Insights into Hybrid Batteries

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Purpose

A hybrid battery, sometimes called a high voltage battery, or traction battery, provides electric current in a hybrid electric vehicle to operate the electric motor that powers the vehicle or supplements power provided by a gasoline engine (depending on vehicle design).

Location

The hybrid battery is typically located at the rear of the vehicle, behind the rear passenger seat or, in sport utility vehicles, under the cargo floor. The hybrid battery in the Chevrolet Volt and Cadillac ELR is a T shaped battery that extends the length of central tunnel in the passenger compartment and then extends laterally under the rear seat.

Operation

Hybrid batteries employ multiple cells linked together to store electricity. They are rated by their capacity to store electrical energy (voltage) and by the maximum rate of current flow to and from the battery (amperage).

Most hybrid vehicles, including models from Toyota, Lexus, Ford, Saturn, earlier Hondas, employ nickel metal hydride (NiMH) batteries. The Chevrolet Volt, Cadillac ELR, 2012 and later Honda Civic hybrid, and 2014 and later Ford Fusion hybrid vehicles employ lithium ion batteries.

Hybrid batteries have from about 160 to 250 individual cells, each typically 1.2 or 1.3 volts, creating total battery voltage from 144 to 330 volts. NiMH batteries are composed of groups of individual cells – each about the size of a typical D cell flashlight battery – linked in series (i.e., positive to negative, negative to positive). These cells are paired into modules that are linked in series to other modules to create the complete hybrid battery.

Lithium ion batteries are similar in configuration, employing individual cells paired and grouped to form modules or blocks. Each cell contains a positive and a negative terminal with manganese cathode used for positive and carbon anode for negative.

Electrical charge and discharge occur as lithium ions are transferred between the terminals. The cells pairs of a lithium ion battery are connected in parallel, with these modules or blocks then connected in series to create a battery pack.

Drivability Symptoms

The most common symptoms of hybrid battery failure are a decrease in fuel economy and a decrease in vehicle performance, particularly acceleration. Recalculation of remaining battery charge displayed on the instrument panel may occur more frequently and displayed “state of charge” will be low more often than is normal.

The gasoline engine will operate more often because the battery requires frequent recharging. Shortly before complete failure, the hybrid battery’s state of charge may fluctuate rapidly between indicated charging and discharging. The instrument panel Malfunction Indicator Lamp (MIL) may also illuminate.

Inspection, Test, and Diagnosis

Hybrid vehicles are equipped with Second Generation On-Board Diagnostics (OBD2) and store Diagnostic Trouble Codes (DTCs) that can be retrieved with a suitable Scan Tool. Malfunctions will either illuminate the MIL or other warning light specific to the hybrid system.

“Generic” codes are shared by all vehicle manufacturers; “enhanced” codes are unique to a specific vehicle manufacturer. Hybrid battery codes were created long after initial implementation of OBD2 in 1996, so retrieval requires a Scan Tool either sufficiently new or updated to recognize these codes. Among the most common hybrid battery generic codes are:

• P3011 through P3029: Battery failure codes indicating excessive internal resistance; the code number identifies the module or cell block
• P0A7D: hybrid battery state of charge low; P0A7E: Hybrid battery over temperature; P0A7F: hybrid battery pack deterioration – either battery resistance excessive or difference between maximum and minimum state of charge values exceeds specifications;
• P0A80: Hybrid battery pack replacement – difference in voltage between battery modules or blocks exceeds specifications;
• P0A95: High voltage fuse;
• P0ABF, P0AC0, P0AC1, P0AC2: Hybrid battery current sensor malfunction;

Hybrid battery failure in Honda NiMH hybrid vehicles is likely to set generic codes P0A7E, P0A7F, P0A9D, P0A9E, P0AC7, P0AC8, P0ACC, P0ACD, or P0ACE and Honda enhanced codes P14535, P1446, P1569, P1570, or P1575. These codes pertain to hybrid battery state of charge or to battery temperature. However, also see “Reprogramming,” below.

Reprogramming

Replacement or repair of a hybrid battery does not require reprogramming. However, Honda issued a technical service bulletin for 2003 through 2008 models recommending reprogramming software controlling hybrid battery charging to correct chronic undercharging leading to premature hybrid battery failure. Honda also extended warranty coverage on these vehicles. See: “Repair/Replacement,” below.

Repair/Replacement

CAUTION: Hybrid battery voltage is lethal. Do not touch any high voltage component of the hybrid electrical system – including the master fuse, master switch, master relay, and any high voltage cables (usually orange in color), unless wearing gloves insulated to withstand 1,000 volts, typically termed “linesman’s gloves.”

Allow ten minutes after disconnecting the hybrid battery for voltage to dissipate from high voltage capacitors that store current within the hybrid electrical system. However, disconnecting the hybrid battery from the vehicle’s hybrid electrical system does not diminish the voltage of the battery itself, which continues to hold lethal voltage. It is essential to follow the detailed procedures specified in the factory service manual for the vehicle to avoid the risk of electrocution.

If a hybrid battery fails within the vehicle manufacturer’s warranty period, it will be replaced by that manufacturer. Mandated warranty periods are 8 years or 100,000 miles in all states except California and ten years or 150,000 miles for new vehicles sold in California. Honda has extended this warranty on 2003 through 2008 models by one year, with an increased mileage limit.

If the warranty has expired, a less expensive alternative to a new NiMH battery is a “reconditioned” battery. Reconditioning is a process of testing individual cells in the hybrid battery to locate those with high voltage and/or low internal resistance and then replacing those cells with used cells meeting voltage and resistance specifications similar to the remaining good cells in the battery. The battery is then cycled through discharge and recharge cycles to restore voltage capacity. Reconditioned batteries are available from the vehicle manufacturer’s dealer network or from aftermarket manufacturers.

The first step in removing a hybrid battery is disconnecting the hybrid battery disconnect switch that isolates the battery from the rest of the hybrid battery circuit. The ignition key should be removed from the vehicle to assure that the vehicle is actually turned off – a hybrid vehicle may be electrically active without emitting any sound. The vehicle’s 12 volt battery and auxiliary battery should then be disconnected. The hybrid battery disconnect switch is then removed or a master switch is turned to the “off” position. Insulated linesman’s gloves must be worn when handling the main fuse or switch.

After disconnecting the hybrid battery, it will be necessary to remove trim panels and, usually, the rear seat to access the hybrid battery. Cooling ductwork, retaining bolts, and electrical connectors to the main relay and battery control module must then be removed before the battery pack and its cover can be removed from the vehicle. Any disconnected or exposed high voltage cables or terminals should immediately be covered with insulating tape. Once the battery pack cover is removed, specific components - including sensors and cables - must be transferred from the old hybrid battery to the replacement battery. The process is reversed for installation.

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