Technical interpretation teaches you how to extend the life of electric vehicle lead-acid batteries

The headache of electric vehicle batteries

For electric bicycles , the development momentum is extremely rapid. In recent years, the actual output has exceeded the social ownership every year, which is an amazing data, indicating that the production and sales of electric bicycles are experiencing the most glorious history. In this prosperous market, it is not too difficult to make money. However, every advantageous industry has its "weakness". If you ask the bosses fighting in this industry what is the most troublesome problem in distributing electric bicycles, the only answer is the short battery life.

Now most manufacturers promise a one-year battery warranty, and of course the merchants advertise it this way, but six months later, problems arise, and a large number of users come back, not for secondary consumption, nor to introduce friends to buy cars, but to replace batteries. Even if the battery manufacturer fulfills its warranty commitment, users will not be satisfied. They will think that this is a quality problem of the electric vehicle, and the dealers have spent a lot of energy, but still cannot avoid complaints. Maybe this problem lies with the battery manufacturer, but the battery manufacturer also has its own difficulties. The design of the battery and the cycle discharge test show that the cycle life of the battery is indeed one and a half years or even two years. The quality is strictly controlled according to the process flow during production, but many batteries will age after half a year. Some manufacturers began to try to replace lead batteries with longer-lasting solid batteries, nickel batteries and even lithium batteries, but the high cost has lost its competitive advantage in the market dominated by office workers, and many pioneering and enterprising manufacturers have been ruthlessly defeated. We all know that the lifespan of many electronic products, such as televisions and computers, can be as long as ten years, but manufacturers only provide a one-year warranty. The lifespan of electric vehicle batteries is at most two years, but battery manufacturers insist on providing a one-year warranty to cover up the unavoidable reality that the lifespan of electric vehicle batteries is not ideal, and to stabilize the consumer confidence of users. This "biting the bullet" warranty method can only hold up for a while in the short term, but problems will always emerge over time. Therefore, many guerrilla-style manufacturers have emerged in this industry. They use a six-month cycle, do not establish a fixed customer base, and seek their own personal interests at the expense of the interests of the entire industry.

So how to extend the life of the battery, how to improve the battery usage environment and so on are all issues that everyone is very disappointed but concerned about. In order to find out how to extend the battery life, we must first figure out the failure mechanism of the battery so as to prescribe the right remedy.

Reasons for short life of lead-acid batteries in electric vehicles

Since 1859, when Frenchman Gaston Plante discovered the phenomenon of lead-acid charging and discharging, lead-acid batteries have been the most widely used products in the battery field. For example, they are used in automobiles, motorcycles, ships, airplanes, and backup power supply equipment. However, we have not heard many complaints about lead-acid batteries from these fields. However, why is it that the same product is truly "complained" when it comes to electric bicycles? Below we explain the reasons for this problem from several aspects.

1. Reasons about the working principle of lead-acid batteries

The charging and discharging process of lead-acid batteries is an electrochemical reaction process. When charging, lead sulfate forms lead oxide, and when discharging, lead oxide is reduced to lead sulfate. Lead sulfate is a substance that is very easy to crystallize. When the lead sulfate concentration in the electrolyte solution in the battery is too high or the static idle time is too long, it will "hug" together and form small crystals. These small crystals will attract the surrounding lead sulfate, forming large inert crystals like a snowball. The crystallized lead sulfate can not only no longer be reduced to lead oxide when charging, but also precipitate and adhere to the electrode plate, causing the working area of ​​the electrode plate to decrease. This phenomenon is called sulfidation, which is often called aging. At this time, the battery capacity will gradually decrease until it cannot be used. When a large amount of lead sulfate accumulates, it will also attract lead particles to form lead branches. The lead branches between the positive and negative plates will bridge and cause the battery to short-circuit. If there are gaps on the surface of the plate or the sealed plastic shell, lead sulfate crystals will accumulate in these gaps and generate expansion tension, which will eventually cause the plate to break or the shell to rupture, causing irreparable physical damage to the battery. Therefore, the main mechanism leading to the failure and damage of lead-acid batteries is the unavoidable sulfation of the battery itself.

2. Reasons for the special working environment of electric bicycles

As long as it is a lead-acid battery, it will be sulfurized during use, but lead-acid batteries in other fields have a longer lifespan than lead-acid batteries used in electric bicycles. This is because the lead-acid batteries in electric bicycles have a working environment that is more prone to sulfurization.

① Deep discharge

Lead-acid batteries used in cars only discharge in one direction when ignited. After ignition, the generator will automatically charge the battery without causing deep discharge of the battery. However, electric bicycles cannot be charged while riding, and often exceed 60% deep discharge. During deep discharge, the concentration of lead sulfate increases, and sulfidation will be quite serious.

