The essential guide to proper battery usage
1. Understanding the memory effect (mainly discussing nickel-cadmium and nickel-metal hydride batteries; those not interested can skip to section 2).
The battery memory effect refers to the reversible failure of a battery, that is, the ability to recover performance after battery failure. The memory effect refers to the tendency of a battery to automatically maintain a specific performance after being subjected to a specific working cycle for a long time. This was first defined in nickel-cadmium batteries. Nickel-cadmium pouch batteries do not have a memory effect, while sintered batteries do. However, current nickel-metal hydride (NiMH) batteries are not bound by this definition of memory effect.
Due to improvements in modern nickel-cadmium battery technology, the aforementioned memory effect has been significantly reduced. However, another phenomenon has replaced this definition: the "crystallization" of nickel-based batteries. Typically, nickel-cadmium batteries are affected by a combination of these two effects, while nickel-metal hydride batteries are only affected by the "crystallization" memory effect, and the impact is smaller than that on nickel-cadmium batteries.
In practical applications, there are strict standards and procedures for eliminating memory effects; improper operation can be counterproductive.
For nickel-cadmium batteries, normal maintenance involves periodic deep discharge: perform a deep discharge (discharge to 1.0V/cell, which is called exercise) once every month (or 30 cycles) on average. Normal use can alleviate the memory effect by using the battery to the last drop or turning off the device. However, this is not exercise because instruments (such as mobile phones) will not be used until 1.0V/cell before turning off. Special equipment or circuitry is required to do this. Fortunately, many nickel-metal hydride battery chargers have this function.
For nickel-cadmium batteries that have not been exercised for a long time, the accumulated memory effect will prevent them from being restored to their capacity through exercise. In this case, a deeper discharge (called reconditioning) is required. This is a process of discharging the battery to 0.4V per cell with a very small current for a long time, which requires specialized equipment.
For nickel-metal hydride (NiMH) batteries, exercising approximately once every three months is sufficient to effectively alleviate the memory effect. Since the cycle life of NiMH batteries is far shorter than that of NiCd batteries, reconditioning is almost unnecessary.
▲Recommendation 1: Discharging the battery before each charge is unnecessary and harmful, as it unnecessarily shortens the battery's lifespan.
▲Recommendation 2: It is not advisable to use a resistor to connect the positive and negative terminals of the battery for discharge. The current cannot be controlled, and it is easy to over-discharge to 0V, or even cause the polarity of the batteries in the series battery pack to reverse.
2. Does the battery need to be activated?
The answer is that the battery needs to be activated, but this is not something the user needs to do. I have visited a lithium-ion battery manufacturing plant, and lithium-ion batteries undergo the following processes before leaving the factory:
The lithium-ion battery casing is filled with electrolyte – sealed – formation, which involves constant voltage charging followed by discharging, repeated several times to ensure the electrodes are fully immersed in the electrolyte and fully activated until the required capacity is achieved. This is the activation process. Capacity grading involves testing the battery's capacity, selecting batteries with different performance (capacity) for classification, categorizing the batteries, and matching their capacity. Lithium-ion batteries delivered to the user in this way are already activated. Commonly used nickel-cadmium and nickel-metal hydride batteries undergo the same formation and activation process before leaving the factory. For some batteries, the activation process requires the battery to be in an open state before sealing; this step can only be completed by the cell manufacturer.
There is a problem here: the time from when the battery leaves the factory to when it reaches the user can sometimes be very long, ranging from one month to half a year. During this time, because the battery electrode materials will become passivated, the manufacturer recommends that the battery be used for the first time for 3 to 5 complete charge and discharge cycles in order to eliminate the passivation of the electrode materials and achieve maximum capacity.
The three national standards for nickel-metal hydride, nickel-cadmium, and lithium-ion batteries promulgated in 2001 all have clear regulations on the testing of initial capacity. The battery can be subjected to five deep charge and discharge cycles, and the test can be stopped when one cycle meets the requirements. This explains the phenomenon I mentioned.
★ So it can also be called "second activation". Users should try to perform several deep charge-discharge cycles on the "new" battery when using it for the first time.
●However, according to my tests (for lithium-ion batteries), for lithium-ion batteries stored for 1-3 months, deep charge-discharge cycles result in almost no capacity increase. (I have a test report on battery activation in the discussion forum.)
3. Do I need to charge for 12 hours for the first three times?
This question is closely related to the battery activation issue mentioned above. Let's assume that the battery has electrode passivation when it leaves the factory and reaches the user. In order to activate the battery, it needs to undergo three deep charge and deep discharge cycles. In fact, this question can be transformed into whether deep charging means charging for 12 hours. My other article, "On the Charging Time of Mobile Phone Batteries," has already answered this question.
