WHAT IS EFFICIENCY IN BATTERIES ?

What are the Factors Affecting Efficiency in Batteries, Which Batteries are the Most Efficient for Industrial Vehicles?

WHAT IS BATTERY EFFICIENCY ?

The value obtained by dividing the energy withdrawn in discharge by the energy given in charge is called the efficiency of the battery. In simpler terms, efficiency is how much energy can be drawn from the battery according to the amount of energy put in after charging.
There are efficiency losses in all battery types. The energy taken out of the battery after a charge is always less than the energy put in. Reactions in the electrochemistry of the cell prevent the efficiency from reaching 100%. Too fast charging and excessive discharge (deep discharge) currents reduce energy efficiency. In addition to energy efficiency, these factors seriously reduce the cycle life and cause batteries to die quickly.
Battery efficiency is especially critical for electric vehicle batteries, battery systems for industrial use and energy storage systems (ESS). Energy efficiency in batteries can be determined in two different ways; coulombic efficiency (CE) and voltage efficiency.
 
Coulomb Efficiency
Faradaric efficiency or coulombic efficiency (CE) refers to the ratio of discharge capacity and charge capacity. It is the ratio between the number of electrons transferred from one electrode of a cell to another during charging and the number transferred back during discharge. To put it more simply, you transfer 1 liter of water into another bottle and transfer it back into the same bottle. When we look at it, we still have 1 liter of water, but the amount of water transferred to the bottle is actually less than the initial amount, and the splashed water droplets are not noticeable. If at the beginning we had 10 units of water in the bottle, after the transfer we have maybe 9.98 units of water left in the same bottle. If we do this water transfer not once but 300-500 times, the amount of water in the bottle will continuously decrease. You can liken the coulombic efficiency and loss of efficiency to this example. A coulomb counter is often used to make these measurements. The higher the coulombic efficiency, the lower the electron loss and the longer the battery life-efficiency. Voltage efficiency is the voltage difference between charging and discharging a battery. All types of batteries must be charged at a higher voltage than the discharge voltage.
 
There are several critical factors that determine efficiency in batteries, which we will basically examine below.
 
Factors affecting efficiency;
- Charge Current
- State of Charge (SOC)
- Internal Resistance
- Temperature
- Aging
 
Charge Current
It is very important to keep charging currents under constant control to maintain battery efficiency and battery life. During charging, changes occur in the internal chemistry of the battery cells. High charging currents can reduce battery life and also cause loss of efficiency. When the charging current is too low, it extends the battery life but reduces the capacity. It is also inefficient. Lithium batteries are chemically more suitable for fast charging with an efficiency of over 99%, but it is important to remember that there is a limit to everything. In some cases, the ability to do business with fast charging may be more important than efficiency. In this case, instant fast charging can be done in lithium batteries.
 
Charge Status
The state of charge is a percentage expression of the energy remaining in the battery, a fuel indicator.
When the battery is in discharge state, the charge percentage and battery voltage decrease, while it increases during charging. Batteries may experience instantaneous voltage drops during demurrage or when the load is hit. As SOC decreases, voltage drops, but Lithium batteries have much less voltage drops than lead-acid batteries.
First of all, all SOC determination methods are based on estimation and do not give a clear result. Minimizing the margin of error is very important for battery efficiency. In conventional batteries, SOC is determined simply by looking at the output voltage, but it is a method with a high error rate. Because the operating ambient temperatures of the batteries, aging factor and relatively memory effect affect this measurement method.
In lithium batteries, due to the flat discharge curve, one of the most accurate ways to estimate SOC is to count current by Coulomb's Law (there are more than 20 estimation methods). Since the discharge curve is parabolic, SOC cannot be estimated from the output voltage in lithium batteries, and if it is done, very inaccurate estimates are obtained.
 
Internal Resistance
Internal resistance is affected by many factors such as aging, currents and cell chemistry. The lower the internal resistance, the higher the battery efficiency and life. Lithium batteries are the energy storage technology with the lowest internal resistance today due to their chemistry. Cell manufacturers use different methods to reduce internal resistance. One of the most well-known methods is to add Vinylene carbonate (VC) additive to the cell electrolyte.
 
Temperature
Lithium batteries should be charged and discharged at 0-45 °C. Lithium batteries produced in industrial standards and purchased from a good manufacturer are much more advantageous compared to other battery types in terms of hot-cold environment behavior and capacity loss. Battery life decreases as the temperature increases. On average, every 10°C temperature increase leads to ~10-15% capacity loss. Especially when the battery temperature rises above 45 °C, the charging current should be gradually reduced. If the battery continues to heat up, the BMS should switch the battery to high temperature protection and cut the inputs and outputs. The battery temperature usually rises during charging, regardless of ambient conditions.
 
Aging
The more aging, the lower the yield. Here, aging should not be considered only in terms of time. One of 2 different batteries with the same production date may have aged much more. Factors such as deep discharge, overcharging, temperature are among the main factors that accelerate aging.
While lithium batteries lose 20% capacity after an average of 2000-3000 cycles, lead-acid batteries become almost unusable after an average of 1000-1500 cycles and are scrapped.
 
 
Which battery is the most efficient for stackers, AGVs, AMRs and industrial vehicles?
Measurements and academic research have proven that lithium batteries have a coulombic efficiency of over 99% compared to all battery types. The efficiency of lead-acid batteries is slightly lower than 90% and around 80% for nickel-based batteries.
Efficiency decreases inversely proportional to the charging current. At 1C charging current (charging in 1 hour), the efficiency of lithium batteries is around 90%, while the efficiency of lead-acid batteries is around 50%.
 
Lithium Batteries;
 
- Compared to lead acid batteries, lithium batteries are much more advantageous and long-lasting in terms of both the number of cycles and capacity losses over time.
- They are completely maintenance-free batteries. Lead acid batteries require weekly, monthly and annual maintenance.
- Suitable for fast charging. In addition, partial / intermediate charging can be done and this has no negative effect on battery life. Lead acid batteries are charged in 8-10 hours and intermediate charging cannot be done.