How should a Ni─MH battery be restored and why is it important? AA element holders. An attempt to restore the capacity of used NiCd and NiMh batteries

Ni-MH batteries (nickel metal hydride) are included in the alkaline group. They are current sources of a chemical type, where nickel oxide acts as the cathode, and a hydrogen metal hydride electrode acts as the anode. Alkali is an electrolyte. They are similar to nickel-hydrogen batteries, but are superior in energy capacity.

The production of Ni-MH batteries began in the mid-twentieth century. They were developed taking into account the shortcomings of outdated nickel-cadmium batteries. NiNH can use different combinations of metals. For their production, special alloys and metals have been developed that operate at room temperature and low hydrogen pressure.

Industrial production began in the eighties. Alloys and metals for Ni-MH are still being manufactured and improved today. Modern devices This type can provide up to 2 thousand charge-discharge cycles. A similar result is achievable due to the use of nickel alloys with rare earth metals.

How are these devices used?

Nickel-metal hydride devices are widely used to power various types of electronics that operate in offline mode. They are usually made in the form of AAA or AA batteries. Other versions are also available. For example, industrial batteries. The scope of use of Ni-MH batteries is slightly wider than that of nickel-cadmium batteries, because they do not contain toxic materials.

IN at the moment sold on domestic market Nickel-metal hydride batteries are divided into 2 groups according to capacity - 1500-3000 mAh and 300-1000 mAh:

1. First used in devices with increased power consumption for short time. These are all kinds of players, radio-controlled models, cameras, video cameras. In general, devices that quickly consume energy.
2. Second used when energy consumption begins after a certain time interval. These are toys, flashlights, walkie-talkies. Battery-powered devices operate that consume moderate amounts of electricity and remain offline for a long time.

Charging Ni-MH devices

Charging can be drip and fast. Manufacturers do not recommend the first one, because it causes difficulties with precise definition stopping the current supply to the device. For this reason, a powerful overcharge may occur, which will lead to battery degradation. with the help quick option. The efficiency here is slightly higher than that of the drip type of charging. The current is set to 0.5-1 C.

How to charge a hydride battery:

• the presence of a battery is determined;
• device qualification;
• pre-charge;
• fast charging;
• recharging;
• maintenance charging.

At fast charging you need to have a good memory. It must control the end of the process according to different criteria independent of each other. For example, Ni-Cd devices have enough voltage delta control. And with NiMH, the battery needs to monitor temperature and delta at a minimum.

For proper operation Ni-MH should remember the “Rule of the Three Ps”: “ Do not overheat”, “Do not overcharge”, “Do not overdischarge”.

To prevent battery overcharging, the following control methods are used:

1. Termination of charge based on temperature change rate . Using this technique, the battery temperature is constantly monitored during charging. When the readings rise faster than necessary, charging stops.
2. Method of stopping charging based on its maximum time .
3. Termination of charge based on absolute temperature . Here the temperature of the battery is monitored during the charging process. Upon reaching maximum value fast charge stops.
4. Negative delta voltage termination method . Before the battery completes charging, the oxygen cycle raises the temperature of the NiMH device, causing the voltage to drop.
5. Maximum voltage . The method is used to turn off the charge of devices with increased internal resistance. The latter appears at the end of the battery life due to lack of electrolyte.
6. Maximum pressure . The method is applied to prismatic batteries large capacity. The level of permitted pressure in such a device depends on its size and design and is in the range of 0.05-0.8 MPa.

To clarify the charging time of a Ni-MH battery, taking into account all the characteristics, you can use the formula: charging time (h) = capacity (mAh) / charger current (mA). For example, there is a battery with a capacity of 2000 milliamp-hours. The charge current in the charger is 500 mA. The capacity is divided by the current and the result is 4. That is, the battery will charge in 4 hours.

Mandatory rules that must be followed for the proper functioning of the nickel-metal hydride device:

1. These batteries are much more sensitive to heat than nickel-cadmium batteries; they cannot be overloaded . Overload will negatively affect current output (the ability to hold and release accumulated charge).
2. Metal hydride batteries can be “trained” after purchase . Perform 3-5 charge/discharge cycles, which will allow you to reach the limit of capacity lost during transportation and storage of the device after leaving the conveyor.
3. Batteries should be stored with a small amount of charge. , approximately 20-40% of the nominal capacity.
4. After discharging or charging, allow the device to cool down. .
5. If in electronic device the same battery assembly is used in recharging mode , then from time to time you need to discharge each of them to a voltage of 0.98, and then fully charge them. It is recommended to perform this cycling procedure once every 7-8 battery recharging cycles.
6. If you need to discharge NiMH, you should stick to the minimum value of 0.98 . If the voltage drops below 0.98, it may stop charging.