★★★The answer is no, it doesn't need to be charged for 12 hours.
Early mobile phone nickel-metal hydride batteries required a trickle charging process, and it might take about 5 hours to reach the perfect full charge, but it didn't need to take 12 hours. The constant current and constant voltage charging characteristics of lithium-ion batteries mean that their deep charging time does not need to be 12 hours.
Some people may ask about lithium-ion batteries: since the current of a lithium-ion battery gradually decreases during the constant voltage stage, is it true deep charging only when the current becomes infinitely small? I once plotted the curve of current decrease versus time during the constant voltage stage and performed curve fitting multiple times. I found that this curve can be approximated to zero current using a function of 1/x. However, in actual testing, due to the self-discharge phenomenon inherent in lithium-ion batteries, this zero current can never be reached.
Taking a 600mAh battery as an example, if the cutoff current is set to 0.01C (i.e., 6mA), its 1C charging time will not exceed 150 minutes. If the cutoff current is set to 0.001C (i.e., 0.6mA), its charging time may be 10 hours. This time cannot be accurately obtained due to the limitations of instrument precision. However, the capacity gained from 0.01C to 0.001C is only 1.7mAh. It is meaningless to exchange more than 7 hours of extra use for this less than three-thousandth of the capacity.
Moreover, there are other charging methods, such as pulse charging, which brings lithium-ion batteries to a limit voltage of 4.2V. It does not have a minimum current cutoff stage, and it is usually 100% charged after 150 minutes. Many mobile phones use pulse charging.
Some people have used the method of charging their phones by first showing that they are fully charged and then charging them with a desktop charger to confirm the full charge level. This testing method is not rigorous. First of all, the green light on the desktop charger is not a reliable indicator of whether the phone is truly fully charged.
★★The only definitive way to determine whether a lithium-ion battery is fully charged is to test its voltage when it is neither charging nor discharging.
The real purpose of reducing the current during the constant voltage stage is to gradually reduce the additional voltage generated by the charging current across the battery's internal resistance. When the current is as small as 0.01C, such as 6mA, the product of this current and the battery's internal resistance (generally within 200 milliohms) is only 1mV. This voltage can be considered to be the battery voltage in the no-current state.
Secondly, the reference voltage of the mobile phone is not necessarily equal to the reference voltage of the charging dock. The mobile phone thinks that the battery is fully charged when it arrives at the charging dock, but the charging dock does not think that it is fully charged and continues to charge.
4. Is there an optimal state for rechargeable batteries?
One theory suggests that rechargeable batteries, when used properly, will reach their optimal state—maximum capacity—within a certain cycle range. This depends on the specific circumstances. Sealed nickel-metal hydride (NiMH) and nickel-cadmium (NiCd) batteries, if used correctly (e.g., with regular maintenance to prevent the formation and accumulation of memory effects), generally reach their maximum capacity around 100-200 cycles. For example, a NiMH battery with a factory capacity of 1000mAh might reach 1100mAh after 120 cycles. I've seen this description in the cycle characteristics diagrams of nickel-based batteries in the technical specifications of almost all Japanese NiMH battery manufacturers.
★ Nickel-based batteries have an optimal state, generally reaching their maximum capacity between 100 and 200 cycles.
For liquid lithium-ion batteries, however, there is no such peak phenomenon in cycle capacity. From the time a lithium-ion battery leaves the factory until it is finally scrapped, its capacity is reduced with each use. When I conduct cycle performance tests on lithium-ion batteries, I have never seen any signs of capacity recovery.
★There is no optimal state for lithium-ion batteries.
It's worth noting that lithium-ion batteries are more susceptible to changes in ambient temperature, exhibiting different performance characteristics. They perform best in environments between 25 and 40 degrees Celsius, while their performance is significantly reduced at low or high temperatures. To ensure your lithium-ion battery fully utilizes its capacity, you must carefully consider the usage environment and prevent extreme temperature fluctuations. For example, placing a mobile phone on the dashboard of a car under direct midday sun can easily cause its temperature to exceed 60 degrees Celsius. Consequently, users in northern regions will experience shorter battery standby times compared to users in southern regions, even under the same network conditions.
5. Is it true that the higher the charging current, the faster the charging?
The article "On the Charging Time of Mobile Phone Batteries" has already discussed this issue. This statement is true for nickel-based batteries with constant current charging, but it is not entirely accurate for lithium-ion batteries.
★★For lithium-ion battery charging, within a certain current range (1.5C~0.5C), increasing the constant current value of the constant current and constant voltage charging method cannot shorten the time to fully charge the lithium-ion battery.
