Knowledge of Electric Vehicle Chargers and Storage Batteries

Classification of Chargers:

Chargers may be categorised into two main types based on whether they incorporate a mains frequency (50Hz) transformer. Freight tricycle chargers typically employ transformers with mains frequency, resulting in larger, heavier units that consume more power yet offer reliability and affordability. Electric bicycles and motorcycles, conversely, utilise so-called switching-mode power supply chargers, which are more energy-efficient and cost-effective but prone to failure. 
The correct procedure for switching-mode chargers is: during charging, connect the battery first, then the mains supply; upon full charge, disconnect the mains supply before removing the battery plug. Removing the battery plug during charging, particularly when the charging current is high (indicated by a red light), can severely damage the charger.
Common switching-mode chargers further subdivide into half-bridge and single-pulse types. Single-pulse chargers are categorised as forward or flyback designs. Half-bridge designs, though higher in cost, offer superior performance and are frequently employed in chargers incorporating negative pulses. Flyback types, being more economical, hold a significant market share.

Regarding Negative Pulse Chargers
Lead-acid batteries have a history spanning over a century. Initially, global practice largely adhered to traditional views and operating procedures: charging and discharging at a rate of 0.1C (where C denotes battery capacity) was believed to prolong lifespan. To address rapid charging challenges, Mr. Max of the United States published his research findings globally in 1967. This involved charging with pulse currents exceeding 1C rate, interspersed with discharge intervals during charging pauses. Discharging facilitates polarisation reduction, lowers electrolyte temperature, and enhances plate charge acceptance capacity.
Around 1969, Chinese scientists successfully developed multiple rapid charger brands based on Mr. Max's three principles. The charging cycle proceeded as follows: high-current pulse charging → interrupting the charging circuit → brief battery discharge → halting discharge → re-establishing the charging circuit → high-current pulse charging...
Around the year 2000, this principle was adapted for electric vehicle chargers. During charging, the circuit remained uninterrupted, employing a low-resistance short-circuit to momentarily discharge the battery. As the charging circuit remained active during short-circuiting, an inductor was series-connected within it. Typically, the short-circuit lasts 3–5 milliseconds within one second (1 second = 1000 milliseconds). As the current within the inductance cannot change abruptly, the brief short-circuit duration protects the power conversion section of the charger. If the charging current direction is termed positive, the discharge naturally becomes negative. Consequently, the electric vehicle industry coined the term ‘negative pulse charger,’ claiming it could extend battery life and so forth.

Regarding Three-Stage Chargers
In recent years, electric vehicles have widely adopted so-called three-stage chargers. The first stage is termed the constant current stage, the second the constant voltage stage, and the third the trickle stage. From an electronic engineering perspective, these are more accurately described as:
- First stage: Charge current limiting stage
- Second stage: High constant voltage stage
- Third stage: Low constant voltage stage During the transition between the second and third stages, the panel indicator lights change accordingly. Most chargers display a red light during the first and second stages, switching to green during the third stage. This transition between stages is determined by the charging current: exceeding a certain threshold activates the first and second stages, while falling below it triggers the third stage. This threshold current is termed the transition current or switching current. 
Early chargers, including those supplied with branded vehicles, though exhibiting indicator changes, were actually constant-voltage, current-limited chargers rather than true three-stage units. Typically, these maintained a single stable voltage value around 44.2V, which was adequate for the high-specific-gravity sulphate batteries of the era.
Regarding the three key parameters of three-stage chargers 
The first critical parameter is the low constant voltage value during the trickle phase. The second is the high constant voltage value during the second phase. The third is the transition current. These three parameters are influenced by the number of batteries, their capacity (Ah), temperature, and battery type. For ease of reference, we shall illustrate using the most common three-stage charger for electric bicycles (three 12V 10Ah batteries in series): 
First, the low constant voltage value during the trickle phase, with a reference voltage of approximately 42.5V. A higher value causes battery dehydration, increasing the risk of overheating and deformation; a lower value hinders full charging. In southern regions, this value should be below 41.5V; for gel batteries, it should be below 41.5V, and slightly lower still in southern areas. This parameter is relatively strict and must not exceed the reference value. 
Next, consider the high constant voltage value in the second stage, with a reference voltage of approximately 44.5V. A higher value facilitates rapid full charging but may cause battery dehydration, with the current failing to decrease sufficiently in the later charging phase, resulting in battery overheating and deformation. A lower value hinders rapid full charging but facilitates transition to the trickle stage. While not as strictly regulated as the first value, it should still not be excessively high. 