Reconditioning of Ni-MH batteries

Due to the “memory effect”, these devices sometimes lose some characteristics and most of their capacity. This happens with multiple cycles complete discharge and subsequent charging. As a result of this operation, the device “remembers” a lower discharge limit, for this reason its capacity decreases.

To get rid of this problem, you need to constantly perform training and recovery. The light bulb or charger discharges to 0.801 volts, then the battery is fully charged. If for a long time If the battery has not undergone the recovery process, it is advisable to perform 2-3 similar cycles. It is advisable to train it once every 20-30 days.

Manufacturers of Ni-MH batteries claim that the “memory effect” takes up approximately 5% of the capacity. You can restore it with the help of training. An important point when restoring Ni-MH, the charger has a discharge function with minimum voltage control. What is needed to prevent the device from being severely discharged during restoration. This is indispensable when the initial state of charge is unknown and it is impossible to guess the approximate discharge time.

If the state of charge of the battery is unknown, it should be discharged under full voltage control, otherwise such restoration will lead to deep discharge. When restoring a whole battery, it is recommended to first full charge to equalize the charge level.

If the battery has been used for several years, then restoration by charging and discharging may be useless. It is useful for prevention during operation of the device. When using NiMH, along with the appearance of the “memory effect,” changes in the volume and composition of the electrolyte occur. It is worth remembering that it is wiser to restore battery cells individually than to restore the entire battery. The battery life is from one to five years (depending on the specific model).

Advantages and disadvantages

A significant increase in the energy parameters of nickel-metal hydride batteries is not their only advantage over cadmium batteries. Having abandoned the use of cadmium, manufacturers began to use a more environmentally friendly metal. It is much easier to resolve issues with .

Due to these advantages and the fact that the metal used in manufacturing is nickel, the production of Ni-MH devices has increased sharply when compared with nickel-cadmium batteries. They are also convenient because in order to reduce the discharge voltage during long-term recharges, a full discharge (up to 1 volt) must be carried out once every 20-30 days.

A little about the disadvantages:

1. Manufacturers limited Ni-MH batteries to ten cells , because with increasing charge-discharge cycles and service life, there is a danger of overheating and polarity reversal.
2. These batteries operate in a narrower temperature range than nickel-cadmium batteries. . Already at -10 and +40°C they lose their performance.
3. Ni-MH batteries generate a lot of heat when charging , therefore they need fuses or temperature relays.
4. Increased self-charging , the presence of which is due to the reaction of the nickel oxide electrode with hydrogen from the electrolyte.

Degradation of Ni-MH batteries is determined by a decrease in the sorption capacity of the negative electrode during cycling. During the discharge-charge cycle, the volume of the crystal lattice changes, which contributes to the formation of rust and cracks during the reaction with the electrolyte. Corrosion occurs when the battery absorbs hydrogen and oxygen. This leads to a decrease in the amount of electrolyte and an increase internal resistance.

It must be taken into account that the characteristics of batteries depend on the processing technology of the negative electrode alloy, its structure and composition. The metal for alloys also matters. All this forces manufacturers to very carefully choose alloy suppliers, and consumers - the manufacturer.

Research into nickel-metal hydride batteries began in the 1970s as an improvement to nickel-hydrogen batteries, since the weight and volume of nickel-hydrogen batteries was not satisfactory for manufacturers (the hydrogen in these batteries was under high pressure, which required a durable and heavy steel body). The use of hydrogen in the form of metal hydrides has made it possible to reduce the weight and volume of batteries, and the risk of battery explosion when overheated has also decreased.

Since the 1980s, NiMH battery technology has improved significantly and commercial use has begun in a variety of applications. The success of NiNH batteries was facilitated by increased capacity (40% compared to NiCd), the use of recyclable materials ("friendly" to the natural environment), and also very long term service, often exceeding the performance of NiCd batteries.

