Story and Photography by Jim Marotta

One of only a few specialized tools necessary to work on a hybrid vehicle -- hybrid electric insulating gloves -- are required for safety when disengaging the battery pack for testing.

The biggest obstacle to servicing hybrids is not the purchase of special tools and equipment, but understanding the system you are trying to service. Most tools necessary to perform hybrid repairs are already in your tool box. While hybrid electric insulating gloves are required for safety when disengaging the battery pack and testing for power prior to repairs, other than that, there are very few hybrid-specific tools required. Always use extreme caution around high-voltage hybrid components: Failure to follow instructions may result in serious personal injury or death.

12-Volt Battery Failure or Discharge

The 12-volt battery on a hybrid vehicle (HV) is subject to the same concerns as all other batteries in terms of service life. Misuse or abuse such as extended periods in a discharged state, significant discharge cycling (dead battery), or more normally, a simple loss of efficiency over time, eventually causes the battery to degrade and fail.

The 12-volt battery runs only the computer and other electronic devices when you turn the hybrid vehicle on. Since the engine is started by the HV battery, the 12-volt batteries are usually rated pretty low in cold cranking amps so they can go bad long before they would ordinarily cause a no-start. This is why some hybrid aficionados advocate preventive replacement of the 12-volt battery, rather than waiting for it to fail.

On most hybrids, a traditional alternator is not fitted to the engine. A DC to DC converter is used to charge and maintain the 12-volt battery and gets its power from the HV battery. As a result of this connection, a bad 12-volt battery can not only drag down the 12-volt system but can result in DC to DC converter overheating and failure.

Most hybrids use wet cell (flooded) lead acid automotive batteries which are mounted in the conventional front-of-engine-compartment location. Service and maintain these batteries like any other automotive battery.

Some newer hybrid models have switched battery positions and are now mounting batteries in the rear of the vehicle. This is where the issue gets confusing. If you have a rear-mounted battery it can be a traditional wet cell (flooded) battery or a new absorbed glass mat (AGM) battery that requires special consideration.

AGM batteries require a charging voltage that does not exceed 14.00 volts at normal temperatures and even lower voltages when summer temperatures soar. Unfortunately, almost all older automotive chargers have a permanently fixed set point voltage that exceeds 14.00 and this spells trouble. Subjecting AGM batteries to 14.6 volts for a prolonged period will eventually destroy them.

Hybrid Battery Pack Failure

Honda HV battery failures are caused by internal shorting of the individual battery cells.

Any battery’s performance will degrade over time and when this happens to an HV battery, vehicle performance is reduced. Hybrid vehicles which rely on HV batteries for power are equipped with internal systems designed to alert the operator only if there is a service issue. Like all batteries, HV batteries fail because their active material and internal structures have reached a point where they have lost sufficient capacity to be considered un-serviceable.

Many first generation Prius and Honda owners are now seeing the end of their HV battery’s life. According to the experts, the Toyota's HV batteries are failing due to electrolyte leakage and the resulting corrosion issues, and the Honda's HV batteries are failing due to internal shorting of the cells.

Toyota HV battery failures are caused by electrolyte leakage and the resulting corrosion issues as seen on this battery terminal.

Unfortunately, there isn't much that can be done to prevent these issues. With the exception of the Gen.1 Prius, which can benefit from cleaning of the corrosion and replacing bus bars on the battery pack, only careful monitoring of the cell module internal resistance and voltage differential between high and low cells while under a load can determine when an HV battery is in decline.

Using a relatively inexpensive and readily available hybrid battery diagnostic tool to perform a routine drive test will show the relationship between a known good battery and the real-time performance observed during the drive. If the battery test results are low or suspect, then a decision can be made about service.

Whether it’s powered by electricity, CNG, gasoline, a biofuel, or diesel, ChiltonDIY’s comprehensive online repair procedures, specifications, diagnostics, and technical bulletins can help you service your car or truck.

A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 25 years experience in the automotive industry.