How to design a good battery charger

update:2017/5/24, view:

How to design an excellent battery charger solution for your battery pack

 

 In the rechargeable battery chemical battery or physical battery during the use of the charger is the successful use of its important device, so rechargeable battery is born, the charger is a key issue, because the charger directly affects the battery directly An important technical indicators: rechargeable battery capacity; rechargeable battery can be used, that is, life. However, until the sixties, the charger technology has not been developed by leaps and bounds, the commonly used method is mainly constant current or constant voltage charging method, the charging effect is not too ideal. This situation until the sixties based on the principle of the minimum blown rate can be found that the battery can accept the charge current size decreases with time according to the law of exponential law, only to confirm the constant current or constant voltage charging is not the most suitable method. Because the constant current charging, the initial charging current is always lower than the acceptable capacity of the battery, resulting in low charging efficiency, charging time is long, and in the latter part of the charge, the final charge current is always higher than the acceptable level of the battery, So the battery internal gas rate is increasing, to the end of the charge, all the charge current all the gas supply, the battery internal voltage increases rapidly, the battery temperature also will rise rapidly, resulting in each charging electrode has active material off, thus greatly Reducing the battery life, and constant voltage charging method in the early charge of the battery is very low internal charge current is too large, and with the time does not fall by the exponential law often deviate from the development curve

 

According to the charging curve research experiment, also put forward the so-called two-stage, three-stage or more paragraph-type charging. The so-called two-stage charging refers to the first battery constant or constant voltage charging, when the battery voltage to a certain extent, and then the trickle charge of the battery: the so-called three-stage general is the first battery constant current charge, to the battery voltage After the voltage threshold is reached, it is converted into the second stage, the so-called voltage-limited charging stage, and converted into the third stage when the charging current is small to some extent. The so-called pressure limiting charge. In addition, due to the actual use of the need, often also want to fast charge the battery. There are two important factors that affect the fast charging of the charger: First, the polarization voltage: Second, the memory effect, in which the polarization voltage is in the charging process, the charge accumulated in the battery electrode and the response voltage, The increase in battery resistance, the effective way to eliminate it is to use negative pulse method at both ends of the battery instantaneous discharge to remove the charge on the electrode. And the resulting pulse charging method: memory effect is not all the batteries have, can be eliminated by a number of charge and discharge can be eliminated. The above two effects should be taken into account in the design of the charge controller.

 

In fact, the charging controller in recent years has taken a lengthy development, an obvious sign is that most of the world's semiconductor manufacturers have produced their own charger chip, and some also with a central processor (CPU). Although there are a variety of charging methods, but also have some effect, but most of the neglect of an important fact that the rechargeable battery is not working in the ideal state, each battery has its own unique personality. Specifically, each rechargeable battery has its own do not have to do other or even the same battery charge and discharge curve, the curve even in the charging process is also dynamic changes, which means that a good charge control mode should be changed, And should be consistent with the battery charge and discharge curve changes is the best. In fact, there is always an optimal charge current and charge mode for each battery at any time of charging. The question is how we can approximate this optimal value.

To sum up, the rules are as follows:

● Timing control;

● voltage control (including the highest voltage: voltage increment: voltage zero increments, etc.);

● temperature control (including the maximum temperature, the maximum temperature rise: temperature change rate, etc.);

● current control and so on.

The better way is to use the comprehensive judgment method, to track each index, and according to the principle of fuzzy mathematics were scored, according to the confidence to make the best judgment.


Initial stage

The initial stage does not belong to the real stage of the charging process, but in the whole charging process, the initial stage is still a very important meet the conditions of the charger after the start of self-test, such as power failure Or repeated, the charging process will be interrupted.If the lack of appropriate intelligent or time off device and battery sealing performance can not be improved, then this phenomenon is often overlooked. Almost all of the charger after the power failure can be Fully restarted, but if the overcharge is not allowed, the charger should be able to carry out a special self-test program to determine whether the battery is already full. The battery status should appear on the charger indicator or similar display device (bad case : For example, only the timer charger, with four hours interval of the fixed time to the battery charge.If the charge three hours and 59 minutes, the power failure failure, when the fault cleared after the power, the charger will start a new round Of the four hours of fixed charge, resulting in overcharging the battery for four hours.This result will often cause damage to the battery.Therefore, the timing of the charger Rarely used.)