Advantages and disadvantages of NiMH batteries

Advantages

・ greater capacity - 40% or more than conventional NiCd batteries
・ much less pronounced “memory” effect compared to nickel-cadmium batteries - battery maintenance cycles can be carried out 2-3 times less often
・ simple possibility of transportation - airlines transport without any preconditions
・ environmentally friendly - can be recycled

Flaws

・ limited battery life - usually about 500-700 full charge/discharge cycles (although depending on operating modes and internal structure There may be significant differences).
・memory effect - NiMH batteries require periodic training (battery full discharge/charge cycle)
・ Relatively short shelf life of batteries - usually no more than 3 years when stored in a discharged state, after which the main characteristics are lost. Storing in cool conditions with a partial charge of 40-60% slows down the aging process of batteries.
・High battery self-discharge
・Limited power capacity - when the permissible loads are exceeded, the battery life is reduced.
・ A special charger with a staged charging algorithm is required, since charging emits large number heat and nickel-metal hydride batteries undergo overcharging.
・ Poor tolerance high temperatures(over 25-30 Celsius)

Construction of NiMH batteries and batteries

Modern nickel-metal hydride batteries have an internal design similar to that of nickel-cadmium batteries. The positive nickel oxide electrode, alkaline electrolyte and design hydrogen pressure are the same in both battery systems. Only the negative electrodes are different: nickel-cadmium batteries have a cadmium electrode, and nickel-metal hydride batteries have an electrode based on an alloy of hydrogen-absorbing metals.

Modern nickel-metal hydride batteries use hydrogen-absorbing alloy compositions such as AB2 and AB5. Other AB or A2B alloys are not widely used. What do the mysterious letters A and B in the alloy composition mean? – The symbol A represents a metal (or a mixture of metals) that releases heat when it forms hydrides. Accordingly, the symbol B denotes a metal that reacts endothermically with hydrogen.

For negative electrodes of type AB5, a mixture of rare earth elements of the lanthanum group (component A) and nickel with admixtures of other metals (cobalt, aluminum, manganese) is used - component B. For electrodes of type AB2, titanium and nickel with admixtures of zirconium, vanadium, iron, manganese are used, chromium.

Nickel-metal hydride batteries with AB5 type electrodes are more widespread due to better cycling characteristics, despite the fact that batteries with AB2 type electrodes are cheaper, have a higher capacity and better power performance.

During the cycling process, the volume of the negative electrode fluctuates up to 15-25% of the original due to the absorption/release of hydrogen. As a result of volume fluctuations, a large number of microcracks appear in the electrode material. This phenomenon explains why a new nickel-metal hydride battery requires several “training” charge/discharge cycles to bring the battery’s power and capacity to nominal. Also, the formation of microcracks has a negative side - the surface area of ​​the electrode increases, which is subject to corrosion with the consumption of electrolyte, which leads to a gradual increase in the internal resistance of the element and a decrease in capacity. To reduce the rate of corrosion processes, it is recommended to store nickel-metal hydride batteries in a charged state.

The negative electrode has excess capacity relative to the positive one in both overcharge and overdischarge to ensure an acceptable level of hydrogen evolution. Due to corrosion of the alloy, the recharge capacity of the negative electrode gradually decreases. As soon as the excess recharge capacity is exhausted, a large amount of hydrogen will begin to be released on the negative electrode at the end of the charge, which will lead to the release of excess hydrogen through the valves of the cell, “boil-off” of the electrolyte and failure of the battery. Therefore, to charge nickel-metal hydride batteries, you need a special charger that takes into account the specific behavior of the battery to avoid the danger of self-destruction of the battery cell. When reassembling the battery pack, ensure that the cells are well ventilated and that you do not smoke near the high-capacity nickel-metal hydride battery that is being charged.

Over time, as a result of cycling, the self-discharge of the battery increases due to the appearance of large pores in the separator material and the formation of an electrical connection between the electrode plates. This problem can be temporarily resolved by deep-discharging the battery several times, followed by a full charge.

When charging nickel-metal hydride batteries, a fairly large amount of heat is generated, especially at the end of the charge, which is one of the signs that the charge needs to be completed. When assembling several battery cells into a battery, a battery monitoring system (BMS) is required, as well as the presence of thermally-opening conductive connecting jumpers between part of the battery cells. It is also advisable to connect the batteries in the battery by spot welding jumpers rather than soldering.

