For product designers, the crucial factor in battery selection is often the run time a battery provides during specific device use. Before finalizing a design, designers typically rely on rated capacities, which are obtained from well-conditioned batteries undergoing a constant-current discharge at room temperature after optimal charging. The standard battery rating, abbreviated as C, is determined under specific conditions, and the discharge rate affects capacity.
In the case of nickel-metal hydride batteries, their rated capacity is usually established at a discharge rate that fully depletes the battery in five hours. This process is repeated up to five cycles to ensure accurate results. Normalizing charge and discharge parameters by the C rate is common, as it allows for fair comparisons of battery performance within a family of various sizes and capacities. This approach helps designers understand how batteries of different sizes and capacities perform relative to their standardized rating.
Internal Resistance
NiMH batteries have a low internal resistance (IR) because of their wound construction, improved contacts, and large electrode surface area. This low IR enables excellent high-rate performance. When fully charged, NiMH batteries typically have an internal resistance of less than 50 milliohms, contributing to their efficient operation.
During discharge, the battery's internal resistance remains relatively constant until it nears the end of its lifespan, at which point it increases sharply. The accompanying graph illustrates the calculated IR during a 750 mA discharge with a 10 mA pulse every 6 minutes. As NiMH batteries age and undergo usage, their internal resistance tends to rise, leading to lower operating voltage and higher voltage during charging. Additionally, cycling the battery over time further increases its internal resistance.
Memory/Voltage Depression
The concern of "memory" or voltage depression, once an issue for devices using nickel-cadmium batteries, is no longer relevant. Memory issues were observed in some applications where nickel-cadmium batteries were regularly partially discharged, causing a drop in the discharge voltage profile by around 150 mV per battery when transitioning from routine to rare discharges. While opinions on the severity of this problem in nickel-cadmium batteries may vary, it is generally attributed to the structure of the cadmium electrode.
With the advent of nickel-metal hydride batteries, which do not contain cadmium, memory problems are no longer a worry. The elimination of cadmium from the battery structure has resolved this issue, offering improved performance and eliminating concerns related to voltage depression or memory effects.
Discharge Termination
To avoid irreversible damage to the battery due to battery reversal
during discharge, it is strongly advised to remove the load from the
battery before it is completely discharged. The usual voltage profile
during a total discharge exhibits a dual plateau, as shown in Figure.
These plateaus occur because of the discharge of the positive electrode
first, followed by the residual capacity in the negative electrode.
When both electrodes are reversed, significant hydrogen gas evolution
takes place. This can lead to battery venting and irreversible damage.
Therefore, it is crucial to prevent total discharge and remove the load
in order to protect the battery from potential harm caused by reversal.
To keep the battery safe, it's crucial to stop using it before it
reaches the second plateau during discharge, where damage can happen.
However, choosing the right voltage to stop the discharge is tricky due
to two factors: high-speed use and situations where multiple batteries
are used together. These factors make it challenging to pick the right
voltage to avoid harming the battery.
Voltage Cutoff at High Rates
Usually, the decision to stop discharging a battery is based on voltage drops, often set at 0.9 volts per battery, which is 75 percent of the nominal mid-point voltage of 1.2 volts per battery. This value works well for most regular discharges in individual battery applications.
However, in cases of high drain-rate usage (1-4C), where the voltage curve takes on a more rounded shape, using a fixed 0.9V/battery cutoff might be too early. This premature cutoff may leave a significant portion of the battery capacity unused. In such situations, a better choice is to use 75 percent of the mid-point voltage at that specific discharge rate as the cutoff. It's important to note that this decision is made solely to prevent damage to the battery.
In certain end applications, it might be justified to choose a higher voltage cutoff, even if it means sacrificing some potential additional capacity. The decision to opt for a higher cutoff voltage could be driven by specific requirements or considerations related to the performance and longevity of the battery in that particular use case. It reflects a balance between extracting maximum capacity and ensuring the overall health and safety of the battery within the context of the end application.
Charging Characteristics
Properly charging nickel-metal hydride (NiMH) batteries is crucial for optimal performance and longevity. Successful charging involves finding a balance between quick, thorough charging and avoiding overcharging, which can harm the battery's lifespan. It's important to use an economical and reliable charger. Refer to the charger manual for additional information on charging techniques and termination.
Compared to nickel-cadmium batteries, NiMH batteries are more sensitive to charging conditions. Insufficient charging can lead to low service, while overcharging can result in a loss of cycle life. NiMH batteries undergo an exothermic charging process involving hydrogen and oxygen. Precautions should be taken to prevent venting, and if venting occurs, manage the vent gases properly.