Charging the battery should be conditions

When the battery is connected to the charger, the charger in the charging phase should be able to sense the display and whether the battery can be charged. Generally, when the charger is powered off, the battery sensing device is usually based on the port of the charger connector Voltage, but when the battery has been repeatedly overused or the port voltage is very low, this sensor device is often a problem.As an alternative, the charger often according to the temperature or voltage transient jump to achieve the above objectives This hardware setting also applies to battery packs.

Once the charger is inductively connected to a rechargeable battery, it must also determine whether the battery has a good charge condition. In this secondary stage [the qualified condition], the battery should be able to detect its basic functional status; open, short, Is to test whether the battery is available, some lead-acid charger type in particular need to be able to show through the LED lights to charge the main charge current [about one-fifth of the fast charge rated current] And allows the battery to reach its specific battery voltage over a fixed charging period.This technique should avoid reversing problems for lead-acid [PbSO4] batteries that have been repeatedly overused.

The detection of peripheral equipment and battery temperature is also one of the necessary conditions at this stage.When the charger detects the battery temperature is too high or too low, generally wait for a set of pre-set time to make it back to normal.As in the set time The charger will automatically change the charging current, this action corresponding to adjust the battery temperature, so the charging efficiency will be greatly increased.Finally, the charger needs to check with the battery connection status is open or short circuit. Usually open state Easy to detect, and if there is a short circuit display, you need to confirm further to avoid the occurrence of the accident.If all the tests are passed, you can charge the battery. This setting is quite lead, please refer to Figure 1

Precharge conditions set [optional type]

Some of the chargers, such as the early nickel-cadmium batteries, are designed with a pre-charge prediction phase to determine whether the battery has been 'fully discharged' before the recharging. The so-called 'full' Of the port voltage to a level of 1V to remove the electrolyte in the dendritic structure, which is why people often misunderstood the 'memory effect' reasons [usually the battery charge capacity is the last discharge capacity of 104-110 percent This so-called 'memory effect' refers specifically to the self-recovery of the internal structure of the electrolyte's internal electrolyte, such as damage, which will reduce the battery life, so the charge and discharge in the normal time period will often solve this problem.

This prediction phase is automatically completed before each charge, or after the remaining half of the battery charge has been displayed [after a connection test or other operation]. The prediction phase generally ranges from one to ten hours. The design of the phase is greater than the ten-hour design is often not used unless the manual method can be used to start detecting the reduced battery capacity.Therefore, due to the nickel Cadmium battery 'memory effect' unknown and all kinds of misunderstanding, the designer should avoid setting the relevant button on the charger to this 'effect' against.

Fast charge phase and charge termination

Fast charge and termination of the use of charging methods to be based on the battery and other design considerations. The following discussion related to today's universal battery charging technology in full use.For detailed advice, with the battery manufacturer application department inquiries.

Nickel cadmium, nickel-metal hydride batteries

The fast charging procedure for nickel cadmium and nickel-metal hydride batteries is very similar, with the main difference being the termination phase of the charge. In each example, when the charger monitors the battery voltage and other data to determine when to terminate the charge, a constant Current. More than 2C of the charging current can be done, but generally the most commonly used C / 2 current.If the charging efficiency is no less than 100%, so with C / 2 current slightly more than two hours of full charge Can be accepted.

During the constant current charging stage, the battery port voltage rises slowly and eventually reaches the vertex [the highest point of the zero voltage.] The nickel-metal hydride battery should terminate the charge at this point [0DV], while the nickel-cadmium battery should A point [the battery port voltage for the first time a slight decrease in the point that is -DD point] end, please refer to Figure 2, allowing a small part of the overcharge. Fast charge, such as more than the two set off point [the highest voltage point and slightly Small-DV point], will cause damage to the battery.This is because the fast charge current is much greater than the normal slow charge caused by the state.