②High current discharge

The current of an electric vehicle cruising at 20 kilometers is generally 4A, which is higher than the working current of batteries in other fields, and the working current of an overspeeding or overloaded electric vehicle is even higher. Battery manufacturers have conducted cycle life tests with 1C charging 70% and 2C discharging 60%. After such life tests, many batteries can reach a life of 350 charge and discharge cycles, but the actual effect in use is far different. This is because high current operation increases the depth of discharge by 50%, and the battery will accelerate sulfurization. Therefore, the battery life of electric three-wheeled motorcycles is shorter because the body of the three-wheeled motorcycle is too heavy and the working current is more than 6A.

③High charging and discharging frequency

Batteries used in the field of backup power supply will only discharge when there is a power outage. If there are 8 power outages a year, to achieve a lifespan of 10 years, they only need to achieve 80 charging cycles. However, it is common for electric vehicles to have more than 300 charging and discharging cycles a year.

④Short-time charging

Since electric bicycles are means of transportation, the charging time is limited. In order to complete a 20A charge at 36V or 48V within 8 hours, the charging voltage must be increased (usually 2.7~2.9V per cell). When the charging voltage exceeds the oxygen evolution voltage (2.35V) or hydrogen evolution voltage (2.42V) of a single cell, the battery will open the valve to vent due to excessive oxygen evolution, causing water loss, increasing the electrolyte concentration, and worsening the sulfide phenomenon of the battery.

⑤ Failure to charge in time after discharge

As a means of transportation, the charging and discharging of electric bicycles are completely separated. It is difficult to recharge in time after discharging. If the large amount of lead sulfate formed after discharging is not recharged for more than half an hour and is reduced to lead oxide, it will be sulfided and form crystals .

In view of the particularity of lead-acid batteries for electric bicycles, various battery manufacturers have adopted a variety of methods. The most typical methods are as follows:

①Increase the number of plates.

The original single-cell design of 5 or 6 sheets was changed to 6 or 7 sheets, 7 or 8 sheets, or even 8 or 9 sheets. The battery capacity was increased by reducing the thickness of the plates and separators and increasing the number of plates.

②Increase the specific gravity of sulfuric acid in the battery.

The sulfuric acid density of floating charge batteries is generally between 1.21 and 1.28, while the sulfuric acid density of electric bicycle batteries is generally around 1.36 to 1.38. This can provide a larger current and increase the initial capacity of the battery.

③ Increase the amount and proportion of lead oxide, the active material of the positive plate.

Increasing lead oxide increases the electrochemical reaction substances involved in discharge, which increases the discharge time and increases the battery capacity.

Through these measures, the initial capacity of the battery meets the capacity requirements of electric bicycles, especially the high current discharge characteristics of the battery are improved. However, as the number of plates increases, the capacity of sulfuric acid decreases, and the battery heats up, resulting in a large amount of water loss. At the same time, the probability of micro-short circuits and lead bridges in the battery increases. Increasing the specific gravity of sulfuric acid increases the initial capacity of the battery, but the sulfidation phenomenon becomes more serious. One of the most basic principles of sealed batteries is that after the positive plate releases oxygen, the oxygen goes directly to the negative plate, is absorbed by the negative plate and reduced to water. The parameter for assessing this technical indicator of the battery is called "sealed reaction efficiency", and this phenomenon is called "oxygen cycle". In this way, the battery loses very little water, achieving "maintenance-free", that is, no need to add water. For this reason, the capacity of the negative plate is required to be larger than that of the positive plate, also known as negative transition. Increasing the active material of the positive plate will inevitably reduce the negative transition, deteriorate the oxygen cycle, increase water loss, and cause sulfidation. Although these measures have increased the initial capacity of the battery, they will cause water loss and sulfidation, and water loss and sulfidation will promote each other, and the final result is the sacrifice of battery life.

There is also the problem of cold welding in the pole group assembly. The place where cold welding is easy to occur is the pole plate. Each cell of the battery has 15 pole plates, which means 15 welding points. A battery has 6 cells, which means 90 welding points. A group of batteries consists of 3 12V batteries, which means 270 welding points. If a welding point has a cold welding, the capacity of the single cell will decrease, and then the single cell will form a backward battery, causing the entire battery to lag behind. The battery will form a serious imbalance, causing this group of batteries to fail prematurely. Even if the cold welding is controlled to one in ten thousand, on average, one group of batteries will have a cold welding for every 37 groups of batteries, which is absolutely not allowed. For batteries with lead-calcium alloy grids, calcium will be precipitated during welding to cover up the cold welding problem. In this way, many battery manufacturers would rather use low-antimony alloy grids instead of lead-calcium alloys. The low-antimony alloy grid has a lower voltage for oxygen and hydrogen evolution, a large amount of battery gas, relatively serious water loss, and the battery is more prone to sulfidation.