Discharge of nickel-metal hydride batteries when low temperatures limited by the fact that this reaction is endothermic and water is formed on the negative electrode, diluting the electrolyte, which leads to a high probability of freezing of the electrolyte. Therefore, the lower the ambient temperature, the less power output and battery capacity. On the contrary, at elevated temperatures during the discharge process, the discharge capacity of a nickel-metal hydride battery will be maximum.

Knowledge of the design and principles of operation will allow you to have a greater understanding of the process of operating nickel-metal hydride batteries. I hope the information gleaned from this article will help extend the life of your battery and avoid possible dangerous consequences due to misunderstanding of the principles safe use nickel metal hydride batteries.

Discharge characteristics of NiMH batteries at different
discharge currents at an ambient temperature of 20 °C

image taken from www.compress.ru/Article.aspx?id=16846&iid=781

Duracell Nickel Metal Hydride Battery

image taken from www.3dnews.ru/digital/1battery/index8.htm

P.P.S.
Scheme of a promising direction for creating bipolar batteries

circuit taken from Bipolar lead-acid batteries

Comparison table parameters various types batteries

Among other batteries, NiMH batteries are often used. These batteries have high technical characteristics, which allow you to use them as efficiently as possible. This type of battery is used almost everywhere; below we will look at all the features of such batteries, and also analyze the nuances of operation and well-known manufacturers.

Contents

What is a Nickel Metal Hydride Battery

To begin with, it is worth noting that nickel-metal hydride is a secondary power source. It does not produce energy and requires recharging before use.

It consists of two components:

• anode – nickel-lithium or nickel-lanthanum hydride;
• cathode – nickel oxide.

An electrolyte is also used to excite the system. Potassium hydroxide is considered the optimal electrolyte. This is an alkaline food source according to modern classification.

This type of battery replaced nickel-cadmium batteries. The developers managed to minimize the disadvantages characteristic of more early types batteries. The first industrial designs were put on the market in the late 80s.

At the moment, it has been possible to significantly increase the density of stored energy in comparison with the first prototypes. Some experts believe that the density limit has not yet been reached.

Operating principle and design of Ni-Mh battery

First, it’s worth considering how a NiMh battery works. As already mentioned, this battery consists of several components. Let's look at them in more detail.

The anode here is a hydrogen-absorbing composition. It is capable of absorbing large amounts of hydrogen; on average, the amount of absorbed element can exceed the volume of the electrode by 1000 times. To achieve complete stabilization, lithium or lanthanum is added to the alloy.

Cathodes are made from nickel oxide. This allows you to obtain a high-quality charge between the cathode and anode. In practice, the most different types cathodes according to technical design:

• lamella;
• metal-ceramic;
• metal felt;
• pressed;
• nickel foam (polymer foam).

Polymer foam and metal felt cathodes have the highest capacity and service life.

The conductor between them is alkali. Concentrated potassium hydroxide is used here.

The design of the battery may vary depending on the purposes and tasks. Most often, these are an anode and a cathode rolled into a roll, between which there is a separator. There are also options where the plates are placed alternately, arranged with a separator. A mandatory design element is a safety valve; it is activated when the pressure inside the battery emergency increases to 2-4 MPa.

What types of Ni-Mh batteries are there and their technical characteristics

All Ni-Mh batteries are Rechargeable Battery (translated as battery). battery of this type are produced different types and forms. All of them are intended for a variety of purposes and tasks.

There are batteries that are almost never used at the moment, or are used to a limited extent. Such batteries include the “Krona” type, it was marked 6KR61, they used to be used everywhere, now they can only be found in old equipment. Batteries of type 6KR61 had a voltage of 9v.

We will analyze the main types of batteries and their characteristics that are used now.

• AA.. The capacity ranges from 1700-2900 mAh.
• AAA.. Sometimes labeled MN2400 or MX2400. Capacity – 800-1000 mAh.
• WITH. Medium-sized batteries. They have a capacity in the range of 4500-6000 mAh.
• D. The most powerful type of battery. Capacity from 9000 to 11500 mAh.

All listed batteries have a voltage of 1.5v. There are also some models with a voltage of 1.2v. Maximum voltage 12v (by connecting 10 1.2v batteries).

Pros and cons of Ni-Mh battery

As already mentioned, this type of battery replaced older varieties. Unlike analogues, the “memory effect” was significantly reduced. They also reduced the amount of substances harmful to nature used during the creation process.

