The traditional lead-acid battery is a tried and tested equipment for storing and transmitting electricity.
However, the Li-ion (lithium-ion) batteries that power your laptop, smartphone, and even your car are now a viable energy solution for forklifts.
What advantages does this technology offer?
First, lithium-ion batteries don’t require the regular downtime that lead-acid batteries require for replacement or cleaning, and maintenance.
These batteries have a longer life span and can be easily charged for short periods of time, with the ability to replenish the charge without having to remove the battery from the forklift.
In addition, they are virtually maintenance-free and do not emit hazardous gases while charging, which means you don’t need to find a separate room to charge the battery.
Key factors when choosing a battery
A comparison of lithium and lead batteries
Today, there are a wide variety of ways to store energy for stationary power systems. These include supercapacitors, compressed air, hydro storage stations, flywheels, and rechargeable batteries. Each technology has its own advantages and disadvantages, which determine its areas of application.
This article examines 2 battery technologies that use chemical conversion to store energy: lead-acid and lithium-ion batteries.
Lead-acid and nickel (alkaline) batteries have been the dominant types of batteries in off-grid renewable energy systems until recently. Nickel batteries (NiCd, NiMH) have almost disappeared from the market due to their high price and environmental damage. Lead-acid batteries have been in use for over 100 years and will be one of the mainstream batteries for the foreseeable future due to their low price and mass production.
Lithium batteries are also a well-established technology and are widely used in gadgets and portable electronic devices. They have yet to find their place in large power systems. They are already widely used in systems where volume, weight, temperature sensitivity, and low maintenance are more important than the initial cost. The diagram below shows the types of batteries used in renewable energy systems.
Basic battery concepts
A fully charged cell has a potential difference between the anode and cathode of about 2 V. During discharge, electrons flow through an external electrical circuit while chemical reactions inside the battery balance the charges. The figure shows the chemical states of a fully charged and a fully discharged lead-acid battery.
Lead-acid battery states
Lead-acid batteries can be divided into 2 categories: liquid electrolyte batteries and sealed batteries (SLA or VRLA). In terms of chemistry, these categories are identical. The difference lies in the technology of performance, which also affects performance characteristics. Liquid electrolyte batteries require the following 3 conditions, which are not required for sealed batteries:
- A defined position to prevent electrolyte leakage
- A ventilated room to remove gases generated during charging and discharging
- Regular maintenance of the electrolyte.
Because of these differences, the complexity and cost of maintenance of liquid electrolyte batteries must be considered, which can offset their lower cost. Sealed batteries are divided into two groups: gel and AGM (Absorbed Glass Mat). They differ in the state of the electrolyte. In gel batteries, a thickening agent is added to the electrolyte, which turns the electrolyte into a gel. An AGM battery uses a glass “sponge” to bind the liquid electrolyte.
Within each category of lead-acid batteries, a distinction is made between “deep cycling” batteries and “buffer mode” batteries with a shallow depth of discharge. “Buffer” sealed batteries are typically used in automobiles as starter batteries – they must deliver powerful pulses of energy for a short time. Stationary power systems use “deep-discharge” batteries, which usually discharge at relatively low currents but over long periods of time.
The concept of lithium-ion batteries was developed in the 1970s. They became widespread in the 1990s. The working principle is that lithium ions shuttle back and forth between the anode and cathode during charge and discharge. The illustration shows the construction of the LiCoO2 variety of lithium-ion batteries.
Reactions in a lithium-ion battery
The characteristics of the chemical processes at the anode, cathode, and electrolyte affect the efficiency of the battery. The design of the lithium-ion battery cell is also affected.
Most commonly, the manufacturer changes the shape and composition of the cathode: they can be LFP, NCM, NCA, Cobalt, or Manganese. More than 90% of lithium anodes consist of graphite; silicon and titanium are used much less frequently.
The electrolyte is usually in liquid form, but in “lithium polymer” batteries the electrolyte is in an absorbed form in a polymer membrane.
This allows the use of a “pouch” instead of the metal case that is normally used with liquid electrolytes in cylindrical and prismatic cells to limit the volume of the battery.
Despite the differences in chemical processes, lithium-ion batteries can be divided into 2 groups: lithium iron phosphate (LFP, LiFePO4) and metal oxide (NCM, NCA, Cobalt, Manganese – Lithium manganese oxide (LiMn2O4) and Lithium nickel and cobalt manganese oxide (LiNiMnCoO2).
LiMn2O4 and LiNiMnCoO2 batteries are medium-sized lithium batteries in terms of size, weight, safety, life, and cost.