Reasons for the production of electric bicycles

Most car controllers have a line loss plug, and many dealers remove the speed limiter to attract customers. Some car manufacturers simply remove the speed limiter before leaving the factory, which can not only attract customers who value speed, but also reduce costs. Such cars have very large currents when driving at high speeds, which will seriously shorten the battery life.

The minimum protection voltage of a 12V lead-acid battery is 10.5V. If it is a 36V battery pack, the minimum retention voltage is 31.5V. At present, the undervoltage protection voltage of the controller used by most car manufacturers is also 31.5V. On the surface, this is correct, but in fact, when the 36V battery pack only has 31.5V voltage, due to the capacity difference of the battery, there will definitely be a battery voltage lower than 10.5V, and the battery is in an over-discharge state. At this time, the capacity of the over-discharged battery drops sharply, and the damage to the battery at this time affects not only the single battery, but also the life of the entire battery pack. In fact, after the battery voltage is lower than 32V and reaches 27V, the increased driving range is less than 2 kilometers, but the damage to the battery is very large. As long as this happens 10 times, the battery capacity will be lower than 70% of the nominal capacity. In addition, some users find that after the battery is under-voltage, after 10 minutes, the battery is no longer under-voltage, so they use power to drive again, which is more damaging to the battery, and most car manuals do not warn users. At present, most controllers have adjustable potentiometers inside, and the vibration drift of this adjustable potentiometer is relatively serious. In the price competition, facing the user group that pays more attention to the appearance of the car, few products use vibration-resistant precision multi-turn potentiometers, so it is not surprising that such controllers drift after vibration.

Reasons for charging equipment

A widely circulated saying in the industry is: Batteries are not broken by use, but by charging. In order to meet the short-term high-capacity charging of electric bicycle batteries, the constant voltage value has to be increased to 2.47V~2.49V in the three-stage constant voltage and current limiting charging. In this way, it greatly exceeds the oxygen evolution voltage of the positive plate and the hydrogen evolution voltage of the negative plate of the battery. In order to reduce the indication of charging time, some charger manufacturers increase the current of constant voltage to floating charge, so that after the charging indication is fully charged, it is not fully charged, and it is compensated by increasing the floating charge voltage. In this way, the floating charge voltage of many chargers exceeds the single cell voltage of 2.35V, so that a large amount of oxygen is still evolved during the floating charge stage. The oxygen cycle of the battery is not good, so it is constantly exhausting during the floating charge stage. The high constant voltage value guarantees the charging time, but at the expense of water loss and sulfidation. The low constant voltage value makes it difficult to guarantee the charging time and the amount of charge. On the basis of improving the battery grid alloy, increasing the gas evolution potential, improving the oxygen cycle performance, and improving the sealing reaction efficiency, the maximum charging voltage is controlled below 2.42V, that is, below the hydrogen evolution potential. Doing so will inevitably lead to a longer charging time. This requires adding a depolarizing negative pulse under the state of high current charging (current limiting charging) to improve the battery's charging acceptance, charge more electricity during high current charging, and shorten the charging time. 70% 2C current charging is when the battery has a relatively large charging acceptance, and the battery is charged with a large current, which causes less damage to the battery. The battery is basically not higher than the serious hydrogen evolution voltage. Once it exceeds the hydrogen evolution voltage, the battery will also lose water quickly. When using this type of charger, continuous charging and discharging must be used. If charging is stopped for a few days in the middle, the battery will produce more serious sulfidation and fail prematurely. When users use the battery, it is impossible to guarantee that it can be charged in time after each use. If it is not charged in time several times within a year, the battery sulfidation will accumulate. Most charger manufacturers say that car manufacturers do not accept chargers that can guarantee battery life due to price factors. It should be admitted that this is the case for most small businesses, but it is true that large and well-developed companies cannot buy good chargers even if they pay high prices. Some charger manufacturers exaggerate certain functions, and the efficacy of the finished products is not as good as they advertise. There are also many functions that are just for selling concepts and have limited effectiveness. Other reasons

Many batteries can achieve relatively good results in single-cell testing. However, for series-connected battery packs, due to the differences in capacity, open-circuit voltage, state of charge, and degree of sulfation, this difference will be magnified in the series-connected battery pack. Single cells in poor condition will affect the entire battery pack, and its life will be significantly reduced.

There are many steps from the time the battery is charged on the production line to the time the user purchases the car and uses it. The interval may even be as long as several months. During this period, because the battery is not recharged, a large amount of lead sulfate produced by self-discharge accumulates and crystallizes. The new battery that the user has just bought may be aged or even reported as depreciated .