Finally, regarding the conversion current, the reference value is approximately 300mA. A higher value benefits battery longevity by reducing thermal deformation, though it hinders rapid charging. A lower value (for laymen) facilitates charging but, due to prolonged high-voltage charging, may cause battery dehydration, leading to thermal deformation. Particularly when individual cells malfunction, if the charging current cannot be reduced below the threshold current, it may damage otherwise healthy cells. The specified reference range permits deviations of ±50mA or even ±100mA, but must not fall below 200mA.
Currently, numerous low-cost flyback chargers are available on the market featuring high constant voltage values of 46.5V, low constant voltage values of 41.5V, and transition currents exceeding 500mA. 
For a charger handling four 12V batteries (48V total), the first two parameters are calculated by dividing the aforementioned voltage reference values by three and multiplying by four. The high constant voltage is approximately 59.5V, and the low constant voltage is approximately 56.5V.
If the battery capacity exceeds 10Ah, the third parameter (current value) should be appropriately increased. For example, a 17Ah battery may require up to 500mA.

Battery failure mechanisms: water depletion; sulphation; anode softening; and shedding of active material from the anode.

Overcharge recovery. If battery lifespan is not a primary concern, this recovery method yields immediate results. Deep discharge and recharge cycles can increase battery capacity, a globally recognised fact. However, this may compromise battery lifespan. Numerous posts on this site focus solely on how overcharging can convert surface α-lead oxide to β-lead oxide on the positive plate, thereby boosting capacity. Employing this approach during repair risks causing irreversible capacity loss. Some batteries returned to manufacturers for refurbishment have been treated using such methods. 
Based on personal practice, I believe that effective over-discharge and over-charge restoration can yield excellent results when strictly limiting current and duration, drawing parallels with the plate formation process during manufacturing. The key lies in discernment, not applying reverse charging uniformly across all cases. Consider a recent case: whilst visiting my acquaintance Lao San's shop, I encountered four 17Ah batteries recently removed from an electric motorcycle. They intended to sell them (for 120 yuan) to a used battery collector. I advised against disposal, suggesting repair was feasible, and took them back for assessment. A brief summary follows: 
Example Three: The four aforementioned batteries were manufactured in Changxing, Zhejiang, though not by Tianneng. As they were freshly removed, no additional testing or charging was performed. Open-circuit voltages were as follows: Unit 1: 13.42V; Unit 2: 13.36V; Unit 3: 13.18V; Unit 4: 12.4V. Evidently, they were low on electrolytes. After opening the casing, each cell in the first three batteries received 6ml plus an additional 4ml of electrolyte, while cell 4 received 6ml plus an extra 2ml. After resting for two hours, charging commenced at 10A initially, reduced to 3A after two minutes, then switched to a step-down mode after half an hour. Gas production gradually began. Cells 1–3 exhibited relatively consistent gas production across all compartments, while cell 4 showed gas production in five compartments at roughly the same time. However, after gas production started, the compartments near the anode still did not produce significant amounts of gas. Charging ceased. Capacity testing revealed that cells 1–3 approached the new condition, while cell 4 yielded only 1.5Ah. Add 4 millilitres of water to each cell of cells 1–3, then charge in steps until all cells produce gas. Charge cell 4 separately for one hour, then discharge at 5A. Monitor terminal voltage: it took 20 minutes to drop from 13.2V to 10.5V, and less than 5 minutes to reach 8.32V. Continue discharging at 5A, maintaining around 8.15V for one hour before stopping the test. Why halt? The conclusion emerged: the cell adjacent to the anode was defective, with a capacity of approximately 1.5Ah. A brief theoretical explanation: the 20-minute drop from 13.2V to 10.5V demonstrated the faulty cell (already significantly below 1.7V) possessed less than 1.5Ah capacity. Continuing the 5A discharge, the faulty cell dropped to 0V. The remaining five healthy cells (10V) reverse-charged the faulty cell. When the faulty cell reached nearly 2V in reverse charge, it stabilised for an extended period. The battery terminal voltage equalled the sum of the five healthy cells minus the reverse voltage of the faulty cell: 10V - 2V = 8V. Further discharge is unnecessary, as it would damage the five good cells. To identify the faulty cell: these batteries have significantly smaller electrolyte filling ports than 10Ah units. Using a homemade lead-plated tool, the faulty cell can be determined within seconds. In this case, five cells exhibited gas evolution, while the cell near the anode did not. Testing confirmed this cell was faulty, with partial cell separation. Isolated treatment restored this cell to 10Ah capacity. The repair is now complete. Cells 1–3 exhibit near-new capacity, while Cell 4 reaches 10Ah (the five functional cells collectively match the near-new capacity of Cells 1–3).

