NiMH stands for Nickel Metal Hydride. Proper charging is key to maintaining performance and longevity. You need to know this technology in order to charge NiMH. The recovery of NiMH cells is a rather complicated process, because the voltage peak and the subsequent drop are smaller, and therefore, the indicators are more difficult to determine. Overcharging leads to overheating and damage to the cell, after which the capacity is lost, followed by loss of functionality.
Design and principle of operation
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, and an insulating foil separator between them. This energy "sandwich" is wound and placed in a battery along with liquidelectrolyte. The positive electrode usually consists of nickel, the negative electrode of metal hydride. Hence the name "NiMH", or "nickel metal hydride".
Benefits:
- Contains less toxins and is environmentally friendly and recyclable.
- Memory effect is higher than Ni-Cad.
- Much safer than lithium batteries.
Flaws:
- Deep discharge shortens life and generates heat during fast charging and high load.
- Self-discharge is higher compared to other batteries and must be taken into account before charging NiMH.
- High level of maintenance required. The battery must be fully discharged to prevent crystal formation during charging.
- More expensive than Ni-Cad battery.
Characteristics of charge/discharge
The Nickel-Metal Hydride cell has many characteristics similar to NiCd, such as the discharge curve (with additional charging) that the battery can accept. It is intolerant of overcharging causing capacity degradation, which is a major problem for charger designers.
Current specifications needed to properly charge a NiMH battery:
- Rated voltage is 1.2V.
- Specific energy - 60-120 Wh/kg.
- Energy density - 140-300 Wh/kg.
- Specific power - 250-1000 W/kg.
- Charge/discharge efficiency -90%.
The charging efficiency of nickel batteries ranges from 100% to 70% of full capacity. Initially, there is a slight increase in temperature, but later, when the charge level rises, the efficiency drops, generating heat, which must be taken into account before charging NiMH.
When a NiCD battery is discharged to a certain minimum voltage and then charged, care must be taken to reduce the conditioning effect (about every 10 charge/discharge cycles), otherwise it will start to lose capacity. For NiMH, this requirement is not required as the effect is negligible.
However, such a recovery process is also convenient for NiMH 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 NiMH batteries. They, like NiCds, require a constant current source. The 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 operating batteries, you need to clearly know what current to charge NiMH batteries with. There are several methods that are used to prevent failure:
- Charging by timer. Use of time fordetermining the end of the 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 cell is charged from a known state, such as when it is fully discharged.
- Thermal detection. Determination of the end of the process is carried out by monitoring the temperature of the element. While the device will get warmer when overcharged, it is difficult to accurately gauge the temperature rise as the center of the battery will be much hotter than the outside.
- Detection of negative delta voltage. NiMH detects voltage drop (5 mV). Before charging NiMH batteries, noise filtering is introduced to reliably capture such a drop to ensure that "parasitic" sensor and other noises do not lead to the end of charging.
Parallel supply of elements
Parallel charging of batteries makes it difficult to qualitatively determine the end of the process. This is because one cannot be sure that each cell or package has the same resistance and therefore some will draw more current than others. This means that a separate charging circuit must be used for each line in the parallel unit. It should be established how much current to charge the NiMH by balancing, for example, using resistors of such a value that they will dominate the control parameters.
Modern algorithms have been developed to ensure accurate charging without the use of a thermistor. Thesedevices are similar to Delta V, but have special measurement methods for detecting full charge, usually involving some kind of cycle where 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 may fill up one at a time, signaling the end of a stage.
It will take several cycles to balance them. When the battery reaches the end of its charge, oxygen begins to form at the electrodes and recombine at the catalyst. The new chemical reaction creates heat that is easily measured with a thermistor. This is the safest way to detect the end of a process during a quick restore.
Cheap way to regenerate
Overnight charging is the cheapest way to charge a NiMH battery at C/10, which is below 10% of rated capacity per hour. This must be taken into account in order to properly charge NiMH. So a 100mAh battery will charge at 10mA for 15 hours. This method does not require an end-of-process sensor and provides a full charge. Modern cells have an oxygen recycle catalyst that prevents damage to the battery when exposed to electric current.
This method cannot be used if the charging speed is over C/10. The minimum voltage required for a complete reaction is dependent on temperature (at least 1.41V per cell at 20 degrees), which must be considered in order to properly charge NiMH. Prolonged recovery does not cause ventilation. It slightly heats up the battery. In order 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 via an LED and also performs a synchronization function.
Fast charging process
Using the timer, you can charge the C/3.33 for 5 hours. This is a bit risky as the battery must be fully discharged first. One way to make sure this doesn't happen is to automatically discharge the battery by the charger, which then starts the recovery process for 5 hours. This method has the advantage of eliminating any possibility of creating a 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 perform a discharge. A power dissipator will be 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 up to 1C, in other words, 100% amp-hour capacity for 1.5 hours. The PowerStream battery charge controller does this in conjunction with a control board that is capable of measuring voltage and current for more complex algorithms. When the temperature rises, the process must be stopped, and whendT/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 detect the end of charge. In practice, they work very well, which is why modern devices use this technology, which includes on and off processes to measure voltage.
