What causes hybrid batteries to degrade and fail?

Eventually all batteries will fail. Hybrid Electric Vehicle (HEV) NiMh batteries have a life cycle. When the hybrid battery does not last as long as the rest of the vehicle, replacing it costs several thousand dollars and creates hazardous environmental waste. Delaying battery failure beyond the service life of the vehicle will reduce the negative environmental impact of hybrid vehicle ownership and save the vehicle owner thousands of dollars.

This graph depicts the typical life cycle of a Hybrid Electric Vehicle (HEV) hybrid battery:


There are three major phases of hybrid battery lifespan. Usable capacity and performance decreasing as the battery ages:

  Typically begins:Typically ends:
Phase 1:Healthy hybrid batteryNew Vehicle3-6 years
Phase 2:Weakened/weakening hybrid battery3-6 years5-11 years
Phase 3:“Failed” hybrid battery (DTC code present)5-11 years 

Each hybrid battery ‘pack’ is comprised of many individual battery cells. Consumer hybrid electric vehicles have anywhere from 120 to 240 individual battery cells. Each cell is about the size of a typical D cell battery. At the beginning of the hybrid battery’s life, each cell easily provides all of the energy needed to propel the vehicle and to stay in sync with other cells in the battery pack. As time progresses, the battery will start to degrade and the cells will fall out of sync with each other. Results of this degradation include:

(1) usable cell capacity is reduced

(2) cells drift out of balance with each other (voltage delta).

The speed of this deterioration is determined by a variety of factors, including vehicle make/model, climate, driving habits, variation in battery cell construction, and more.

This battery deterioration means that the individual cells within the battery pack can have a different charge level and capacity – they are no longer ‘in sync’ with each other. Some cells can be ‘full’ while other cells in the same battery pack are ’empty’. The overall battery pack performance is limited by the vehicle’s battery management system to the weakest cell when discharging and the strongest cell while charging. As a result, the wider the cell imbalance, the narrower the usable range of the hybrid battery becomes. The car’s battery management system uses only a limited range of the battery cells actual physical capacity (80%-40% for Toyota and 80%-20% for Honda). As the usable capacity decreases and the cells become further and further out of balance with each other, the vehicle us able to use less and less of the batteries actual capacity. The vehicle cannot correct this imbalance problem on its own. The car cannot force the cells back into balance with each other.

If no attention is given to the hybrid battery, it will ‘fail’ and the check engine light/hybrid battery light will be illuminated. If this happens you are not helpless! It may not be too late. Your car needs help – battery care from Prolong Battery Systems.

How do I know if my hybrid battery is failing?

There are several subtle cues displayed by a hybrid vehicle that can alert you to degrading hybrid battery capacity & performance. If not acted upon, the battery will ultimately display a fault code for battery ‘failure’. By paying attention to the warning signs and performing preventative care on the battery before it ‘fails’, you can easily improve battery health and prolong its life. Prolong Battery Systems will improve hybrid battery performance, save you thousands of dollars in avoided replacement hybrid battery costs, and benefit the environment by requiring one less hybrid battery to be manufactured. (Note: The symptoms described below are applicable to Toyota vehicles. Honda vehicles display similar symptoms. Feel free to give us a call and we can describe the Honda symptoms for you.)

The first indication of decreased battery performance is a reduction on fuel economy and diminished vehicle performance. The vehicle does not get the same fuel mileage that it used to and it feels sluggish – lacking in power during acceleration. If equipped, EV mode may be less frequent and for shorter duration then when the vehicle was newer. This happens because as the hybrid battery weakens, the vehicle is forced to use less of the battery and more of the gas engine. It also spends more time charging the hybrid battery in the background (i.e. not shown on the dash). Both of these behaviors reduce fuel economy. This can last for several months as the battery weakens and fuel economy/performance progressively decreases. Note: sometimes under inflated tires can cause similar symptoms. We recommend checking your tire pressure to be sure that is not the cause of decreased fuel economy.