When battery manufacturers are performing warranty work, they do not completely eliminate recycled batteries. After the batteries are returned, the battery manufacturers will re-test the batteries for charge and discharge, and often find that more than 60% of the single cells do not meet the return conditions. The reason is that in the series battery pack, individual batteries lag behind, resulting in the decline of the function of the entire battery pack, which causes the entire pack to be returned. Many battery manufacturers match, replenish water, remove sulfur, and package the returned batteries before providing them to users again to increase the effective service life of the batteries, reduce the scrap rate, and reduce the losses of the battery manufacturers' partial claims. Therefore, many dealers have already felt that the batteries provided by the manufacturers are obviously "inferior to the previous generation."

How to solve the problem of battery sulfation

To reduce the sulfidation of the battery and extend the service life of the battery, we must first improve the working environment of the electric bicycle. Reduce the weight of the vehicle body, remove unnecessary decorative parts, set a proper speed limit, do not carry heavy objects, and recharge the electric bicycle when it is not used for a long time. It is best to charge it in time after each discharge, and do a good job of undervoltage protection to prevent the battery from over-discharging. For the nominal 24V undervoltage protection, it should be set at 21.5V~22V, for the nominal 36V undervoltage protection, it should be selected 32.5V±0.5V, and for the nominal 48V, it should be set at 44V~45V. Such a voltage only reduces the endurance by less than 2 kilometers, but it can effectively extend the service life of the battery. Go to a professional maintenance point regularly every three months to inspect the battery and replenish water in time. These methods are simple and easy, with low economic costs, but it is difficult to strictly abide by them. Therefore, special equipment can be used for desulfurization maintenance. These methods are:

1. Use desktop rapid desulfurization equipment

台式快速除硫设备的工作原理是高电压大电流脉冲充电，通过负阻击穿消除硫化。这种方法速度快，见效快，可以获得暂时的消除硫化的效果，但是，高电压大电流能击除硫也能除活性物质,在消除硫化中带来严重失水和正极板软化的问题，对电池产生致命的损伤，经过这类设备除硫两次后的电池基本都会报废。另外，目前的专业维修点进行一次除硫收费基本在60~80元之间，最多能延长电池寿命半年，并没有为用户来显著的经济利益。目前,市场上的专业电池维护店主都已经明白了这种方法的危害.于是,又出现了脉冲放电除硫的设备,其实,根本原理并没有变,只是从恒高压恒大电流变成了瞬时峰值高压,还是会损伤极板活性物质,用过这类产品的朋友应该很清楚这点。

2. Choose a desulfurizing charger

At present, there are three working principles for desulfurizing chargers. One is similar to the working principle of desktop rapid desulfurizing equipment, which uses high voltage and high current pulse charging to remove sulfur through negative resistance breakdown. As mentioned above, this method will cause fatal damage to battery life and has been rejected by the market. The second is to use fast pulse front charging and discharging pulses, using instantaneous peaks to interfere with the sulfation of the battery during the charging process. The other is to periodically use a 10% to 20% overcharge method to reduce the lead sulfate crystals in the battery. Both chargers can remove sulfur during charging, but will cause undercharging or overcharging, and ignore the fact that the battery discharge process is the most important sulfation process. Therefore, the effect is not ideal. Most users will give up this repeated investment desulfurization method after they have a charger equipped with an electric vehicle.

3. Use online lead-acid battery extender

The online lead-acid battery extender is connected in parallel with the battery to prevent and eliminate sulfation for 24 hours. This method is relatively slow and takes a long time to repair, often more than 120 hours, but it can prevent and eliminate sulfation during both the charging and discharging process, and the repair effect is very good. Because of the low voltage and low current, the extender will not have a strong impact on the battery plate to cause water loss and softening. This is a maintenance method that users can persist with a one-time investment, especially for new batteries with good quality. It can extend the battery life by 2 to 5 times. Moreover, once invested, it can accompany the electric automatic car. The extender can continue to be used when the battery is replaced next time, which can save users a lot of economic costs. If the user replaces the battery once a year, a set of batteries costs 280 yuan, and the user will spend 2,800 yuan on battery replacement in 10 years. According to conservative calculations, if the extender is used to extend the battery life by twice, nearly half of the battery cost can be saved in 10 years.

This approach is of great significance. First, it brings tangible economic benefits to users and reduces their troubles. Second, it improves the reputation of the car manufacturer and provides conditions for expanding production. Third, it solves the difficulty of battery warranty for electric car dealers, reduces complaints, improves credibility, and increases profit points. At the same time, it also adds sales solutions to facilitate transactions in store sales. Fourth, it can greatly reduce the cost of claims for battery manufacturers. Fifth, it improves the image of electric bicycles and expands the development of the overall electric bicycle market. Sixth, it improves the utilization rate of batteries , which is beneficial to environmental protection.