Why Hybrid And Electric Vehicle Batteries Fail Over Time

Batteries rely on electrochemical reactions to supply electric power. The amount of power that they can supply is therefore pegged on these electrochemical reactions. The electrochemical constituents degrade over time and this explains why hybrid and electric vehicle batteries fail over time. Besides that inherent setback, when these batteries are used in HEVs/EVs, time produces undesirable effects that further reduces the performance of the battery. These undesirable effects include the loss of rated capacity, reduced charge acceptance, an increase in internal resistance, faster temperature rise during operation, and a higher rate of self-discharge. Of these effects, the loss in rated capacity is the highest contributor to battery degradation.

Most battery manufacturers consider a battery end of life to be at the moment when it can only deliver 80% of its rated amp-hour capacity. This doesn’t mean that the battery can not be used. It will still be in use but at a reduced capacity. Watch out for our next article on how you can tell that your battery is operating at a reduced capacity. While still at this it is worth noting that the CYCLE LIFE of a battery is defined as the number of discharge cycles that the battery is capable of delivering before its nominal capacity falls below 80% of its initial rated capacity.

Factors Influencing the life of a battery

A lot of research has been done in this regard and it has shown that the life of a battery is influenced by a number of factors. The main factors include Depth of Discharge (DOD) of battery cycles, Overcharging and Over-discharging, Extreme Temperature, Rate of charge, or discharge.

Extreme Temperature

Temperature has a double effect on the life of a battery. It can increase the efficiency of the battery and significantly shorten its life. Temperature increases the efficiency of the battery only in the range when an increase in temperature leads to a decrease in the battery’s internal resistance. Beyond this range, increase in temperature increases that rate of chemical reactions which include even the unwanted chemical reactions. The unwanted chemical reactions leads to permanent damage of the battery chemical component explaining why hybrid and electric vehicle batteries fail over time. For instance, in Ni-MH batteries, an increase of 10 °C above a battery temperature of 18 °C will double the rate of unwanted chemical reactions. An increase in the operating temperature of the battery to about 40 °C reduces the cycle life of the battery by about 60%.

The influence of temperature on battery Cycle Life

You do not need to worry about maintaining the battery temperature in the appropriate levels. HEVs and EVs have Battery Control Modules (BCM) that maintain the temperature at these levels using the battery cooling system. What you need to care about is the cooling system air intake and vents points positions so that you keep them clear (clean and with no obstructions). If you block these passages, the BCM will not manage to maintain the temperature at optimum and this will cause the battery cycle life to be reduced. It is important to have the cooling systems serviced after some time to ensure it works appropriately.

Overcharge and Over-discharge

The active chemical composition of the materials making up the battery cells defines the amount of charge present in that cell. These charges define the limits of the potential difference (voltage) between the terminals of those cells. Trying to increase the amount of charge that can be stored in these cells by overcharging, will cause irreversible chemical reactions which destroy the chemical composition of the cells materials. The irreversible chemical reactions include the inactive cell components being broken down into forms that can not be recombined. The result is a permanent damage of the cell.

Similarly, attempting to extract more charge from the cell will also lead to irreversible chemical reactions as the cell tries to deliver the demanded charge. This will again cause permanent damage to the cell. The figure below shows how the electrodes of a NiMH cell would reverse when subjected to significant over-discharge. Overcharging also results in battery mechanical failures such as battery swelling, interruptions of current paths and internal short circuit.

Nickel-Metal Hydride Cell Polarity Reversal Voltage Profile

You again don’t need to worry about this since the BCM has been programmed in such a way that it doesn’t allow overcharge or over-discharge. The only time when overcharge and over-discharge is likely to occur is when the battery is being serviced or reconditioned. Battery reconditioning is likely to cause hybrid and electric vehicle batteries to fail over time if it is done without the right equipment. Every vehicle battery charger and discharger is programmed for particular model of vehicle(s). If you take your hybrid battery for reconditioning ensure that they have the specific equipment for your vehicle model.

Depth of Discharge (DOD)

The Depth of Discharge defines the amount of Amp-hours (Ah) removed from the cell during discharge as a percentage of the cell rated capacity. It is the opposite of State of Charge (SOC). For instance, if during discharge, 75Ah are removed from a cell with a rated capacity of 100Ah, the depth of discharge is 75% and the state of charge is 25%. Research has shown that the higher the DOD, the shorter the cycle life of the battery. This relationship is exponential as shown in the figure below.

Relationship between Depth of Discharge and Cycle Life

As a HEV or EV user, this is also another thing that do not need to worry about. The manufacturer figured it out for you and has programmed the BCM in such a way that it only allow a certain DOD level that will allow the battery to be of optimal use while still trying to increase the cycle life.

Charge and Discharge Rate

Current is defined as the rate of flow of charge through a point over a given time. Therefore, by considering the current application on a cell, we are considering the charge and discharge rate. Research has shown that high current application is hard on the cells expected lifetime. Drawing high amounts of current reduces the conductivity between the plates. The reduced conductivity leads to uneven current distribution during discharge and charging hence exerting stress on the cells. This is similar to mechanical fatigue and is another reason why hybrid and electric vehicle batteries fail over time.

Again, there is nothing you can do about this as a HEV or EV user. Just leave the BCM to do its thing. The only time you can be careful about this is when you take your battery for servicing or reconditioning. Ensure that the equipment that your mechanic is using is specific to your battery model.

This is why hybrid and electric vehicle batteries fail over time

And what exactly do the factors discussed above do to the battery to cause its degradation? They reduce the usable capacity of the cells and cause the cells to drift out of balance with each other. The cells drift out of balance when they discharge/charge to different levels. The battery performance is limited by the BCM to the strongest cell when charging and the weakest cell when discharging. What this means is that the battery stops charging when the strongest cell is fully charged. Similarly, it stops discharging when the weakest cell is fully discharged. The result is a reduction in the usable capacity of the battery. The higher the battery cell imbalance, the less the usable battery capacity available. This adds to the limitation by the battery management system of about 80-40% for Toyota and 80-20% for Honda.

If no attention is given to the hybrid battery, it will ‘fail’. 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 battery reconditioning. Just give us a call, we will do it and your battery will be back to capacity. We have the right charger and discharger for almost all the hybrids in the Kenyan market.

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