The second warning that your hybrid battery is failing is called negative battery recalibrations. This is initially seen when parking the car for a few days (such as over a weekend) or when parking the vehicle overnight. Just before the vehicle is parked, the dash battery display shows a full or near full hybrid battery. When the car is started the next morning (or a few mornings later) the battery display shows an empty or near empty battery. The empty battery indication can be displayed immediately when the car is started or drop from full to empty in the first few minutes of operating the vehicle. This occurs most often in warmer climates or seasons, but as the battery degrades will happen in any climate. This is a clear sign of a weak battery that needs attention soon. If ignored, the battery will soon progress to the point of failure. If treated with Prolong Battery Systems before the battery degrades further, the success rate for recovering the pack and restoring it to good operating condition is nearly 100%.

The third and final warning the vehicle displays of a failing hybrid battery is negative recalibrations that occur while the vehicle is being operated. These recalibrations often occurs on warm days or when idling the vehicle for longer periods (such as waiting for a parking spot or railroad crossing), but as the battery weakens can happen at any time. These are seen as the dashboard battery charge display suddenly dropping from near full to near empty. This drop is followed by the gasoline engine revving to a higher than normal idle speed while force charging the battery from empty to full. This event is easily visible on the dashboard battery display and can be audibly heard by the loud, unusual high idling of the gasoline engine. This can be very unsettling behavior as the vehicle seems to ‘take over’ and ignore driver inputs while force re-charging the battery. This is a red flag, serious battery issues are imminent. In these cases, we strongly recommend using the Prolong Battery Reconditioning Package to perform a preventative battery reconditioning as soon as possible. Ignoring these symptoms for more than a few weeks will result in the battery displaying a failure code and being stranded on the side of the road. If treated with Prolong Battery Systems as soon as these symptoms become present, the success rate for recovering the pack and restoring it to good operating condition is nearly 90%.

The next phase of battery failure is a failure light on the dash and cell failure. Our products can still help vehicle owners in this situation, but the success rate depends on how long it has been since the failure first appeared. If this describes your car, please give us a call to discuss your specific situation and learn how much our products can help you and your hybrid vehicle battery.

What are Prolong™ Battery systems?

Prolong™ Battery Systems consist of the Prolong Battery Charger and Prolong Battery Discharger products.

Prolong Battery Charger is an HEV power grid charger that very slowly charges the pack above its actual physical maximum capacity (i.e. above 100%). It is like a trickle charger for the hybrid battery. The slow rate of charge is intentional, so the battery pack is not damaged while using the charger. When each individual cell reaches its maximum capacity, the extra energy delivered by the charger is converted by the cells into heat. Cells that are not yet full continue to charge, until all cells reach their peak physical capacity. This forcing of all cells to their maximum capacity slowly and carefully re-balances the entire battery pack.

The heat that is created during the charging and balancing process must be removed from the battery pack. All Prolong Battery Chargers have PATENT PENDING integrated battery cooling fan control that drives the hybrid battery cooling fan to easily dissipate this heat without using the vehicle’s 12V Aux battery system. Without proper hybrid battery cooling fan control, a charger can easily overheat and destroy the hybrid battery. NEVER use an HEV battery charger that does not provide for hybrid battery cooling while charging.

OurProlong Battery Discharger products enable deep discharge of the hybrid battery to recondition the cells and restore lost usable capacity. Over several years, crystal deposits/voltage depression form inside the cells due to this repeated shallow cycling and a memory effect is created. This memory effect reduces usable battery capacity, effectively shrinking the size of the hybrid battery and shortening its life. The only way to break down the memory effect and restore lost battery capacity is via a slow, controlled deep discharge battery reconditioning process. The reconditioning process increases the usable battery pack capacity and extends its life.

What is battery reconditioning?

