Problems and maintenance of electric vehicle batteries

1 Causes of problems with electric vehicle batteries

1.1 Caused by the battery itself

Why do we say that? In the previous issue, we learned 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. When the crystallized lead sulfate is charged, it will not only not be reduced to lead oxide, but will also precipitate and adhere to the electrode plate, causing the working area of ​​the electrode plate to decrease. This phenomenon is called sulfidation. 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 that the battery itself cannot avoid sulfation.

1.2 Reasons for battery production

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.

④There is also the problem of cold soldering during the assembly of the pole group .

The plate is prone to cold welding. Each cell of the battery has 15 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 cold welding, the capacity of the cell will decrease, and then the cell will lag behind, 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 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 grids with low-antimony alloys have lower oxygen and hydrogen evolution voltages, large battery gas output, relatively serious water loss, and the battery is more prone to sulfidation.

From the above we can see why some batteries are good and some are bad, and some manufacturers produce batteries with longer life under the same conditions of use.

1.3 Causes caused by the electric vehicle usage environment itself

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 motorcycles is shorter, because the body of electric motorcycles is too heavy, the motor power is large, and the working current is more than 8A when cruising. Some even reach 10A.

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 10-year lifespan, they only need to achieve 80 cycles of charging and discharging. However, it is common for electric vehicles to be charged and discharged more than 300 times a year. Some people may even charge several times a day, but the charging time is very short, and they use it before it is fully charged.

④Short-time charging

Since electric bicycles are transportation tools, they don’t have much time to charge. To complete a 20 ampere-hour charge at 36 or 48 volts within 8 hours, the charging voltage must be increased (usually 2.7 to 2.9 volts per cell). When the charging voltage exceeds the oxygen evolution voltage (2.35 volts) or hydrogen evolution voltage (2.42 volts) of a single cell, the battery will open the valve to exhaust gas due to excessive oxygen evolution, causing water loss, increasing the electrolyte concentration, and aggravating 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 them in time after discharging. The large amount of lead sulfate formed after discharge will sulfide and crystallize if it is not charged and reduced to lead oxide for more than half an hour.

1.4 Reasons for Electric Bicycle Production

Most cars have a speed limiter plug in their controllers. Some car manufacturers simply remove the speed limiter before shipping, which can both attract customers who value speed and reduce costs. However, such cars draw very large currents when driving at high speeds, which can 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.

1.5 Reasons for charging equipment

A widely circulated saying in the industry is: Batteries are not damaged 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 being 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 high 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 batteries, it is impossible to guarantee that they can be charged in time after each use. If there are several cases of not charging in time within a year, the sulfidation of the battery will accumulate. Some charger manufacturers exaggerate certain functions, and the efficacy of the finished product is actually not as good as it is advertised.

1.6 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."

2 Electric vehicle battery maintenance

Electric car batteries need to be replaced after a year or two. How can we extend the life of electric car batteries?

(1) Do not overload or overweight your bike (such as carrying passengers or heavy objects).

If the instrument shows low battery while riding, you should ride the bike manually, because deep discharge will greatly reduce the battery life.

(2) Pay attention to the charging method

When charging a new battery for the first time, you must charge it for a long time to ensure that it is fully charged. For lead-acid batteries, no matter how far you travel, you must charge them immediately after use. Do not wait until the battery is completely exhausted before charging. If the car is not used for a long time, you must also ensure that it is charged once a month. This can protect the battery and extend its service life.

(3) Avoid sudden braking

Frequent sudden braking will affect the braking flexibility and consume battery capacity; the vehicle speed should not be too fast. The faster the speed, the greater the damage to the battery.

(4) Do not use the motor to start the car directly when it is stationary. It is best to use your foot to start the car while assisting with power.

When going on a bridge, uphill, or against the wind, be sure to use the pedals to assist, so as to avoid impact damage to the battery, which will affect the battery's mileage and service life.

(5) Maintenance of electric vehicle batteries during use

a) When starting, going up a bridge, climbing a slope, or driving against the wind, you should use human power to avoid instantaneous large current discharge as much as possible.

b) When acceleration is required, turn the speed control handle slowly to avoid accelerating directly to the fastest gear.

c) When road conditions permit, the electric vehicle should be driven at the highest speed possible.

d) Try to avoid frequent braking and starting, and use the pedals more when the road is crowded.

e) The power indicator shows that the battery is out of power. After a period of time, you will find that the battery has a small voltage again, which is called the rebound voltage. Users should avoid riding with this rebound voltage.

(6) Charging and maintenance of electric vehicle batteries

a) Charge at night, the average time is about 8 hours, avoid overcharging and undercharging.

b) When charging, do not unplug the charger immediately when the indicator light shows full charge. Allow to float charge for another 2-3 hours.

c) Try to fully charge the battery at one time. If you really need to ride during charging, you should fully charge the battery immediately after riding.

d) Frequent charging is beneficial to the cycle life, but the chargers currently in circulation on the market have defects such as high failure rate, poor reliability and low precision. Therefore, sometimes frequent charging will affect the service life of the battery. Discharging the battery and then charging it may cause over-discharge of some cells. The charging acceptance capacity of the over-discharged battery will be greatly reduced, causing insufficient charging failure. In addition, since the charger is under heavy load for a long time after the battery is fully discharged, it is easy to damage the charger. Therefore, it is more reasonable to charge the battery once when 50-70% of the power is discharged, which is good for the battery life.

(7) When maintaining electric vehicle batteries, be careful to avoid storing them with low power.

The battery is in a low-power state when it is not charged in time after use. In the low-power state, sulfation occurs, and lead sulfate crystals are attached to the plates, blocking the ion channels and causing the battery capacity to decrease. The longer the battery is idle in the low-power state, the more serious the damage will be. Especially in summer, it is important to charge it in time. If the storage time exceeds one month, it should be recharged once.

(8) Electric vehicle batteries should also be maintained in a timely manner

The battery needs maintenance when the following conditions occur

a) Long-term charging without green light

b) The battery gets hot while charging

c) Battery capacity drops too quickly

(9) Electric vehicle battery maintenance requires regular inspection

During use, if the mileage of an electric vehicle suddenly drops by more than ten kilometers in a short period of time, it is very likely that at least one battery in the battery pack has a short circuit phenomenon such as broken cells, softened plates, and falling off of active materials on the plates. At this time, it should be checked, regenerated, and matched in time. This can relatively extend the life of the battery pack and save costs to the greatest extent.

(10) When maintaining an electric bicycle battery, you should also pay attention to the following: the battery should be firmly installed on the electric bicycle to prevent the battery from being damaged by vibration while riding; dust and dirt should be removed from the battery cover regularly, and the battery should be kept dry and clean to prevent the battery from self-discharging.