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A Practical Lithium Ion Battery Charger for use with Solar Panels

Mon, 04/04/2011 - 12:12pm
John Carpenter & Karthik Kadirvel, Texas Instruments
John_CarpenterKarthik_KadirvelIncreasingly, the number of battery operated devices use rechargeable lithium ion batteries as the power source. Common charging solutions for these batteries include wall adapter based chargers and universal serial bus (USB) based chargers. While these chargers provide a low cost solution for charging batteries, these chargers suffer from a common drawback for use with solar power in that they depend on a constant power source for their normal operation. Chargers which can work with both solar power and USB or wall power would be better suited. Solar chargers also have the added benefit of providing a mobile charging solution that a wall adapter or a USB charger cannot provide and the solar power can be used to extend the device operation time.

Some of the considerations to developing a practical charging solution are described. The primary reason for these considerations is because the solar panel behaves as a high output impedance power source whose power output is time varying and dependent on the ambient conditions. This is in contrast to a wall adapter or a USB source which is a low output impedance power source. Two key considerations for solar charging solutions are reverse leakage protection and preventing solar panel collapse.

• Prevent solar panel collapse

The charger circuitry must be designed to operate within the bounds of the source. In the case of a solar panel, the current capability and open circuit voltage is dynamic and is dependent on light conditions.

A charger for solar power must not inadvertently collapse the solar panel while trying to maintain charging current required for charging the battery. The solar panel may not provide the full current used to charge the battery for a given light condition. When this occurs, the panel voltage will start collapsing. The charger must be able to detect the voltage decrease from the solar panel and immediately reduce the current drawn from the panel to prevent the voltage from collapsing.

• Reverse leakage protection

Reverse leakage is the phenomenon in which charge stored in the battery is lost back to the energy source. Reverse leakage can occur when the battery voltage is higher than the source. When this occurs, the source becomes a load on the battery instead of charging the battery. Therefore, design of a solar charging solution must be capable of addressing this problem. One common method to address the problem is to use back to back switches as seen in figure 1.

Figure 1. Typical application schematic of bq24210 IC

The Texas Instruments bq24210 IC helps address all the requirements of a solar charger and for USB/Wall adapters for charging a single cell lithium ion battery. The IC has an input voltage regulation loop which prevents the collapsing of the solar panel and at the same time maximizes the power extracted from the solar panel. It also has all the circuitry needed to charge a lithium ion battery using the standard charging profile using a wall adapter, USB source and solar input. Protection features such as battery over voltage protection, battery over temperature protection and prevention of solar panel collapse are also included. Digital outputs which can drive external light emitting diodes (LEDs) to indicate power good and charger enabled are also present.
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