Battery reconditioning is a maintenance process used to increase the usable battery pack capacity and extends its service life. Battery reconditioning is not a one-time event. If performed once and never repeated, the benefits of battery reconditioning will be short lived. Proper battery reconditioning as a recurring maintenance process can significantly extend the service life of the hybrid battery pack.

The battery reconditioning process is performed by first charging & balancing the battery pack with a Prolong Battery Charger. Then a Prolong Battery Discharger is connected to the battery to provide load that discharges the battery. The battery is carefully discharged well below the level that the vehicle would allow during normal operation. This deep discharge drains the battery beyond the voltage depression “floor” to break down the memory effect within the cells. As this happens, material is recovered within the cells and the usable capacity increases. After the deep discharge is complete, the Prolong Battery Charger is reconnected to the battery to charge the cells back to full and rebalance them again. Ideally, this charge -> discharge -> charge process is repeated three times for maximum benefit. Increases in usable battery capacity of 15%-25% or more are common.

For more information about process of battery reconditioning, refer to our Battery Reconditioning Overview page.

I heard battery reconditioning is a temporary fix that won’t last, what makes this different?

Battery reconditioning and preventative maintenance balancing is not a onetime event! People who think reconditioning is not a good idea have usually only experienced it as a one time event, not an ongoing maintenance/service of the hybrid battery. A one-time battery reconditioning will typically last 8-15 months and not deliver lasting results. Avoid service centers that claim to restore aged batteries with a one-time battery reconditioning service!

Prolong Battery Systems’ patent pending technology allows for easy unlimited battery reconditioning for the remaining life of the vehicle. Once the vehicle specific Prolong Car Harness is installed (a one time process ranging from 1- 2.5 hours depending on vehicle), our products enable Plug-and-Play battery maintenance in seconds. Routine battery maintenance can significantly extend the life of your battery. Depending on battery condition when Prolong Battery systems are installed and the battery begins being serviced, several years of incremental life are attainable.

Will a Prolong Hybrid Battery Charger and/or Discharger extend the life of my battery?

Over half of our customers have already received a hybrid battery DTC trouble code before purchasing a Prolong Battery Reconditioning Package. In over 90% of these cases, our customers report significant improvement when using our Prolong Battery Charger. With routing reconditioning and regular charging, the DTC light can often be eliminated and battery life significantly extended.

Unfortunately, in any one specific case, it is not possible to say for sure how much the charger will help. The amount of improvement depends entirely on the condition of the underlying battery pack. Even if one were to have to eventually replace part or all of the hybrid battery, a Prolong Battery Reconditioning Package is still a good investment as it will prevent the replacement hybrid battery from returning to the deteriorated state of the current pack. We have many customers that are buying brand new hybrid battery packs and adding a Prolong Battery Charger on day one to eliminate the chance of cell imbalance returning.

Show me proof. Have any before/after tests?

We are proud to provide conclusive evidence that the Prolong battery reconditioning process is highly effective at extending the life of healthy batteries and recovering ‘failing’ batteries.

Click here for independent before-and-after testing of the Prolong reconditioning process using Toyota Techstream software on a 2008 Toyota Prius.

In addition, here are before-and-after battery reconditioning tests we have performed on several different vehicles:


Should a Prolong Battery Charger be used as preventative maintenance?

Yes. The sooner a Prolong Battery Charger is installed on the vehicle and used regularly, the greater the benefit it will deliver.

When installed and used regularly on a healthy hybrid battery, a Prolong Battery Charger will significantly delay the degredation of the hybrid battery cells. This battery erosion is what leads to failed battery packs and hybrid battery replacement. A Prolong Battery Charger will deliver:

  • Better vehicle performance since the re-balanced hybrid battery delivers more assist when accelerating.
  • Higher fuel economy, since the car is spending less time charging the battery in the background.
  • Increased EV function (for equipped vehicles).


Prolong Battery Systems can help with all of the following DTC trouble codes:

Toyota Prius
Honda Civic/Insight

How frequently should I use the Prolong Battery Charger/Discharger on my hybrid battery?