The charger will still monitor the battery temperature and voltage when the charge is greater than the C / 2 current [the charging time does not exceed two hours.] The battery temperature will rise very quickly when the battery is full, so the temperature monitoring element will start another In addition to the temperature off, other monitoring data, including the charged time, the maximum limit of the battery protection voltage, can be involved in the charger off the off point. Therefore, the successful design of the charger automatically shut down should be based on these monitoring factors [temperature off, charging time off, the maximum battery voltage off, etc.] combination.

Note: When the battery is first charged, it will produce a specific effect of mimicking the shutdown state, so the charger will introduce a one-to-five buffer period before activating the shutdown mode monitoring curve. In addition, the charge-off state is below C \ 8 current is difficult to monitor when charging data, because the voltage and temperature curves [V / t and T / t] are very small compared to other system data. For fast charge safety, these systems Hardware and software configuration will always cause the charger to turn off the wrong operation.

 

Lithium Ion Battery

Lithium-ion battery charging curve and nickel-ion battery is very different compared to the charge off to determine the maximum safety value will not move after the lithium charger to charge voltage control in less than 0.75% of the precise range, and its maximum charge The efficiency is controlled by the charge current limit, which is very similar to the desktop desktop DC power supply.When the fast charge starts, the battery port voltage is relatively low, the charging current is equivalent to its limited current value.

During the charging process, the battery voltage rises slowly. Finally, the charge current reaches the set peak and the charge voltage rises to the float voltage level of 4.2V per section


When the battery is charged to the float voltage level, the charger can terminate the charge, but this way ignores the charge peak top off operation, such as in the float voltage stage, the charger will start the timer operation, After the buffering period, the charging is terminated automatically. Alternatively, the charge current can be monitored at a lower current level [typically with a 0.05 limit, and some manufacturers recommend using a relatively high 100mA current minimum] Charging peak turn - off loops often also rely on this technique.

In recent years, considerable progress has been made in lithium-ion batteries and their chargers, as well as on the understanding of this new battery, and the earliest lithium-ion batteries have some safety-level shortcomings in terms of user use These problems will not appear in today's successful design of the system. Manufacturers of the proposal will not be static and will not be exactly the same, and lithium-ion battery technology will continue to move forward.

Lithium polymer battery charging requirements and lithium-ion battery requirements are the same, can be treated according to the same.

The latest occurrence of lithium iron phosphate battery voltage than lithium-ion battery voltage is about 0.4V lower, charging constant voltage voltage set value of 3.7V / section is appropriate. Charge the way with the lithium-ion battery is basically the same (note that only the voltage is lower!)


Lead-acid batteries.

Lead-acid [PbSO4] batteries can be charged with current limiting, or they can be charged with a more general and simple pressure-limiting charge. The pressure-limited charge has a similar function to the former, and high accuracy is not critical. Battery float voltage of the current limiting voltage source [about 2.45V or so].

After a preset operation, the charger confirms the start of the fast charge and continues to the minimum charge current [this process is similar to a lithium-ion battery charger] and then quickly terminates. The charger needs a Vf float voltage for maintenance [usually 2.25V voltage]. Lead acid [PbSO4] battery allows the use of this floating charge in the non-deterministic phase maintenance


In the high temperature environment, the lead-acid [PbSO4] battery for fast charge current according to the 0.3 percent Celsius standard temperature coefficient to reduce the maximum charge when the maximum battery temperature is set at 50'C or so, Can exceed this temperature.

 

Selectable trickle charge [for batteries other than lithium ion batteries]

In addition to lithium-ion batteries, other battery chargers are often provided with a selectable trickle charge stage. This stage can be used to compensate for the self-discharge of the battery after the charge. Lead acid [PbSO4] battery self-discharge rate [The number of percentages per day], while the lithium-ion battery is the lowest. Lithium-ion battery self-discharge rate is sometimes low to have no need to maintain. However, nickel cadmium [NiCds] batteries often, indefinitely need C / 16 Charge current for maintenance. For nickel-metal hydride [NiMH] batteries, the safe continuous discharge current is often around C / 50, and trickle charge is generally not recommended.

Continued discharge current is often around C / 50, and trickle charge is generally not recommended.


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