Key Features of NiMH Battery Charger Circuits

A good NiMH battery charger circuit should have the following key features:

1. Regulated constant current charging – NiMH batteries should be charged using a constant current, typically between 0.1C to 1C (where C is the battery capacity in Amp-hours). The charger circuit should regulate the charging current to stay at the desired level.

2. Overcharge protection – NiMH batteries can be damaged if overcharged. The charger circuit should detect when the battery is fully charged and stop or greatly reduce the charging current to prevent overcharging. Common methods to detect a full charge include:

3. Negative delta V (-ΔV) detection
4. Peak voltage detection
5. Temperature rise detection

6. Timer cutoff

7. Reverse polarity protection – The charger circuit should prevent damage if the battery is inserted with reversed polarity.

8. Indicator LEDs – The charger should have LED indicators to show charging status, full charge detected, errors, etc.

9. Support for multiple cell counts – For flexibility, the charger circuit should be able to charge battery packs with different numbers of cells in series (e.g. 1-10 cells). This requires the charger to adapt its output voltage based on the battery configuration.

NiMH Battery Charging Process

NiMH batteries are typically charged in three stages:

1. Constant Current (CC) Stage – The battery is charged at a constant regulated current, usually between 0.1C to 1C. During this stage, the battery voltage gradually rises.

2. Top-Off Stage – When the battery reaches full charge, the charger switches to a lower top-off current (usually C/10 or lower) for a set time (e.g. 1-2 hours) to ensure the battery is fully charged and equalized.

3. Standby Stage – After top-off is complete, the charger goes into a standby or trickle charge mode, supplying a very low current (C/300 or lower) to counteract the battery’s self-discharge.

The charging process can be visualized in this graph:

Typical NiMH Charger Circuit Design

A basic linear NiMH charger circuit usually consists of the following key components:

• Voltage regulator – Generates the regulated charging voltage. Typically a linear regulator like LM317 for simplicity.

• Constant current regulator – Regulates the charging current. Often a simple transistor current source.

• Full charge detector – Detects when the battery is fully charged and signals to switch to top-off mode. Can be a comparator monitoring battery voltage or temperature.

• Top-off timer – Controls duration of the top-off stage, usually a 555 timer IC.

• Standby trickle current regulator – Supplies a very low standby current after charging is complete to maintain the full charge.

• Reverse polarity protection – A P-channel MOSFET or diode to prevent reverse current if battery is connected backwards.

• Indicator LEDs – Driven by the charger control logic to indicate status.

Here is a simplified schematic showing the key components:

                  +---------+
  DC Input >------|Voltage  |       
                  |Regulator|--+    
                  +---------+  |    
                               |    
                  +---------+  |     +-------+
                  |Current  |--+----|Battery|
                  |Regulator|       +-------+
                  +---------+           | 
                               +---------+  
                               |    
                  +---------+  |    
                  |  Full   |--|    
                  | Charge  |       
                  |Detector |  +----------+
                  +---------+  |Top-off   |
                               |  Timer   |
                               +----------+
                                     |    
                               +----------+
                               | Standby  |
                               | Current  |
                               |Regulator |
                               +----------+
                                     |    
                               +----------+
                               | Reverse  |
                               | Polarity |
                               |Protection|
                               +----------+
                                     |    
                                 +------+
                                 | LEDs |
                                 +------+

More advanced charger designs can include additional features such as:
– Switch-mode CC/CV converter for higher efficiency
– Microcontroller-based control and monitoring
– Battery fuel gauging
– USB or wireless connectivity

But the basic design remains similar at the core.

Frequently Asked Questions (FAQ)

1. What is the proper charging current for NiMH batteries?

NiMH batteries are typically charged at a constant current between 0.1C to 1C, where C is the battery’s capacity in Amp-hours (Ah). For example, a 2000mAh battery would be charged at 200mA to 2A.

Slower charge rates (e.g. 0.1C) are gentler on the battery and produce less heat, potentially extending battery life. Faster charge rates (e.g. 1C) charge the battery more quickly but generate more heat. 0.5C is a good balance for most applications.

2. How do I know when a NiMH battery is fully charged?

There are a few methods to detect a fully charged NiMH battery:

• Negative Delta V (-ΔV) – The battery voltage peaks and then begins to drop slightly when the battery is full. Detecting this voltage drop is a reliable indicator of a full charge.

• Peak Voltage – NiMH cell voltage typically peaks around 1.5V when fully charged. A charger can detect when this voltage is reached. However, the exact peak voltage varies with temperature and charge rate.

• Temperature – NiMH battery temperature rises more quickly when the battery is nearly full. A temperature sensor can detect this increased rate of temperature rise. However, this method is less reliable than -ΔV.

• Timer – For a known battery capacity and charge rate, a simple timer can be used to estimate when the battery is fully charged. However, this is not as reliable as the other methods since battery capacity can change over time, and it risks overcharging the battery if the timer is set too long.

The most reliable and commonly used method is -ΔV detection.

3. What happens if I overcharge a NiMH battery?

Overcharging a NiMH battery can cause irreversible damage, reducing the battery’s capacity and cycle life. When a NiMH battery is overcharged, the excess energy begins to break down the water in the electrolyte into hydrogen and oxygen gas. This increases the internal pressure and temperature of the battery.

Mild overcharging leads to reduced capacity over repeated cycles. Severe overcharging can rupture the battery’s safety vent, leaking electrolyte which permanently damages the battery. In extreme cases, the increased pressure can even cause the battery to explode.

To prevent accidental overcharging, a well-designed NiMH charger should use reliable full-charge detection and cut off or reduce the charging current when the battery is full.

4. Can I leave NiMH batteries on the charger after they are fully charged?

Yes, it is generally safe to leave NiMH batteries on a smart charger after they are fully charged. A proper NiMH charger will automatically switch to a very low trickle current (usually C/300 or lower) when the battery is full to maintain the charge without causing damage. This standby trickle charge counteracts the battery’s natural self-discharge.

However, it’s best not to leave NiMH batteries on the charger for extended periods (many days or weeks) as this can still cause gradual overcharge and reduced battery life over time. If possible, it’s better to remove the batteries from the charger once they are charged and store them in a cool place until ready to use.

5. Why do my NiMH batteries get hot when charging?

It’s normal for NiMH batteries to get somewhat warm during charging, especially at higher charge rates (0.5C or above). This is because the charging process is not 100% efficient – some of the input energy is converted to heat instead of being stored in the battery. The heat generated is proportional to the square of the charging current (P=I^2*R), so higher currents produce more heat.

Typically a NiMH battery should not get hotter than about 45°C (113°F) during normal charging. If the battery is getting very hot (too hot to comfortably hold), that is a sign of a problem such as:

• Charging current is too high
• Battery is defective or worn out, with increased internal resistance
• Charger is malfunctioning and not cutting off when the battery is full
• Poor electrical connection between charger and battery, causing high resistance

If abnormal heating is observed, stop charging immediately and investigate the cause. Continued overheating can permanently damage the battery and even pose a fire hazard in extreme cases.

In summary, NiMH battery charger circuits play a crucial role in properly charging and maintaining NiMH batteries to ensure optimal performance and long life. A good charger should provide regulated constant current charging, reliable full charge detection, and overcharge protection. By understanding how NiMH batteries are charged and what features to look for in a charger circuit, you can design an effective and safe charging system for your NiMH battery application.