The advantages include the following nuances.

• Work well at low temperatures. This is especially important for equipment used outdoors.
• Reduced “memory effect”. But still he is present.
• Non-toxic batteries.
• Higher capacity compared to analogues.

Batteries of this type also have disadvantages.

• Higher self-discharge value.
• More expensive to produce.
• After approximately 250-300 charge/discharge cycles, the capacity begins to decrease.
• Limited service life.

Where are nickel metal hydride batteries used?

Due to their large capacity, such batteries can be used everywhere. Whether it is a screwdriver or a complex measuring device, in any case, such a battery will easily provide it with the required amount of energy.

In everyday life, such batteries are most often used in portable lighting devices and radio equipment. Here they show good performance, maintaining optimal consumer properties long time. Moreover, both disposable and reusable cells can be used, regularly recharged from external sources nutrition.

Another application is instruments. Due to their sufficient capacity, they can also be used in portable medical equipment. They work well in blood pressure monitors and glucometers. Since there are no voltage surges, there is no influence on the measurement result.

Many measuring instruments in technology have to be used outdoors, including in winter. Here metal hydride batteries are simply irreplaceable. Due to their low response to negative temperatures, they can be used in the most difficult conditions.

Operating rules

It must be taken into account that new batteries have a fairly high internal resistance. To achieve some reduction in this parameter, you should discharge the battery to zero several times at the beginning of use. To do this, you should use chargers with this function.

Attention! This does not apply to disposable batteries.

You can often hear the question to how many volts can you discharge a Ni-Mh battery. In fact, it can be discharged to almost zero parameters, in which case the voltage will not be enough to maintain the operation of the connected device. It is even recommended to sometimes wait until the battery is completely discharged. This helps reduce the “memory effect”. The battery life is accordingly extended.

Otherwise, the operation of batteries of this type does not differ from analogues.

Is it necessary to pump Ni-Mh batteries?

An important stage of operation is pumping up the battery. Nickel metal hydride batteries also require this procedure. This is especially important after long-term storage in order to restore capacity and maximum voltage.

To do this, you need to discharge the battery to zero. Please note that electric shock is required. As a result, you should get the minimum voltage. This way you can revive the battery, even if quite a lot of time has passed since the date of manufacture. The longer the battery has been sitting, the more charging cycles are required. Typically, it takes 2-5 cycles to restore capacitance and resistance.

How to restore Ni MH battery

Despite all the advantages and features, such batteries still have a “memory effect”. If the battery begins to lose performance, then it should be restored.

Before starting work, you need to check the battery capacity. Sometimes it turns out that it is almost impossible to improve the performance, in which case you just need to replace the battery. We also check the battery for malfunction.

The work itself is similar to pumping. But here they do not achieve a complete discharge, but simply reduce the voltage to a level of 1v. It takes 2-3 cycles. If during this time it was not possible to achieve the optimal result, the battery should be considered unusable. When charging, you need to maintain the Delta Peak parameter for a specific battery.

Storage and disposal

It is worth storing the battery at a temperature close to 0°C. This is the optimal state. It is also necessary to take into account that storage should occur only during the expiration date, these data are indicated on the packaging, but different manufacturers decoding may vary.

Manufacturers worth paying attention to

All battery manufacturers produce Ni-Mh batteries. In the list below you can see the most famous companies offering similar products.

• Energizer;
• Varta;
• Duracell;
• Minamoto;
• Eneloop;
• Camelion;
• Panasonic;
• Irobot;
• Sanyo.

If you look at the quality, they are all about the same. But we can highlight Varta and Panasonic batteries; they have the most optimal price-quality ratio. Otherwise, you can use any of the listed batteries without any restrictions.

The main difference between Ni-Cd batteries and Ni-Mh batteries is the composition. The base of the battery is the same - it is nickel, it is the cathode, but the anodes are different. For a Ni-Cd battery, the anode is cadmium metal; for a Ni-Mh battery, the anode is a hydrogen metal hydride electrode.

Each type of battery has its pros and cons, knowing them you can more accurately select the battery you need.

Will the old charger fit the new battery if I change the Ni-Cd to a Ni-Mh battery or vice versa?

The charging principle for both batteries is absolutely the same, so the charger can be used from the previous battery. The basic rule for charging these batteries is that they can only be charged after they are completely discharged. This requirement is a consequence of the fact that both types of batteries are subject to the “memory effect”, although with Ni-Mh batteries this problem is minimized.