RC Lithium Polymer Batteries (RC LiPo). LiPo batteries are the smallest, cheapest, lightest, and most powerful lithium batteries. Their disadvantages include their short lifespan and tendency to ignite into giant fireballs, so we are not considering them in this article.
All lithium-ion batteries can withstand a deep discharge. Battery life increases significantly if the depth of discharge is no more than 80% of rated capacity.
Comparison of lithium-ion batteries with lead-acid batteries
The typical allowable discharge rate indicates that lead-acid batteries must have a higher rated capacity than lithium batteries to store the same amount of energy.
Because of the great differences in technical and economic characteristics, choosing the best type of battery depends on the specific situation. Below we will discuss these parameters in more detail.
Comparison in terms of number of cycles
Lithium-ion batteries have many more possible charge-discharge cycles, especially for deep discharges. The difference also increases with increasing temperature. The number of cycles for each type of battery can be increased by limiting the depth of discharge (DoD), discharge current, and temperature, but lead-acid batteries are generally much more sensitive to these factors.
The figure shows the dependence of the residual capacity on the number of charge-discharge cycles for different types of batteries at moderate temperatures (about 25°C).
Service life in cycles at moderate operating temperatures
Since the number of cycles depends on the depth of discharge, the figure shows different curves for different depths of discharge of lead-acid batteries. It can be seen that AGM batteries must be discharged by 30% in order to be comparable in terms of the number of cycles to lithium batteries with a 75% depth of discharge. This means that the rated capacity of AGM batteries must be about 2.5 times that of lithium batteries in order to provide a comparable service life and amount of stored energy.
Number of cycles, hot climate
In hot climates, at average temperatures of about 33°C, the differences between AGM and lithium-ion batteries are magnified. The number of cycles for lead-acid batteries is reduced by half, while it remains stable up to 45°C for lithium-ion batteries. The illustration below shows the difference.
Lead-acid batteries give up less capacity as the discharge current increases. This must be taken into account when designing the power system. The shorter the discharge time, the lower the capacity of the SK battery.
For example, a 100 Ah VRLA battery will only give 80 Ah on a 4-hour discharge. On the other hand, a 100 Ah lithium-ion battery will give 92 Ah even with a 30-minute discharge. Figure shows that lithium-ion batteries are especially beneficial in systems where the discharge time is less than 8 hours.
Dependence of the capacity output on the discharge current
Both types of batteries reduce their useful capacity when the temperature drops. The graph below shows the change in the capacity of different batteries as temperatures drop to -20°C. As you can see lithium-ion batteries lose significantly less capacity. The capacity loss of lead acid batteries is affected by the discharge rate, so the graph shows two different discharge rates, a 2-hour, and a 10-hour rate.
Dependence of battery capacity on temperature
Lead-acid batteries are not as environmentally friendly as lithium-ion batteries. They require many times as much raw materials as lithium-ion batteries to produce the same useful energy storage capacity. The environmental impact of raw material extraction for lead-acid batteries is much higher; lead production also consumes a lot of energy, which in turn releases pollutants into the environment. Despite the high toxicity of lead, finished lead-acid batteries pose no danger to human health. On the positive side, more than 97% of the lead from used batteries can be recycled.
The manufacturing process of lithium batteries also has some environmental concerns. The main components of a lithium battery require the mining of lithium carbonate, copper, aluminum, and iron. Lithium mining is particularly resource-intensive, but fortunately, the lithium in the battery makes up a smaller fraction of its total mass. The environmental impact of aluminum production and the copper required for its production is much higher. The lithium-ion battery recycling industry is in its infancy right now, but it is already clear that much of the material can be recycled and used to make new batteries.
Both lead-acid and lithium-ion batteries can go into a “temperature spike” where the temperature of the cell rises rapidly and a release of electrolytes, harmful gases, and even a fire can occur. This is more likely to happen with lithium-ion batteries because of their much higher energy density. Numerous safety measures are taken during battery production to prevent the onset of overheating, but it has not yet been completely eliminated.
When considering replacing batteries in an existing system with a different type of battery, the most important factor is voltage. The figure below shows the voltage of 3 batteries with a nominal voltage of 24V. The rated voltage of the LiNMC battery is technically 25.9V and for the LFP it is 25.6V.
The graph shows that the voltage of lithium-ion batteries agrees well with that of similar lead-acid batteries over almost the entire operating voltage range. Lithium batteries will require equipment that can operate at higher voltages. Most modern equipment can handle these voltages and has adjustments for both lead and lithium batteries.
