Hybrid battery packs have 28 to 40 NiMH modules assembled in series. A Prius type prismatic module consists of six individual NiMH cells (1.2 V each) assembled in series. Detailed specifications for these modules are available.
7.2 V hybrid battery prismatic module with individual cell marked 1 – 6
28 prismatic modules with a series nominal voltage of 202 V
NiMH modules undergo shallow depth-of-discharge (less than 10%) because they are used for power assist, not electric drive. Unused electrode material undergoes structural changes that effectively reduces amp hour (Ah) capacity. This reversible failure mode (“memory effect”) takes modules out-of-balance with neighboring modules, especially those that are near the pack center. Out-of-balance modules can undergo voltage reversal during discharge. Voltage reversal at the anode and cathode permanently damages the module. High-rate charging generates gaseous oxygen at the positive electrode that can pressurize and rupture the module casing. Modules, reconditioned by a series of deep charge-discharge cycles with tapered currents, will recover lost capacity and have an extended lifetime. Overheating of hybrid batteries causes venting of vaporized electrolyte water. This permanent failure mode increases the internal resistance of the module. Such modules substantially increase reconditioning time. Modules suffering from permanent failure modes are identified and removed before full reconditioning.