on August 22th 13,906

NiMH Battery Charging: How to Charge Them?

Nickel-metal hydride (NiMH) batteries are based on older designs, the nickel-hydrogen battery. Both types use a nickel-hydroxide positive electrode and potassium hydroxide (KOH) as the electrolyte, similar to Nickel-Cadmium batteries. The difference is that instead of cadmium at the negative electrode, nickel-hydrogen batteries use hydrogen gas. To make storing hydrogen easier, scientists created metal-hydride compounds that can absorb and release hydrogen without the need for high-pressure containers. This led to the development of NiMH batteries.

This article explores how NiMH batteries charge and discharge, and how this differs from NiCd batteries, regarding overcharging. Overcharging affects NiMH batteries differently, impacting their lifespan and performance. We will also cover charging methods to maximize NiMH battery life and efficiency. By using the right charging techniques, you can boost battery performance and protect their health. This shows the importance of using the right tools and methods to keep NiMH batteries working well.

Catalog

Figure 1: NiMH Batteries

NiMH Charge/Discharge Characteristics

NiMH (Nickel-metal hydride) batteries behave differently from NiCd (Nickel-cadmium) batteries, when it comes to overcharging. Overcharging can cause capacity loss in NiMH batteries, that is why they require more advanced charging controls. While NiCd batteries show a clear voltage spike when fully charged, NiMH batteries only show a small voltage change, making it harder to detect full charge based on voltage alone. As a result, chargers must use temperature sensors to sense the heat that builds up once the battery is fully charged, helping to prevent overcharging.

NiMH batteries charge efficiently, similar to other nickel-based batteries, up until they reach about 70% of their capacity. After this point, charging becomes less efficient, and the battery starts to heat up. NiMH batteries charge differently depending on the manufacturer, making it hard to create a universal charger. As a result, advanced chargers that adjust to small voltage and temperature changes are required for longer battery life.

Figure 2: High-Rate Discharge of NiMH Battery

NiMH Charge Methods

Charging NiMH batteries requires careful control of the charging current to avoid overcharging. Several methods have been developed to achieve this. One common method is timer-based charging, where charging runs for a set amount of time based on the assumption that the battery starts empty. However, this approach risks overcharging, if the battery has lost capacity over time.

Figure 3: Timer-based Charger

Another method involves detecting heat. Charging stops once the battery starts to get noticeably warmer. While effective, this method doesn’t work well with slow chargers since the temperature change is too small.

Figure 4: Peak-Detecting NiMH Battery Fast Charger

A more sophisticated approach is the negative delta voltage (NDV) method, borrowed from NiCd charging. It looks for a slight drop in voltage to indicate the battery is full. For NiMH batteries, though, this voltage drop is very small, so advanced circuitry is a must to avoid errors.

Figure 5: Battery Charger with Negative Delta Voltage (NDV)

Many advanced chargers combine these techniques using NDV, temperature detection, and timers to improve accuracy. Some chargers also use a step-differential approach, starting with a high charge rate and gradually lowering it. This helps manage heat better and adapts to the specific needs of the battery.

NiMH Trickle Charging

NiMH batteries are more sensitive to overcharging than NiCd batteries, so trickle charging must be done at lower rates, usually around 0.05C. If trickle charging is too fast or lasts too long, it can damage the battery over time. To avoid this, it's better to periodically recharge the battery rather than leave it on trickle charge for extended periods.

In devices like cordless phones, where the battery stays connected to the charger, it's important to manage NiMH batteries carefully to avoid overcharging. Replacing the battery regularly helps keep the device running smoothly.

How to Charge NiMH Batteries?

Step 1

Choose a charger specifically made for NiMH batteries. The best option is a smart charger, equipped with a microprocessor and a thermistor. These components help prevent overcharging by tracking the battery’s capacity and temperature throughout the process. Smart chargers cost around $20 to $30 USD and can be found at electronics shops. Make sure the charger, whether fixed or with adjustable current settings, is compatible with your battery’s specifications.

Figure 6: Smart Charger for NiMH Batteries

Step 2

Carefully take the battery out of its device. For smaller, standard-sized batteries, this might involve simply popping them out. Larger battery packs may need wires to be unplugged or screws removed, using tools like a screwdriver. If you’re unsure how to proceed, refer to your device's manual for guidance.

Figure 7: Removing the Battery from the Device

Step 3

Look for the milliamp-hour (mAh) rating on the battery to know its capacity. This rating is need to know the correct charging approach. If the information isn't visible, a quick online search with the battery model number can help.