Recommended frequency of use depends on the age and condition of the battery. If the vehicle is newer and the battery still performing well, performing a charge & balance once every 4-6 months and/or anytime the car sits unused for more than a week is recommended.

If the battery is degraded, charging alone will not yield optimal results. Performing a full battery reconditioning 2-3 times per year, as well as a charge & balance every 8-12 weeks (and/or anytime the car sits unused for more than five days) is recommended. Customers with highly degraded batteries report that they charge every couple weeks, sometimes weekly or more, depending on how much ‘life support’ is needed to keep the battery going.

What is the difference between the Standard and Discharge Ready Prolong Battery Charger systems?

Standard Prolong Battery Charger systems are designed to be used as preventative maintenance on healthy hybrid batteries. The Standard system allows for charging and balancing of the hybrid battery, but does not support reconditioning (deep discharging of the battery). By using our Standard charger system as preventative maintenance, battery degradation can be significantly delayed and often eliminated for the service life of the vehicle.

For batteries that are already showing signs of deterioration, the Discharge Ready version of our Prolong Battery Charger System is recommended. The Discharge Ready versions are compatible with our Prolong Battery Dischargers. Many of our customers opt to perform in-car battery reconditioning, which requires both the Charger and Discharger. The Discharge Ready option by itself does not allow you to discharge the hybrid battery; a Prolong Hybrid Battery Discharger is required.

In Summary:

  • The Prolong Standard Charger supports preventative maintenance charging and balancing.
  • The Prolong Discharge Ready Charger supports preventative maintenance charging and balancing and is forward compatible with discharging (to enable battery reconditioning) with the subsequent purchase of a discharger.
  • The Prolong Battery Reconditioning package supports preventative maintenance charging and balancing and battery reconditioning. No additional purchases are required to maintain your hybrid battery.

What about a timer? Should I use an HEV power grid charger with a timer for vehicle storage?

This is not a good idea. Repetitive shallow cycling of a hybrid battery when charged daily for a fixed time period will build a memory effect that can reduce battery capacity. If the timer is left on too long the battery will be overcharged, or rebalanced, on a daily basis. This is hard on the battery and can significantly shorten battery life. If the timer is not left on long enough, the battery will not only shallow cycle each day but will still self-discharge down to zero such that a full charge & balance will be required when the vehicle is placed back in service. All of the above scenarios are bad for the battery. Hybrid Automotive and quality focused hybrid battery builders agree that daily shallow cycle charging is not a good idea for hybrid battery health.

The proper way to care for your hybrid battery when storing it for long periods of time is to allow the battery to self discharge. The self-discharge does not harm the battery. When it is time to place the vehicle back into service, simply charge and balance the cells once for 20-24 hours at the end of the storage cycle. The practice is much better for the long-term health of the battery.

If you still want to use a timer with your Hybrid Automotive Prolong Battery Charger, we offer the Belkin Wemo WiFi control unit that will allow full programmable control of your Prolong Battery Charger.

What are the technical specifications?

Bundle SKU:Charger included in bundle:Discharger included in bundle:

Prolong Battery Charger technical specifications:

Input Voltage:
Input Current (@120V)
Input Frequency:
Input Leakage:
Output 1 Voltage:
Output 1 Current:
Output 2 Voltage:
Output 2 Current:
88 ~ 264VAC
0 ~ 4A
47 ~ 63Hz
2ma 240vac


Charger SKU:Output voltage:Battery size:
CO-I10-180 Volts DC120 cells maximum
CO-I20-128 Volts DC84 cells maximum
CO-PR0-250 Volts DC168 cells maximum
CO-P10-340 Volts DC228 cells maximum
CO-HC20-195 Volts DC132 cells maximum
CO-CR0-295 Volts DC204 cells maximum
CO-HH0-360 Volts DC240 cells maximum

Maximum ambient temperature: 100 degrees Fahrenheit (38 degrees Celsius)