How to charge NiMH batteries correctly

NiMH means nickel metal hydride. Proper charging is the key to maintaining performance and durability. You need to know this technology in order to charge NiMH. The restoration of NiMH elements is a rather complicated process, because the voltage peak and subsequent drop are smaller, and therefore, the indicators are more difficult to determine. Excessive charging leads to overheating and damage to the element, after which capacity is lost with the subsequent loss of functionality.

Device and principle of operation

A battery is an electrochemical device in which electrical energy is converted and stored in chemical form. Chemical energy is easily converted into electrical energy. NiMH works on the principle of absorbing, releasing and transporting hydrogen inside two electrodes.

NiMH batteries consist of two metal strips that act as positive and negative electrodes, as well as an insulating foil separator between them. This energy “sandwich” is wound and placed in a battery with liquid electrolyte. The positive electrode usually consists of nickel, the negative - of metal hydride. Hence the name "NiMH", or "nickel-metal hydride."

Benefits:

1. It contains less toxins and is environmentally friendly, recyclable.
2. The memory effect is higher than that of Ni-Cad.
3. Much safer than lithium batteries.

Disadvantages:

1. Deep discharge shortens service life and generates heat during fast charge and high load.
2. Self-discharge is larger compared to other batteries, it must be considered before charging NiMH.
3. A high level of maintenance is required. The battery must be completely discharged to prevent the formation of crystals during charging.
4. More expensive than a Ni-Cad battery.

Charge / discharge characteristics

A nickel metal hydride cell has many characteristics similar to NiCd, for example, the discharge curve (taking into account the additional charge) that the battery can take. It is intolerant of overcharging, which causes a decrease in capacity, which is a serious problem for developers of chargers.

Current characteristics required to properly charge a NiMH battery:

1. The rated voltage is 1.2V.
2. Specific energy - 60-120 Watt-hour / kg.
3. The energy density is 140-300 W-hour / kg.
4. Specific power - 250-1000 W / kg.
5. Charging / discharging efficiency - 90%.

The charging efficiency of nickel batteries ranges from 100% to 70% of full capacity. At first, a slight increase in temperature occurs, but later, when the charge level rises, the efficiency drops, generating heat, which must be taken into account before charging NiMH.

When the NiCD battery is discharged to a certain minimum voltage and then charged, measures must be taken to reduce the conditioning effect (approximately every 10 charge / discharge cycles), otherwise it will begin to lose capacity. For NiMH, this requirement is not required, since the effect for it is negligible.

Nevertheless, such a recovery process is also convenient for nickel-metal hydride devices; it is recommended to consider it before charging NiMH batteries. The process is repeated three to five times before they reach full capacity. The conditioning process of rechargeable batteries ensures that they will last for many years.

NiMH Recovery Methods

There are several charging methods that can be used with nickel metal hydride batteries. They, like NiCds, require a direct current source. Speed ​​is usually indicated on the cell body. It should not exceed technological standards. The limits of charging boundaries are clearly regulated by manufacturers. Before using batteries, you need to clearly know how to charge NiMH batteries. There are several methods that are used to prevent failure:

1. Charging by timer. Using time to determine the end of a process is the easiest way. Often an electronic timer is built into the device, although many devices do not have this feature. The approach assumes that the element is charged from a known state, for example, when it is completely discharged.
2. Thermal detection. Determination of the end of the process is carried out by observing the temperature of the element. Despite the fact that the device heats up when overcharged, it is difficult to accurately assess the temperature increase, since it will be much hotter in the center of the battery than outside.
3. Negative delta voltage detection. NiMH detects a voltage drop (5 mV). Before charging NiMH batteries, a noise filtering is introduced to reliably fix such a drop in order to ensure that the “spurious" sensor and other noises do not lead to the end of charging.

Parallel Power Elements

Parallel charging of batteries makes it difficult to accurately determine the end of the process. This is due to the fact that you cannot be sure that each cell or packet has the same resistance, and therefore some of them will consume more current than others. This means that you need to use a separate charging circuit for each line in the parallel unit. It is necessary to establish how to charge NiMH with current, determining the balance, for example, using resistors of such resistance that will dominate the control of parameters.