Figure 8: Battery Capacity Label

Step 4

Align the batteries properly in the charger, making sure the positive and negative ends are placed in the correct slots. For battery packs, connect the wires to the appropriate charger port.

Figure 9: Putting the Batteries into the Charger

Step 5

If you want to charge your battery gently, use a C/10 rate. To find this, divide the battery's capacity (in mAh) by 10. This slower charging method protects the battery from overheating and prolongs its life, though it takes longer.

Figure 10: Charge the Battery at C/10

Step 6

For fully depleted batteries, use a faster C/3.33 rate along with a charger that includes a built-in timer. The timer ensures the charging stops once the battery reaches full capacity, preventing overcharging.

Figure 11: Use a timer charger at C/3.33

Step 7

If you need a quicker charge, set the charger to a 1C rate. This provides a faster charge but requires close monitoring, as the battery could overheat or suffer damage if left unattended for too long.

Step 8

To determine how long to charge the battery, use the formula (Battery Capacity x 1.2) ÷ C-rate. NiMH batteries need a bit more energy than they release, so this calculation helps you estimate the correct charging time.

Figure 12: Calculating How Long to Leave the Battery on the Charger

Safe Charging Practices

It's important to use a charger specifically designed for NiMH cells since they aren't compatible with NiCd chargers.

Only charge your batteries at room temperature. Charging when the battery is hot such as right after use or near heat sources can lead to overheating and damage.

Unplug the charger once the battery is fully charged. Leaving the battery on charge for too long can shorten its lifespan. Regularly check the battery's charge level or use the timer on your charger to avoid overcharging.

Fast charging can be a good option if the battery supports it, as it allows easier monitoring of the charging process and keeps the temperature lower up to about 70% of the battery's capacity.

Store batteries at about 40% charge in a cool, dry place when not in use. If you're not using them for an extended period, partially discharge them or use the discharge function on your charger to keep them in good condition.

Once a NiMH battery can no longer hold a charge, after about 500 cycles, it's time to recycle it. Many electronic stores have battery recycling programs where you can drop off old batteries for proper disposal.

Similarly, you can receive expert guidance on NiMH battery charging from engineers specializing in custom lithium ion battery packs.

Conclusion

Properly managing NiMH batteries with the right charging methods is important for getting the best performance and longest life from them. As discussed in this article, picking the right charger, knowing the battery's capacity, and following the suggested charging speeds help keep the battery in good shape. Using advanced technologies like temperature sensors and negative delta voltage detection also helps prevent overcharging. By following these tips and using safe charging practices, users can make sure their NiMH batteries stay dependable and last through many uses. Taking care of your batteries not only helps your device but also reduces electronic waste and supports more sustainable technology use.

Frequently Asked Questions [FAQ]

1. What is the best way to charge a NiMH battery?

To optimally charge a Nickel-Metal Hydride (NiMH) battery, use a smart charger designed for NiMH cells. These chargers automatically adjust the charging rate to prevent overcharging, and enhancing battery life. Begin by inserting the batteries into the charger correctly, aligning the positive and negative ends as indicated.

2. Can I charge NiMH batteries with any charger?

Not all chargers are suitable for NiMH batteries. It's required to use a charger marked for NiMH batteries. Using a charger designed for other types of batteries, like Nickel-Cadmium (NiCd), can lead to improper charging and may damage the NiMH cells due to incompatible charging profiles.

3. How long should NiMH batteries be charged?

The charging time for NiMH batteries depends on the charger and the capacity of the batteries. Generally, a typical charge can take about 4 to 8 hours with a standard charger. Smart chargers that can adjust the charge rate based on the battery's capacity and condition might complete charging faster and more safely.

4. Can you leave a NiMH battery charging overnight?

Leaving NiMH batteries to charge overnight is safe if using a smart charger equipped with an automatic shut-off feature to prevent overcharging. Without this feature, there's a risk of overcharging and can reduce battery life and effectiveness.

5. Is it better to charge NiMH fast or slow?

Slow charging is preferable for NiMH batteries. It tends to be safer and extends the life of the batteries by reducing heat build-up and stress on the cells. Fast charging can be convenient but should be done with a compatible charger that can handle quick charging without damaging the batteries.

6. What is the charge limit of a NiMH battery?

NiMH batteries do not have a fixed "charge limit" in the traditional sense. They can be recharged hundreds of times, but their overall lifespan and performance will degrade gradually with each charge cycle. It's important to avoid deep discharging and to recharge the batteries before they are completely depleted to maximize their lifespan.