How to properly store Ni-Cd and Ni-Mh batteries?

The best place to store a battery is in a cool, dry room, since the higher the storage temperature, the faster the battery self-discharges. The battery can be stored in any condition other than completely discharged or fully charged. The optimal charge is 40-60%%. Once every 2-3 months, you should recharge (due to the presence of self-discharge), discharge and charge again to 40-60% of the capacity. Storage for up to five years is acceptable. After storage, the battery should be discharged, charged and then used normally.

Can I use batteries with a larger or smaller capacity than the battery from the original kit?

Battery capacity is the operating time of your power tool on battery power. Accordingly, there is absolutely no difference in battery capacity for a power tool. The actual difference will only be in the charging time of the battery and the operating time of the power tool from the battery. When choosing a battery capacity, you should proceed from your requirements; if you need to work longer using one battery, choose a larger one. capacious batteries, if the supplied batteries are completely satisfactory, then you should choose batteries of equal or similar capacity.

It all started with the fact that my camera point-and-shoot device flatly refused to work with batteries freshly removed from the charger - four AA-size NiMH batteries. Take them as usual and throw them away. But for some reason this time curiosity prevailed over common sense (or maybe it was the toad that spoke), and I wanted to understand whether it was possible to squeeze at least something else out of these batteries. The camera is very hungry for energy, but there are also more modest consumers - wireless mice or keyboards, for example.

Actually, there are two parameters that are interesting to the consumer - the battery capacity and its internal resistance. There are also few possible manipulations - discharge and charge. By measuring the current and time during the discharge process, you can estimate the battery capacity. According to the difference in battery voltage to idling and under load, internal resistance can be estimated. By repeating the discharge-charge cycle (i.e., performing the “training”) several times, you can understand whether this action makes sense at all.

Accordingly, the following plan was formed - we are making a controlled spark gap and charger with the ability continuous measurement process parameters, perform simple arithmetic operations on the measured values, repeat the process the right number once. We compare, draw conclusions, and finally throw away the batteries.

Measuring stand

The stand was made from what was found within a radius of three meters. If someone wants to repeat it, it is not at all necessary to follow the diagram exactly. The choice of element parameters can be quite wide, I will comment on this a little later.

The discharge unit is a controlled current stabilizer based on op-amp IC1B (LM324N) and field effect transistor Q1. Almost any transistor, as long as there are enough permissible voltages, currents and power dissipation. And they are all small here. Resistor feedback and at the same time part of the load (together with Q1 and R20) for the battery - R1. Its maximum value must be such as to provide the required maximum discharge current. If we assume that the battery can be discharged to 1 V, then to ensure a discharge current of, for example, 500 mA, resistor R1 should not be more than 2 Ohms. The stabilizer is controlled by a three-bit resistive DAC (R12-R17). Here the calculation is as follows - the voltage at the direct input of the op-amp is equal to the voltage at R1 (which is proportional to the discharge current). We change the voltage at the direct input - the discharge current changes. To scale the DAC output to the desired range, there is a trimming resistor R3. It is better if it is multi-turn. The values ​​of R12-R17 can be any (in the region of tens of kilo-ohms), the main thing is that the ratio of their values ​​is 1/2. No special accuracy is required from the DAC, since the discharge current (voltage on R1) is measured directly by the instrumentation amplifier IC1D during the process. Its gain is K=R11/R10=R9/R8. The output is fed to the microcontroller ADC (A1). By changing the values ​​of R8-R11, the gain can be adjusted to the desired value. The voltage on the battery is measured by the second amplifier IC1C, K=R5/R4=R7/R6. Why control the discharge current? The point here is basically this. If you discharge with a constant high current, then due to the high internal resistance of worn-out batteries, the minimum permissible voltage of 1 V (and there is no other reference point for stopping the discharge) will be reached before the battery actually discharges. If you discharge with a constant low current, the process will take too long. Therefore, the discharge is carried out in stages. Eight steps seemed enough to me. If the hunt is more/less, then you can change the bit depth of the DAC. In addition, by turning the load on and off, you can estimate the internal resistance of the battery. I think that the controller operation algorithm during discharge does not require further explanation. At the end of the process, Q1 is locked, the battery is completely disconnected from the load, and the controller turns on the charge unit.