Battery voltage comparison
Lead-acid and lithium-ion batteries have advantages and disadvantages. There are many factors to consider when deciding which technology to choose – initial cost, service life, weight, volume, temperature sensitivity, convenience and cost of maintenance, product availability, etc. There is no definite answer right now, but the lower cost and availability of lithium batteries increase their appeal and make them the choice in a number of situations, especially in hot climates and when deep discharging and charging daily.
The safety of lithium batteries is significantly better than lithium iron phosphate batteries, which is why they are the most widely used in stationary power systems.
Why overpay for a lithium battery?
When you choose a battery, the first thing that may come to your mind is to use a lead-acid battery. The reasons for this choice are at first glance obvious – low cost and simplicity of design (no BMS control board required).
However, there are weighty arguments that may encourage you to abandon the use of lead-acid batteries in favor of lithium-ion battery. Let’s break them down in order.
1. Weight Difference
It’s no secret that leads batteries weigh more than lithium-ion batteries, but this is often overlooked, prioritizing the price advantage. However, for an electric bike, low weight may be more important than price, since you will be lifting it up and down stairs (maybe even every day) and sometimes loading it into a car for transportation.
Also, the extra weight is not good for acceleration and braking dynamics. More weight – more inertia – more wear on the brake pads. Also, the use of heavy lead batteries increases the load on the frame and suspension system of the bicycle.
Consider, for example, a 36 V battery with a capacity of 7-7.5 Ah. The lead-acid version is three 12 V 7Ah batteries connected in series, while the lithium-ion version is 30 18650 cells of 2500 mAh and a BMS board, weighing about 100 grams.
2. Energy density
Li-ion batteries have several times the energy density of lead batteries. In our case, the lead-acid battery has an absolute capacity of 252 Wh (7 Ah * 36 V) at a weight of 6.9 kg. That is, the energy density is 36.5 Wh/kg.
At the same time, a lithium-ion battery with an absolute capacity of 270 Watt (7.5 Ah * 36 V) weighs 1.6 kg, that is, its energy density is 168 Watt/kg, which is more than 4 times higher than that of a lead battery.
3. Service life and maintenance
When using the battery its capacity gradually decreases, and the rate of this reduction depends on several factors, including the depth of discharge, operating temperature, and others.
The service life of the battery, subject to the rules of its storage and operation, is determined by the number of charge-discharge cycles. For lead-acid batteries, this characteristic is in the range of 200 to 1000 cycles, for lithium-ion batteries – in the range of 1000 to 4000 cycles.
In addition, lead batteries have a higher self-discharge and require periodic maintenance – recharging and adjusting the electrolyte density. Li-ion batteries are virtually maintenance-free because the balancing function, that is, the alignment of the cell voltages, is performed by a battery management card (BMS, Battery Management System).
The efficiency of lead batteries in correct operation is approximately 80-90%. In other words, if it takes 100 watts of energy to charge the battery, it will give only 80-90 watts on discharge.
With lithium-ion batteries, this parameter reaches 95-97%, which means that practically all the energy expended during charging will be returned when the load is connected.
5. Charge Current and Charge Time
The optimum charging current for lead batteries is considered to be a current of 10% of the battery capacity in Ampere Hours (0,1C), i.e. in the above example with a capacity of 7 Ah the optimum charging current is 0,7 A. For fast charging, a current of 0.2C is allowed (in our case 1.4A). In both cases, this current should be reduced towards the end of the charging process.
For Li-ion batteries, the charging current is between 0.2C to 1C (in the case of fast charging), i.e. for a battery with a capacity of 7 Ah, this will be between 1.4 A and 7 A. Charging is carried out first by a constant current, and then, when reaching the upper voltage, the current is gradually reduced.
In most cases, it takes 8-16 hours to fully charge a lead battery, while it takes only 2-4 hours to charge a lithium-ion battery. This time advantage makes it possible to significantly recharge a lithium-ion battery even during a stop at a cafe for lunch if you go on a long trip.
6. Effects of high temperatures
When operated at high temperatures, the battery degrades quickly. But what is meant by high temperature and where does it come from?
When a load, such as a bicycle motor, is connected to a battery, current starts flowing through the battery. Each battery has its own internal resistance. Accordingly, the current flowing leads to the gradual heating of the battery – depending on the current strength, of course. The heavier the load (higher the current), the faster the heating occurs.
If the load is too high for a given battery, it heats up quite quickly and the process of accelerated degradation begins. The result of this process is a significant reduction in battery capacity, and as a consequence, the need for battery replacement.
Let’s summarize all of the above. If you have already decided to use a lead-acid battery on an electric bike, use it. But if, after all, the arguments in this article have changed your mind, and inclined you to buy a lithium-ion battery, you will definitely be satisfied with its significantly lower weight, compact size, no need for maintenance, longer life, and charging speed.