Plugged In: Explaining Electric & Hybrid Vehicle Tech (October)
With Hybrid Electric Vehicles (HEV) or Electric Vehicles (EV), it is important to have the right power source that will be acceptable to the operation of the design of the vehicle.
In this article, I will explain the major types of accumulators that are used.
I expect NiMH and Lithium-ion to be the main source for the next 10 years and the 2020s will see a great many new EVs and PHEVs on the market.
A word or two on battery life.
Batteries are not only used as an initial power source but also as a stabiliser, absorbing ripple voltages that occur when charging and discharging. I do not like saying an automotive battery is ‘flat’ – rather it is dead if it has charge below 11 Volts or when it is discharging or charging.
Manufacturers consider the battery at its end-of-life when it is at 80 per cent of its rated capacity. That means if a battery can go 100km on a full charge, when at 80 per cent it will travel just 80km. The battery will still deliver usable power – it just means that, for example, the internal combustion engine (ICE) of an PHEV will be used more to charge the battery.
The time needed to charge a completely discharged battery can be estimated by using the reserve capacity rating in minutes divided by the charging rate (hours needed to charge battery = Reserve capacity/Charge current). For example, if a 10A charge rate is applied to a discharged battery that has a 90-minute reserve capacity, the time needed to charge the battery will be 9 hours (90minutes/10A = 9 hours).
Types of Batteries Overview
Though the lead acid battery is more than 160 years old, it is still the major choice for low voltage vehicles. Improvements have been made, of course, such as in sealed low-maintenance batteries in which some of the active materials have been changed to reduce gassing and increase reliability.
Valve Regulated Lead Acid (VRLA) battery
The most popular battery used in the industry is the Valve Regulated Lead Acid (VRLA) battery. Unlike the traditional flooded electrolyte lead acid battery, the VRLA provides a path for oxygen generated at the positive electrodes to reach the negative electrodes, where it recombines to form lead sulfate.
GEL CELL BATTERY
A gel cell battery is a VRLA battery with a gelified electrolyte. The sulfuric acid is mixed with silica fume (gas), which makes the resulting mass gel-like and immobile. Gel batteries reduce the electrolyte evaporation and spillage common to wet cell batteries and provide better resistance to temperature, shock, and vibration.Chemically they are the same as wet batteries except that antimony in the lead plates is replaced by calcium.
ABSORBED GLASS MAT BATTERY
An Absorbed Glass Mat (AGM) battery is like a flooded lead acid battery except the electrolyte is held in glass mats as opposed to freely flooding the plates. Very thin glass fibres are woven into a mat to increase surface area enough to hold sufficient electrolyte on the cells for their lifetime. The fibres that comprise the fine glass mat do not absorb, and are not affected by, the acidic electrolyte they are in.
In a Lighting, Starting, Ignition (LSI) battery, cell components when active are Lead Dioxide, Sponge Lead, and Sulphuric Acid. These are held in grids to form positive and negative plates in parallel, with cells then connected in series Separators between the plates are made of micro porous plastic and these stop plates from shorting out.
A 12 Volt battery consist of 6 cells (2.1Volt) and each cell has plates of Lead Dioxide (Positive) and Sponge Lead (Negative). The electrolyte is a diluted solution of sulfuric acid by distilled or deionised water (60% distilled water and 40% sulfuric acid).
Specific gravity (SG) is a measurement defining the concentration of electrolyte acid. A fully charged cell at 25c would be in the range 1.240 to 1.260 (pronounced 12/40, 12/60). This is an approximate volume ratio of acid to water of 1:3. Pure sulphuric acid has SG of 1.835, water a nominal 1.0.
Always add acid to water, but never top up a battery with sulfuric acid – the ratio will change and the chemical reaction will increase can cause explosion.
The main source of lithium is from salt pans and salt lakes which produce the soluble salt lithium chloride. Australia is a main supplier of this resource and worldwide reserves are estimated at 30 million tons.
About 0.3 kg of lithium is required per kW/h of battery energy, and even with the expected increase in EVs using lithium, it is estimated that there is enough to last 1000 years.
Lithium-ion cells are considered non-hazardous and contain elements that can be recycled. The anode is a graphite structure and the cathode is layered metal oxide. Lithium is deposited between these layers. The electrolyte is composed of non-aqueous organic lithium salts and acts purely as a conducting medium and does not take part in the chemical action.
When the battery is charging, the lithium–ions move from anode to cathode and take on electrons – the number of ions determines the energy density of the cell. Discharging is this process in reverse – lithium-ions release the electrons to the anode and move back to the cathode.
Lithium movement is slower during the charging process if the temperature is cold. If the charging current is too high it will cause elemental (electroplating) lithium to be deposited on top of the anode, covering the surface. This is known as lithium plating.
Lithium polymer batteries have polymer gel instead of a liquid electrolyte.
Nickel Metal Hydride
The nickel metal hydride batteries evolved from nickel hydrogen batteries used to power satellites in the 1970s. These batteries were expensive and have low volumetric energy because they required high-pressure hydrogen storage tanks, but they did offer high-pressure energy density, higher life cycle, and long calendar life compare to anything else of the time.
The modern nickel metal hydride (NiMH) electric vehicle battery was invented by the GS Yuasa Corporation and, in these batteries, the negative electrode has been replaced with a metal compound to store hydrogen.
Metal hydride cell chemistry depends on the ability of some metals to absorb large quantities of hydrogen. Metals or alloys are used for the negative electrodes, while the positive electrode is nickel hydroxide. The electrolyte – potassium hydroxide – takes no part in the reaction but serves to transport the hydrogen between the electrodes.
NiMH batteries are known as alkaline batteries due to the pH (greater than 7) nature of the electrolyte. Potassium hydroxide works very well for this application because it does not corrode the other parts of the battery and can be housed in a steel container.
Because it doesn not take part in the chemical reaction, the potassium hydroxide concentration stays constant at any given state of charge (SOC). These factors help the NiMH battery achieve high power performance and excellent life cycle.
NiMH battery during charging: hydrogen ions (an atom or a group of atoms that has electric charge), travel from the positive electrode to the negative electrode where they are absorbed into the metal hydride material.
NiMH battery during discharge: when the battery is discharged, this process reverses, with the hydrogen ions (protons) traveling from the negative electrode back to the positive electrode. The density of the electrodes changes during the charge-discharge process, but this is kept to a minimum as only protons are exchanged during battery cycling.
Electrode stability due to minimal density changes is one reason why the NiMH battery has very good cycle life.
SOC of a NiMH battery cannot be measured using cell voltage alone. Instead, SOC is determined using a complex calculation based on battery temperature, output current, and cell voltage (NiMH cells produce 1.2volts, and modules come in packs of 6 cells). Accurate SOC measurements are critical for maximising NiMH battery performance and service life. This is worked out by the HV ECU.
High operating temperatures can lower performance and cause damage to a NiMH battery pack. Consequently, all current HEVs use air-cooling to control temperature. Sensors are mounted in various locations in the battery pack housing to send data to the module responsible for controlling battery temperature. These inputs are used to help determined battery charge rate and cooling fan operation.
In some Ford vehicles, the air conditioning system is used to circulate cool air over the battery pack.
I hope you have found this article of interest, In the next issue of Motor Trader, I will discuss battery terms and how we measure battery Energy – kW/h, Power kW, C rate, Ah, and Rated peak power.
Until next time. Paul.
9 Oct 2018