6. Is the output current of a direct charger equal to the charging current?
This brings us to the discussion of phone charging methods. For phones where charging management is integrated, the output of a direct charger (actually called a power adapter) is set to something like 5.3V 600mA.
A. The charging management uses a switching method (high-frequency pulse width adjustment PWM method). With this charging method, the phone does not fully utilize the output capacity of the direct charger. The direct charger operates in the constant voltage range, outputting 5.3V. At this time, the actual charging current is adjusted by the phone's charging management and will definitely be less than 600mA, generally between 300 and 400mA. Therefore, the output current of the direct charger that you see is not the phone's charging current. For example, many Motorola direct chargers output 5.0V 1A, but only about 500mA is actually used to charge the battery, since the phone's battery capacity is only 580mAh.
★At this point, the output current indicated on the direct charger is not equal to the actual charging current.
B. When charging management is in pulse mode, the phone fully utilizes the current-limiting current of the direct charger, which is 600mA on the battery. In this case, the output current of the direct charger is the charging current.
Of course, the above refers to the constant current stage of lithium-ion batteries or the charging of nickel-metal hydride batteries.
If the phone doesn't have its own charging management system and has moved charging control to the direct charging function (as is the case with many CDMA phones), there's not much to say about it. The output specifications are clearly stated, for example, 4.2V 500mA. These are the constant current and constant voltage specifications for a lithium-ion battery.
7. Does each charge-discharge cycle reduce the lifespan by one?
Cycle life is the use of a battery. We are using a battery and we are concerned about the duration of use. In order to measure how long a rechargeable battery can be used, the definition of cycle life is defined. Actual user use varies greatly, and tests under different conditions are not comparable. To make comparisons, the definition of cycle life must be standardized.
The national standard specifies the following test conditions and requirements for the cycle life of lithium-ion batteries: Charge the battery for 150 minutes at a constant current and constant voltage of 1C under a constant current and constant voltage environment at 25 degrees Celsius, and then discharge it to a cutoff voltage of 2.75V using a constant current and 1C discharge regime. This constitutes one cycle. The test ends when any discharge time is less than 36 minutes, and the number of cycles must be greater than 300.
explain:
A. This definition specifies that cycle life testing is conducted using a deep charge-deep discharge method.
B. It is stipulated that the cycle life must exceed 300 cycles after execution in this mode, and the capacity must still be above 60%.
In fact, different cycle regimes yield drastically different cycle counts. For example, if all other conditions remain the same, but the constant voltage is changed from 4.2V to 4.1V for the same battery model during cycle life testing, the battery is no longer in deep charge mode, and the cycle life can be increased by nearly 60%. If the cutoff voltage is increased to 3.9V for testing, the cycle count should increase several times.
The idea that each cycle reduces lifespan by one has been discussed by many friends. I'm just adding some further explanation. When discussing the number of cycles, we must not ignore the conditions under which the cycle occurs.
● It's meaningless to discuss cycle count without considering the rules, because cycle count is a means of testing battery life, not the end goal!
▲ Misconception: Many people like to let their mobile phone lithium-ion batteries run until they automatically shut down before recharging, which is completely unnecessary.
In reality, users cannot use batteries according to the national standard testing mode. No mobile phone will shut down at 2.75V. Moreover, its discharge mode is not a high-current constant discharge, but a mixture of GSM pulse discharge and normal low-current discharge.
Another way to measure cycle life is by time. Some experts suggest that the lifespan of a typical civilian lithium-ion battery is 2 to 3 years. Considering practical factors, such as setting the lifespan at 60% capacity and taking into account the aging effect of lithium-ion batteries (see point 9), I believe that using time to describe cycle life is more reasonable.
The charging mechanism of lead-acid batteries is similar to that of lithium-ion batteries. It uses a current-limiting and voltage-limiting method and is used by shallow charging and discharging. Its lifespan is expressed in terms of time, not number of cycles, such as 10 years.
★★★Therefore, there is no need to turn off the device to recharge lithium-ion batteries. Lithium-ion batteries are inherently suitable for use by charging them at any time, which is one of their biggest advantages over nickel-metal hydride batteries. Please make good use of this feature.
8. Is higher battery capacity always better?
For different battery models (especially different sizes), the higher the capacity, the longer the usage time. Ignoring factors such as size and weight, higher capacity is of course better.
However, even with the same battery model and nominal capacity (e.g., 600mAh), the actual measured initial capacity can differ: for example, one is 660mAh and the other is 605mAh. Does that mean the 660mAh one is better than the 605mAh one?