Modern algorithms have been developed to ensure accurate charging without the use of a thermistor. These devices are similar to Delta V, but have special measurement methods for detecting a full charge, usually involving some cycle when the voltage is measured over a time interval and between pulses. For multi-element packets, if they are not in the same state and are not balanced in capacity, they can be filled one at a time, giving a signal about the end of the stage.

To balance them, it will take several cycles. When the battery reaches the end of the charge, oxygen begins to form on the electrodes and recombine on the catalyst. A new chemical reaction creates heat that is easily measured by a thermistor. This is the safest way to determine the end of a process during quick recovery.

Cheap way to regenerate

Night charging is the cheapest way to charge a nickel metal hydride battery at C / 10, which is below 10% of the nominal capacity per hour. This must be considered in order to properly charge NiMH. Thus, a 100 mAh battery will be charged at 10 mA for 15 hours. This method does not require a process end sensor and provides a full charge. Modern elements have an oxygen recirculation catalyst that prevents damage to the battery when exposed to electric current.

This method cannot be used if the charging speed exceeds C / 10. The minimum voltage required for a complete reaction depends on the temperature (at least 1.41 V per cell at 20 degrees), which must be taken into account in order to properly charge NiMH. Prolonged recovery does not cause ventilation. It slightly heats the battery. To preserve the service life, it is recommended to use a timer with a range of 13 to 15 hours. The Ni-6-200 charger has a microprocessor that reports the state of charge through the LED, and also performs a synchronization function.

Quick charge process

Using the timer, you can charge C / 3.33 for 5 hours. This is a bit risky, as the battery must first be fully discharged. One way to make sure that this does not happen is to automatically discharge the battery by a charger, which then starts the recovery process for 5 hours. The advantage of this method is to eliminate any possibility of creating negative battery memory.

Currently, not all manufacturers produce such chargers, but the microprocessor board is used, for example, in the C / 10 / NiMH-NiCad-solar-charge-controller charger and can be easily modified to discharge. A power dissipation unit is required to dissipate the energy of a partially charged battery within a reasonable amount of time.

If a temperature monitor is used, NiMH batteries can be charged at a speed of up to 1C, in other words, 100% capacity in ampere hours for 1.5 hours. The PowerStream battery charge controller does this in conjunction with a control board that can measure voltage and current for more complex algorithms. When the temperature rises, the process should be stopped, and when the dT / dt value should be set to 1-2 degrees per minute.

There are new algorithms that use microprocessor control when using the -dV signal to determine the end of charge. In practice, they work very well, so modern devices use this technology, which includes on and off processes to measure voltage.

Adapter specifications

An important issue is battery life or the total cost of the system’s life. In this case, manufacturers offer microprocessor controlled devices.

Algorithm for an ideal charger:

1. Soft start. If the temperature is above 40 degrees or below zero, start by charging C / 10.
2. Option. If the voltage of a discharged battery is higher than 1.0 V / cell, discharge the battery to 1.0 V / cell, and then proceed to fast charging.
3. Fast charge. At 1 degree, until the temperature reaches 45 degrees or dT does not indicate a full charge.
4. After completion of the fast charge, charge at C / 10 for 4 hours to ensure full charge.
5. If the voltage of a charged NiMH battery rises to 1.78 V / cell, stop operation.
6. If the fast charge time exceeds 1.5 hours without interruption, it is stopped.

Theoretically, recharging is a charge rate that is high enough to keep the battery fully charged, but low enough to avoid recharging. Determining the optimal charging speed for a particular battery is a little difficult to describe, but it is generally accepted that it is about ten percent of the battery capacity, for example, for Sanyo 2500 mAh AA NiMH, the optimal charging speed is 250 mA or lower. It must be taken into account in order to properly charge NiMH batteries.