Charge block. Also a current stabilizer, only uncontrolled, but switchable. The current is set by the reference voltage source on IC2 (2.5 V, accuracy 1% according to the datasheet) and resistor R21. In my case, the charge current was classic - 1/10 of the nominal battery capacity. Feedback resistor - R20. You can use any other reference voltage source - depending on your taste and availability of parts. Transistor Q2 operates in a more rigid mode than Q1. Due to the noticeable difference between Vcc and battery voltage, significant power is dissipated across it. This is the price to pay for the simplicity of the circuit. But the radiator saves the situation. Transistor Q3 serves to forcibly turn off Q2, i.e., to turn off the charge unit. Controlled by signal 12 of the microcontroller. Another reference voltage source (IC3) is needed for the controller’s ADC to operate. The measurement accuracy of our stand depends on its parameters. LED1 - to indicate the process status. In my case, it does not light up during the discharge process, lights up when charging and flashes when the cycle is completed.
The supply voltage is selected to ensure that the transistors open and operate in the required ranges. IN in this case For both transistors, the gate unlocking voltage is quite high - about 2-4 V. In addition, Q2 is “backed up” by the battery voltage and R20, so the gate unlocking voltage starts from approximately 3.5-5.5 V. In turn, LM323 cannot raise the output voltage above Vcc minus 1.5 V. Therefore, Vcc must be quite large and in my case it is 9 V.

The charge control algorithm was based on the classic version of monitoring the moment the voltage on the battery begins to drop. However, in reality everything turned out to be not entirely true, but more on that later.
All measured values ​​during the “research” process were written to a file, then calculations were made and graphs were drawn.

I think that everything is clear with the measuring stand, so let’s move on to the results.

Measurement results

Graphs on the axes: time, hours (X) and power, W (Y) for the best and worst batteries. It can be seen that the stored energy (the area under the graphs) is significantly different. In numerical terms, the measured battery capacities were 1196, 739, 1237 and 1007 mAh. Not a lot, considering that the nominal capacity (which is indicated on the case) is 2700 mAh. And the spread is quite large. What about internal resistance? It was 0.39, 0.43, 0.32 and 0.64 Ohm, respectively. Terrible. It is clear why the soap dish refused to work - the batteries are simply not able to deliver a large current. Well, let's start training.

Cycle one. Again the output power of the best and worst battery.

Progress is visible to the naked eye! The numbers confirm this: 1715, 1444, 1762 and 1634 mAh. The internal resistance also improved, but very unevenly - 0.23, 0.40, 0.1, 0.43 Ohm. It would seem there is a chance. But alas, further discharge/charge cycles did not yield anything. The capacitance values, as well as the internal resistance, varied from cycle to cycle within about 10%. Which lies somewhere close to the limits of measurement accuracy. Those. Long training, at least for my batteries, did nothing. But it became clear that the batteries retained more than half of their capacity and would still work at low current. At least some savings on the farm.

Now I want to dwell a little on the charging process. Perhaps my observations will be useful to someone who is planning to design a smart charger.
Here is a typical charge graph (on the left is the battery voltage scale in volts).

After the start of charging, a voltage dip is observed. In different cycles it may be greater or lesser in depth, slightly different in duration, and sometimes absent. Then, for about 10 hours, there is a uniform increase and then an almost horizontal plateau. The theory states that with a low charge current there is no voltage drop at the end of the charge. I was patient and still waited for this fall. It’s small (it’s almost invisible to the eye on the chart), you have to wait a very long time for it, but it’s always there. After ten hours of charging and before the decline, the voltage on the battery, although it increases, is extremely insignificant. This has almost no effect on the final charge; no unpleasant phenomena such as heating of the battery are observed. Thus, when designing low-current chargers There is no point in providing them with intelligence. A timer for 10-12 hours is enough, and no special accuracy is required.

However, this idyll was disrupted by one of the elements. After about 5-6 hours of charging, very noticeable voltage fluctuations occurred.

At first I attributed this to a design flaw in my stand. The photo shows that everything was assembled using a hinged installation, and the controller was connected with rather long wires. However, repeated experiments have shown that such nonsense consistently occurs with the same battery and never occurs with others. To my shame, I did not find the reason for this behavior. Nevertheless (and this is clearly visible on the graph) the average voltage value is growing as it should.

Epilogue

That's all. Thank you for your attention.