The reality is that high-capacity batteries may have increased initial capacity by adding more components to the electrode material, while reducing components for electrode stabilization. As a result, after dozens of cycles, high-capacity batteries quickly degrade in capacity, while low-capacity batteries remain robust. Many domestic cell manufacturers often use this method to obtain high-capacity batteries, but after six months of use, the standby time is abysmal.
The AA nickel-metal hydride batteries used by the public (that is, size 5 batteries) are generally 1400mAh, but some are labeled with ultra-high capacity (1600mAh), and the reason is the same.
★ Increasing capacity comes at the cost of sacrificing cycle life. Without modifying battery materials, manufacturers cannot truly "increase" battery capacity.
9. Is it okay to store a fully charged battery?
One very bad characteristic of lithium-ion batteries is their aging (or time-related aging). After a period of storage, even without cycling, some of the capacity of a lithium-ion battery will be permanently lost. This is because the positive and negative electrode materials of lithium-ion batteries begin their degradation process as soon as they leave the factory. Different temperatures and battery charge states result in different aging consequences. The following data is taken from reference [1] and is listed as a percentage of capacity:
Storage temperature -- 40% charge state ------- 100% charge state
0 degrees Celsius ------- 98% (one year later) ----- 94% (one year later)
25 degrees Celsius ------ 96% (one year later) ------ 80% (one year later)
40 degrees Celsius ------ 85% (one year later) ------ 65% (one year later)
60 degrees Celsius -- 75% (after one year) -- 60% (after three months)
Therefore, the higher the storage temperature and the more fully the battery is charged, the greater the capacity loss. Long-term storage of lithium-ion batteries is not recommended. Instead, manufacturers should recycle them like rotten food, and users should pay close attention to the battery's production date.
If users have spare batteries, experts recommend storing them at a charge level of 40% and a storage temperature of 15 degrees Celsius or lower.
Nickel-metal hydride and nickel-cadmium batteries are almost unaffected by this aging effect; nickel-based batteries stored for a long time can recover their original capacity after several deep charge and discharge cycles.
10. Is it helpful to charge for an extra hour after the green light on the charging dock comes on?
The green light is just an indicator; whether the battery is truly fully charged depends on the charger's control and judgment of the charging process. Let's take a 4.2V lithium-ion battery as an example to discuss this issue.
First, there's the control. The control of the battery output is first constant current, then constant voltage (the current gradually decreases).
Next comes the judgment: if the current is less than a certain value, a green light is displayed. Because the accuracy of analog-to-digital conversion and the voltage accuracy itself are limited, the charging dock is usually set to this current value of 50mA. When the green light is displayed at this point, it means that the battery is indeed less than 10% full (according to my measurements, current lithium-ion batteries can reach 95% capacity when charged with a 50mA cutoff, greatly improving charging acceptance). The question now is what the charging dock does next:
A. If the charging circuit is completely shut off and constant voltage charging is not continued, then leaving it on the charging dock for another 10 hours will not help. Many charging dock designs are like this, such as TI's (Texas Instruments) BQ2057 series charging chips and Linear Technology's (Linear Technology) LT1800 series.
B. The charging dock continues constant voltage charging, and the voltage is strictly controlled not to exceed 4.2V. Undoubtedly, charging for an extra hour will increase the battery capacity.
C. The charging dock continues to charge, but its current control is terrible. It accidentally causes the battery to exceed 4.2V and continues to rise. Because lithium-ion batteries cannot absorb any overcharge, continuously applying current to the battery will cause this result, and overcharging occurs. This is obviously a poorly designed charging dock, such as the common "egg charger" that costs around ten yuan and can charge both lithium-ion and nickel-metal hydride batteries.
D. There is another type of charging management chip, such as Maxim's 1679 chip. Like many mobile phone charging management chips, it uses pulse charging. When it displays a green light, it means that the lithium-ion battery is 100% charged. Of course, if it is left to stand for another hour, it will not overcharge, which is obviously a waste of time.
Users don't actually know what the charger is doing after the green light comes on. It could be A, B, or D—any of those possibilities exist. The charger's instruction manual doesn't specify these things. Excluding defective chargers, we should actually trust qualified and original chargers. If the green light is on, why not remove it and use it? This doesn't have a significant impact on the user. Incomplete charging doesn't affect the cycle life (as mentioned in point 7 above), and 95% capacity is acceptable. Unless an enthusiast can deeply analyze how their charger is charging, we might as well remove it and use it after the green light comes on.
11. Does a cradle charger provide a fuller charge than a direct charger?
★There is no such thing as a cradle charger charging the battery more fully than a direct charger, nor is there such a thing as a direct charger charging the battery more fully than a cradle charger. What matters is whether their charging methods can charge the battery to its maximum capacity the fastest.