Adapter specs
An important issue is battery life, or the total lifetime cost of the system. In this case, manufacturers offer devices with microprocessor control.
Algorithm for the perfect charger:
- Soft start. If the temperature is above 40 degrees or below zero, start by charging C/10.
- Option. If the discharged battery voltage is higher than 1.0V/cell, discharge the battery to 1.0V/cell, and then proceed to fast charging.
- Fast charging. At 1 degree until the temperature reaches 45 degrees or dT indicates full charge.
- After fast charging is completed, charge at C/10 for 4 hours to ensure a full charge.
- If the voltage of a charged NiMH battery rises to 1.78V/cell, stop operation.
- If the fast charging time exceeds 1.5 hours without interruption, it will be stopped.
Theoretically, recharging is a charge rate that is fast enough to keep the battery fully charged, but slow enough to avoid overcharging. Determining the optimal recharge rate for a specific batterya little hard to describe, but it is generally accepted that it is about ten percent of the capacity of the battery, for example, for Sanyo 2500 mAh AA NiMH, the optimal recharging rate 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 the so-called "fast chargers" for 5 or 8 hours. The problem with these instruments is that they don't really 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 applied incorrectly, battery life will be shortened in several ways:
- When fully charged or partially charged batteries are inserted into the device, it cannot sense it, so it fully charges the batteries it was designed for. So, battery capacity drops.
- Another common situation is to interrupt the charging cycle in progress. However, this is followed by a reconnection. Unfortunately, this causes a full charge cycle to be restarted, even if the previous cycle is almost complete.
The easiest wayTo avoid these scenarios, use an intelligent microprocessor-controlled charger. It can detect when the battery is fully charged, and then, depending on its design, either turn off completely or switch to trickle charge mode.
iMax B6 smart devices
In order to charge the NiMH iMax, you will need a dedicated charger, as using the wrong method can render the battery useless. Many users consider iMax B6 to be the best choice for NiMH charging. It supports the process of up to 15 cell batteries, as well as many settings and configurations for different types of batteries. The recommended charging time should not exceed 20 hours.
Typically, the manufacturer guarantees 2000 charge/discharge cycles from a standard NiMH battery, although this may vary depending on usage conditions.
Working algorithm:
- Charging NiMH iMax B6. It is necessary to connect the power cord to the outlet on the left side of the device, taking into account the shape at the end of the cable to ensure that the connection is correct. We insert it all the way and stop pressing when a sound signal and a welcome message appear on the display screen.
- Use the silver button on the far left to scroll through 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: charge, discharge, balance, fast charge, storage andothers.
- Two central control buttons will help you select the desired number. By pressing the far right button to enter, you can go to the voltage setting by scrolling again with the two center buttons and pressing enter.
- Use multiple cables to connect the battery. The first set looks like lab wire equipment. It often comes bundled with crocodile clips. Sockets for connection are located on the right side of the device near the bottom. They are easy enough to spot. This is how you can charge NiMH with iMax B6.
- Then you need to connect the free battery cable to the end of the red and black clamps, creating a closed loop. This can be a little risky, especially if the user makes the wrong settings for the first time. Press and hold the enter button for three seconds. The screen should then inform that it is testing the battery, after which the user will be asked to confirm the mode setting.
- While the battery is charging, you can scroll through the various screens on the display using the two center buttons that provide information about the charging process in different modes.
Tips for optimizing battery life
The most standard advice is to completely drain the batteries and then recharge them. Although this is a treatment for the "memory effect", care must be taken in nickel-cadmium batteries, as it is easy to damage them due to over-discharging, which leads to "pole reversal" and irreversible processes. In some cases, battery electronics are madein a way that prevents negative processes by shutting down before they happen, but simpler devices like flashlights don't.
Required:
- Be ready to replace them. Nickel-metal hydride batteries don't last forever. After the end of the resource, they will stop working.
- Buy a smart charger that electronically controls the process and prevents overcharging. Not only is this better for batteries, but it also uses less power.
- Remove the battery when recharging is complete. Unnecessary time on the device means that more jet energy is being used to charge it, thus increasing wear and tear and using more power.
- Do not drain batteries completely to prolong their life. Despite all advice to the contrary, a complete discharge will actually shorten their life.
- Store NiMH batteries at room temperature in a dry place.
- Excess heat can damage batteries and cause them to drain quickly.
- Consider using a low battery model.
Thus, you can draw a line. Indeed, NiMH batteries are more prepared by the manufacturer for today's environment, and properly charging batteries using a smart device will ensure their performance and longevity.
