to be cooler than the cabin of a parked car in a hot soak. The Energi also will have a preconditioning mode using A/C for cabin cooling during plug-in, enabled through the Sync or MyFord Touch modules.
Ford has seen no durability problem with its battery pack from hot soak itself, Portalatin told AEI. So long as the vehicle is parked, peak temperatures in the battery compartment don’t affect battery capacity or longevity. Once the Energi is in use and the A/C is turned on (which can be assumed in very hot weather), pack temperatures quickly lower to an acceptable level.
This approach contrasts with the pre-2010 Ford Escape HEV, which used nickel-metal hydride batteries susceptible to deterioration if operating temperatures exceeded 140ºF (60ºC). It employed a second, rear HVAC system with an evaporator and a refrigerant flow control circuit that would chill the fan-driven airflow to the battery pack.

Innovative algorithm

In addition, C-Max Energi has algorithms that can adjust the EV range for the driver’s chosen route or operating choices, one of which is particularly innovative.
It can recognize a familiar route that is very close to the maximum EV range of the vehicle and do “smart discharge.” Using GPS from Sync to determine location along the known route, the system controls power output to extend range without changing the maximum percentage of discharge. This may make it possible for the car to reach its destination without gasoline engine operation. Other algorithms, similar to those in competitive PHEVs, permit the driver to choose when to use plug-in power, including reserving it, such as for lower-speed urban operation.
Ford limits the Energi to a 3.3 kW charge rate with a Level 2 (208-240-volt) system, which is part of the thermal balance with the protection from air cooling. The small battery pack takes just 2.5 hours for full recharge. By comparison, the Focus EV, with its active liquid cooling, accepts a Level 2 charge at a 6.6 kW rate.
Motor electronics for all full and plug-in hybrids rely on liquid cooling, typically with a dedicated electric-pump circuit, under the hood.