A graph (Fig. 12) illustrates the typical behavior of a NiMH battery being charged at the C rate. It emphasizes the importance of charge control and highlights battery characteristics used to determine when to apply charge control. During charging, voltage initially spikes, gradually rises until full charge, and then peaks before trending down. The charge process generates heat, causing a positive slope in the temperature curve. Overcharging leads to a significant temperature increase and pressure rise, posing a risk of physical damage to the battery without proper safety measures.
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Recommended Charging Rates
For NiMH batteries, it's generally preferred to use a moderate-rate smart charger that takes about 2 to 3 hours. These chargers have built-in circuitry to protect the batteries from overcharging. Extremely fast charging, less than 1 hour, can negatively impact the battery's cycle life and should be used sparingly, only as needed.
Alternatively, slow overnight chargers with timer-based functions are also acceptable and can be a cost-effective option compared to smart chargers. A charger applying a 0.1 C rate for 12 to 14 hours is well-suited for NiMH batteries. Additionally, a maintenance or trickle charge rate of less than 0.025 C (C/40) is recommended. Using very small trickle charges is preferred to minimize the adverse effects of overcharging, contributing to the overall health and longevity of the NiMH batteries.
Self Discharge
NiMH batteries gradually lose power over time due to internal reactions within the battery. After 12 months of storage, they typically retain about 50% to 80% of their capacity. Higher temperatures can speed up this self-discharge process. To maximize battery performance during storage:
1. Keep them at the lowest feasible temperatures (ideally between -20°C to 30°C).
2. Store batteries without any connections to avoid loaded storage effects.
3. Store them in a charged state, except for large volumes.
4. Keep them in a clean, dry, and protected environment to minimize physical damage.
5. Follow good inventory practices, using a "first in, first out" approach to reduce the time batteries spend in storage.
Capacity Recovery After Storage
When you store batteries, they usually provide full capacity upon the first use after being taken out and charged with standard methods. However, if batteries are stored for a long time or in high temperatures, it might take more than one cycle to reach their original capacities. If you plan on prolonged storage and quick restoration, it's a good idea to consult with the manufacturer.
For consumer-use batteries meant for extended storage, especially if they're fully discharged, it's recommended to remove them from the device. Some electronic devices, even when turned off, can have a low-level drain on batteries. This small current might sustain memory functions, sensor circuits, or keep switch positions. To avoid potential issues, eliminate these loads when storing batteries for a long time.
When nickel-metal hydride batteries are stored under load, a small amount of electrolyte might seep around seals or vents. This seepage can lead to the formation of potassium carbonate crystals, affecting the battery's appearance. While such incidents are rare, for applications requiring extended battery storage, it's suggested to electrically isolate the battery positively. This can be done by using insulating tape over the positive terminal or by removing the battery from the device.
Factors Affecting Life
The lifespan of nickel-metal hydride (NiMH) batteries can be significantly influenced by how consumers use them, especially in terms of the charger choice that ensures proper charging without overcharging. Effective control of overcharging, exposure time, and charge rate is crucial for enhancing battery life, which is typically expected to be two to five years.
Several factors impact battery life:
1. **Degree of Overcharge:** Some overcharging is necessary to ensure all batteries are fully charged and balanced. However, maintaining full charge currents for extended periods can reduce the battery's life.
2. **Exposure to High Temperatures:** Higher temperatures, especially during charging, accelerate chemical reactions within the battery and contribute to aging. Charge acceptance is reduced at higher temperatures, making it challenging to sense the transition from charge to overcharge.
3. **Battery Reversal:** Discharging NiMH batteries to the point of reversal, especially if done routinely, can shorten battery life.
4. **Prolonged Storage under Load:** Keeping a load on a battery beyond full discharge can lead to irreversible changes in battery chemistry, promoting issues like creep leakage.
The life of NiMH batteries involves both abrupt failure events and gradual deterioration. Abrupt failures are rare and can result from mechanical events causing a short or an open circuit. Battery deterioration takes the form of oxidation of the negative active material, leading to mid-point voltage depression, and deterioration of the positive active material, resulting in reduced capacity. Both phenomena result in a loss of usable capacity, requiring application design adjustments or proper initial battery selection.
The specific mechanism determining battery life may vary based on application parameters and battery characteristics. Ongoing development work aims to reduce negative electrode oxidation, minimizing mid-point voltage depression as the battery ages.
