Battery Chemistry, Life and Warranty
Batteries are chemical systems which store and move electrons from one half cell to another half cell via two connected electrodes with a corresponding movement of charged ions between the cells. As the electrons are attracted from one electrode to the other, they are passed through an electrical circuit where they can be made do work such as driving an electric motor, lighting a bulb etc. When a battery is re-charged, electrons are pumped, using an external power source, into the battery in the opposite direction to the discharge direction. During this process, the chemical systems in each half cell are restored to their original balance and the electrical potential is reset ready to discharge electrons again. Lithium chemistry is chosen for its ability to generates very high negative electrode potentials (ie voltages) in the anode while Permanganate (MnO4- ) is typically chosen at the cathode for its ability to generates very high positive electrode potential helping to generate high cell voltages of 4Volts giving the battery a high energy density. In comparison, a lead battery typically can only generate cell voltages of 2V.
Like a tap, the volume of electrons flowing (ie current measured in Amperes) together with the voltage (similar to water pressure) from the battery provides the power to drive an Electric Vehicle. The battery will discharge at a higher or lower rate depending on what level of power the driver requires by pressing the accelerator pedal. Plugging the EV in for charging and therefore moving the electrons in the opposite direction, the battery can also be charged at a higher rate depending on which power source is used (Domestic, On-Street or Fast Charge).
| ||Fig. 3 Cross Section of Lithium Ion Battery Cell in Discharging and Charging Cycles|
Batteries can lose their capacity to store energy (measured in Kilowatt Hours (kWh – same Unit as on Electricity Bills) or Amp Hours (Ah)) as they age through daily use. This aging occurs mainly because of a build up of deposits on the electrodes, an increase of unwanted chemical impurities in the electrolyte and internal corrosion of the components. The speed at which these deteriorations may occur is governed significantly by how the battery is used in service and can depend typically on factors such as:
- number of charging cycles
- Depth of Discharge on each cycle (DOD is the % of the battery’s storage capacity used between each charge)
- rate of charging/discharging – how quickly the energy is put in to or extracted from the battery
- level of over-charging or over-discharging – voltage with respect to cell voltage operating limits
- operating and storage temperatures
Modern Lithium batteries are constructed with up to 100 individual battery cells. Each Lithium cell (see Fig. 3 below) can be charged and operated within a voltage range of 3 - 4.2 Volts. In a Lithium Ion cell, Lithium is contained in atomic layers of crystal structures of carbon graphite (LiC6 ) at the anode and Lithium Permanganate (LiMnO4 (or LiCoO)) at the cathode. The battery consists of a set the described set of layers which are needed to conduct electrons and positively charged Lithium Ions. As in Fig 3, modern Li Ion Batteries consist of a series of layers which may be either coiled inside each other in a cylinder or pressed together in rectangular flat layers.
When the battery is releasing its energy in Discharge mode, the higher electrode potential LiC6 electrode transfers electrons to the lower electrode potential LiMnO4 cathode. This causes the Li+ ion to be released from the Lithium-Carbon graphite crystal (referred to as intercalation) which then is free to travel through the electrolyte towards the cathode. The electrolyte allows movement of the ions but is resistant to the conduction of electrons. Therefore it is far easier for the electrons to travel via the connecting wire outside of the cells towards the cathode creating a Direct Current (DC). On its route, it passes through the electric motor where it provides the electromotive force necessary to drive the wheels. Arriving at the cathode, the Li+ ions enter the LiMnO4 atomic crystal layers where they join with the electron in this new configuration.
In order then to Charge the battery, an Alternating Current (AC) from the electricity grid is converted into a DC current through a rectifier and a transformer. This DC current is applied in reverse to the battery as indicated in Fig 3. The electrode potential of the battery is reversed and electrons flow back into the “anode” and out of the “cathode” causing the Li+ ions to reverse direction and re-enter the carbon graphite forming a crystalline alloy of Lithium and Carbon as before.
When selecting Lithium and electrolyte chemistry combinations, manufacturers must choose the optimum combinations to ensure high energy to weight storage ability while ensuring stability and resistance to degradation. Unlike Lead batteries, Lithium batteries can be partially charged without degrading the storage potential of the battery which is useful if a quick charge is needed within a limited amount of time.
Batteries will generally have a thermal protection/charge protection system to prevent over-charging or over-discharging of the individual cells. Modern electric vehicles use sophisticated battery management systems to prevent or minimise the effect on life due to each of the above listed risk areas. The system will also ensure the optimum battery pack temperatures are maintained during operation of the battery.
All EVs registered for this scheme are required to provide a minimum warranty of 3 years (or 100,000 km) on the vehicle and battery. Each manufacturer can provide information to the Consumer on battery life estimates specific to their battery and battery management system. When considering these, it is important to note the operating condition which the manufacturer has assumed such as Depth of Discharge, Charge/Discharge Rate and temperature. So for example, an EV which can retain 80% of its storage capacity after 5 years means that the vehicle range in km could be reduced by 20% at that time.
Vehicle suppliers in many cases source their batteries separately from battery manufacturers. Some Li battery manufacturers are currently offering batteries with claimed performance of up to 1,000 cycles with 100% depth of discharge for each cycle claiming an 80% battery life remaining (ie battery capacity/kWh storage) after this number of cycles. For example considering what this could mean in practical terms, an EV with a range of 160km which was used to commute each day a round trip distance of 40km and was run each day until 100% fully discharged, would need to be plugged in and fully recharged every 4th night. A 1,000 charging cycle battery would therefore provide 4,000 days of continuous service which is 11 years of useful service after which time the battery may still have 80% of its storage capacity available depending largely on how it was driven and charged. In order to conserve battery life by reducing the depth of discharge on each charging cycle and with early Consumer’s cautious of vehicle range, it is envisaged that vehicles will be plugged in more frequently than this thereby improving the potential range beyond the rated battery life provided in this example.