Method for checking sulphation without opening the cover
Here is a method to determine sulphation without opening the battery: Charge the battery using an adjustable constant current source set to approximately 0.05C. Note that sulphation is indicated by the following conditions. Taking a 12V battery as an example: the initial voltage exceeds 15V (with a greater deviation indicating more severe sulphation), and as charging time increases, the voltage decreases, approaching 15V. If switched to constant voltage charging, the current will show an increasing trend. This is based on my practical experience, whereas standard literature typically only mentions symptoms like excessive heat generation, premature gas evolution, and reduced capacity. I have demonstrated this diagnostic method on-site to several visiting university students specialising in the field, comparing lead-acid batteries with varying degrees of sulphation. The adjustable constant-current source is my 1978 design, the ‘New Star Multifunctional Charger’, included in the appendix of my textbook Black and White Television Installation. Originally utilising a 36V transformer with discrete linear components, it was later upgraded to an integrated circuit linear design with electronic switch-controlled constant current.

Assessing water loss without opening the casing

Determining water loss without opening the cover requires two simultaneous conditions: 1) The open-circuit voltage of a 12V battery exceeds 13.2V. 2) Reduced capacity. Even primary school pupils can grasp these principles. The underlying theory involves two key points: 1) Open-circuit voltage correlates with sulphuric acid concentration; water loss increases acid concentration, raising terminal voltage. 2) Water loss lowers the electrolyte level, reducing the amount of reacting material and diminishing capacity. Further clarification on conditions: The aforementioned values refer to the open-circuit voltage of a 12V electric vehicle battery half an hour after charging. For automotive batteries, the values should be lower. Even for electric vehicle batteries, the brand matters—for instance, Panasonic batteries have lower values due to their lower sulphuric acid specific gravity compared to Zhejiang Changxing batteries. It also states that one should not be dogmatic: for example, a battery with seemingly standard voltage but low capacity typically has five cells lacking water, with one cell partially detached.

Irreparable Standards
Irreparable standards (for batteries with normal use and lead sulphation):
1.  Irreparable if exhibiting external deformation, cracking, or leakage.
2.  Irreparable if showing internal breakdown, mechanical damage, or overcharged plates turning carbon black; characteristic symptoms: voltage rises rapidly during charging and drops significantly after standing.
3.  Irreparable if exhibiting poor CEL (Cell Error Light), single-cell failure, or internal self-discharge. (For removable batteries on forklifts, individual cells may be replaced and the battery restored.)