Battery Damage Processes

The most common cause of premature battery failure is overcharging. The types of chargers that most often cause it are so-called “fast devices” for 5 or 8 hours. The problem with these devices is that they really do not have a process control mechanism.

Most of them have simple functionality. They charge at full speed for a fixed period of time (usually five or eight hours), and then turn off or switch to a lower “manual” speed. If they are used properly, then everything is in order. If they are not used correctly, the battery life is reduced in several ways:

1. If fully charged or partially charged batteries are inserted into the device, it cannot feel it, therefore it fully charges the batteries for which it is intended. So, the battery capacity drops.
2. Another common situation is the interruption of the charging cycle in the process. However, a reconnection follows. Unfortunately, this leads to restarting the full charge cycle, even if the previous cycle is almost complete.

The easiest way to avoid these scenarios is to use an intelligent microprocessor-controlled charger. It can determine when the battery is fully charged, and then, depending on its design, either turn off completely or switch to recharging mode.

IMax B6 Smart Devices

In order to charge NiMH iMax, you will need a special charger, since using the wrong method can make the battery useless. Many users find the iMax B6 the best choice for charging NiMH. It supports the process of up to 15 cell batteries, as well as many settings and configurations for different types of batteries. Recommended charging time should not exceed 20 hours.

As a rule, the manufacturer guarantees 2000 charge / discharge cycles from a standard NiMH battery, although this number may vary according to operating conditions.

Work algorithm:

1. Charge NiMH iMax B6. You must plug the power cord into a power outlet on the left side of the device, taking into account the shape at the end of the cable to make sure that the connection is made correctly. We insert it all the way and stop pressing when a sound signal and a welcome message appear on the display screen.
2. Use the silver button in the far left corner to view the first menu and select the type of battery to be charged. Pressing the leftmost button will confirm the selection. The button on the right will scroll through the options: charging, discharging, balance, fast charging, storage and others.
3. Two central control buttons will help you select the desired number. By pressing the rightmost button to enter, you can go to the voltage setting, again scrolling with the two central buttons and pressing enter.
4. Use several cables to connect the battery. The first set looks like equipment for laboratory wires. It often comes complete with crocodile clips. Sockets for connection are located on the right side of the device near the bottom. They are easy enough to detect. This is how NiMH can be charged with the iMax B6.
5. Then you need to connect a free battery cable to the end of the red and black terminals, creating a closed loop. This can be a little risky, especially if the user first makes the wrong settings. Press and hold the enter button for three seconds. Then the screen should inform that it is checking the battery, after which the user will be asked to confirm the mode setting.
6. While the battery is charging, you can scroll through the various display screens using the two center buttons, which provide information about the charging process in various modes.

Battery Optimization Tips

The most standard tip is to completely discharge the batteries and then charge them. Although this is a treatment of the “memory effect”, one needs to be careful in nickel-cadmium batteries, as it is easy to damage them due to excessive discharge, which leads to “reversal of the poles” and to irreversible processes. In some cases, the electronics of the batteries are designed in such a way that prevents negative processes, turning off before they happen, but simpler devices, for example, for flashlights, do not.

It is necessary:

1. Be prepared to replace them. Nickel-metal hydride batteries do not last forever. After the end of the resource, they will stop working.
2. Buy a smart charger that electronically controls the process and prevents overcharging. This is not only better for batteries, but also consumes less power.
3. Remove the battery when recharging is complete. Unnecessary time on the device means that more “jet” energy is used to charge it, therefore, wear increases and more energy is consumed.
4. Do not discharge the batteries completely to extend their life. Despite all the advice to the contrary, a complete discharge actually reduces their service life.
5. Store NiMH batteries at room temperature in a dry place.
6. Excessive heat can damage the batteries and cause them to run out quickly.
7. Consider using a low battery model.

Thus, a line can be drawn. Indeed, nickel-metal hydride batteries are more prepared by the manufacturer for working in modern conditions, and the correct charging of the batteries using a smart device will ensure their performance and durability.