Develop Affordable Mixed-Signal Battery Chargers

As battery-powered electronic devices gain in ubiquity and power, the need for easily adaptable battery- charger designs arises. With just standard components, battery- charger designs can become more flexible and more cost-effective. A mixed-signal design facilitates the addition of new, unique features to the system. It also makes it possible to add differentiating features.

Many disparate battery chemistries are used for rechargeable portable applications, including lithium-ion (Liion), nickel metal hydride (NiMH), nickel cadmium (NiCd), and lead acid batteries. Li-ion batteries have the highest energy density of all battery types, making them the most portable of all rechargeable technologies. NiMH batteries are popular because they’re safe and environmentally friendly. It’s possible to design a mixed-signal, universal battery charger that can charge both of these battery chemistries.

The rate of charge or discharge is expressed in relation to battery capacity. Known as the C-rate, this rate of charge equals a charge or discharge current. It’s defined as:

I = M x CN

A battery discharging at a Crate of 1 delivers its nominal rated capacity in one hour. For example, if the rated capacity is 1000 mAh, a discharge rate of 1 C corresponds to a discharge current of 1000 mA. Similarly, a rate of C/10 corresponds to a discharge current of 100 mA.

Preferred charge profile (Li-ion and nimh)

Li-ion battery chemistries use a constant, or controlled, current and a constant-voltage algorithm that can be broken up into four stages: trickle charge, constantcurrent charge, constant-voltage charge, and charge termination (Fig. 1). The preferred algorithm for NiMH consists of trickle charge, constant current, top-off charge, and charge termination (Fig. 2).

• Stage 1, Trickle Charge: Trickle charge restores charge to deeply depleted cells. For Li-ion batteries, when the cell voltage is below approximately 3 V, the cell charges with a constant current of 0.1 C maximum. For NiMH batteries, trickle charge conditions weak batteries when the cell voltage is less than 0.9 V per cell.
• Stage 2, Constant- Current Charge: For Li-ion and NiMH batteries, after the cell voltage rises above the trickle-charge threshold, the charge current increases to perform constant-current charging. The constant-current charge should range from 0.2 to 1.0 C.
• Stage 3, Constant Voltage: For Li-ion batteries only, constant-current charge ends and the constant- voltage stage begins when the cell voltage reaches 4.2 V. To maximize performance, the voltage-regulation tolerance should be better than +/-1%.
• Stage 4, Charge Termination: The continuation of trickle charging isn’t recommended for Li-ion batteries. Instead, charge termination is a good option. For NiMH batteries, a timed trickle charge ensures 100% of battery capacity use. When the timed top-off charge is complete, charge termination is then necessary.

For Li-ion batteries, one of three methods—minimum charge current, a timer, or a combination of the two—typically terminates charging. The minimum charge-current approach monitors the charge current during the constant-voltage stage and terminates the charge when the charge current diminishes in the range of 0.02 to 0.07 C. The timer method determines when the constantvoltage stage begins. Charging then continues for two hours, and the charge terminates. Charging in this manner replenishes a deeply depleted battery in roughly two-and-a-half to three hours.

Advanced chargers employ additional safety features. For example, with many advanced chargers, the charge stops if battery temperature is less than 0°C or greater than 45°C.

For NiMH batteries, charge termination is based on a –dV/dt reading of the battery pack, a +dT/dt (delta temperature versus time), or a combination of both. In this case, temperature sensing is a possible safety precaution, as well as a termination method.

System considerations

To recharge any battery quickly and reliably, a high-performance charging system is required. Key system parameters ensure a reliable, cost-effective solution.

• Input Source: Many applications use very inexpensive wall cubes for the input supply. Output voltage depends heavily on the wide-ranging ac input voltage, as well as on the load current drawn from the wall cube. Applications that charge from a car adapter can experience a similar problem. The output voltage of a car adapter typically will range from 9 to 18 V.
• Output Voltage- Regulation Accuracy: For Li-ion batteries, output voltage- regulation accuracy is critical to maximizing battery- capacity usage. A small decrease in output-voltage accuracy results in a large decrease in capacity. However, the output voltage can’t be set arbitrarily high because of safety and reliability concerns.
• Charge Termination Method: Overcharging is the Achilles’ heel of Li-ion and NiMH cells. Accurate charge termination methods are essential for a safe and reliable charging system.
• Cell Temperature Monitoring: The temperature range over which a rechargeable battery should be charged is typically 0°C to 45°C. Charging the battery at temperatures outside of this range may cause the battery to overheat. During a charge cycle, pressure inside the battery increases, causing it to swell. Because temperature and pressure are directly related, the combination of high temperature and high pressure inside the battery can lead to mechanical breakdown or venting inside the battery. Charging the battery outside of the 0°C to 45°C range also may harm battery performance or reduce its life expectancy.