Solar Chargers for Lithium-ion Batteries - Electronic Products

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Solar chargers for lithium-ion batteries

Using small panels to keep Li-ion batteries going

BY KARTHIK KADIRVEL, Design Engineer;

UMAR LYLES, Design Engineer;

and

JOHN CARPENTER, Systems Engineer

Texas Instruments

www.ti.com

In recent years, there has been a marked increase in

the number of battery-operated small form factor

devices such as tablet PCs, handheld video games,

standalone movie players, digital photo frames, etc.

Typically, these devices use rechargeable Li-ion

batteries.

Common charging solutions include wall adapter-based

chargers and USB-based chargers. While these

chargers provide a low-cost solution, they do depend

on mains power for their operation. The dependence on mains power increases ones power bill and increases

emission of greenhouse gases. Furthermore, since they depend on mains power they are not as portable as one

might like. To extend battery life in a more environmentally friendly way, chargers that can harvest ambient light

energy using solar panels would be ideal. Solar chargers could also have the added benefit of aiding mobility.

In this article, some of the unique considerations for developing a solar charging solution are described. The solar

panel behaves as a high-output impedance power source with time varying voltage and current that is dependent on

ambient conditions, requiring unique design considerations. This is in contrast to a wall adapter or a USB source,

which is a low-output impedance power source at a fairly predetermined voltage and current. Some key

considerations for solar charging solutions are maximum power point tracking (MPPT), reverse leakage protection,

techniques for charge termination, and preventing solar-panel collapse.

Maximum power point tracking

Maximum power point (MPP) is the operating region of a solar cell where maximum power can be extracted. This

region can be understood from the plot in Fig. 1, which shows the typical output current and power versus panel

voltage of a two-cell solar panel along with the MPP. The MPP is a function of ambient temperature and light and,

therefore, is time varying. Chargers that depend on solar power must have appropriate circuitry to track the MPP as

the conditions change.

Maximum power point tracking (MPPT) schemes range from simple open-loop techniques, where the panel voltage

is maintained at a fixed fraction of the open-circuit voltage, to complex microcontroller-based techniques where theinput and output power is measured and the panel voltage is appropriately adjusted. Selecting a MPPT scheme for

charging solutions is a tradeoff between cost and efficiency, and is very application specific.

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Fig. 1: Output current and output power as a function of voltage for a two-cell solar panel.

Reverse leakage protection

Reverse leakage is a phenomenon where charge stored in the battery is lost back to the energy source and occurs

when the battery voltage is higher than the source. This condition does not occur when using a wall adapter or a

USB source because their voltage is always guaranteed to be above that of the battery.

In the case of a solar panel, the voltage across the panel can go below the battery voltage when the panel does not

have enough light. Figure 2a shows a simple USB based charger connected to a battery with switch S1 capable of

disconnecting the battery.

In the case of a solar panel, if the same arrangement is used, the body diode of the switch can turn on if the

solar-panel voltage goes below the battery voltage. One common method to address this problem is to use

back-to-back switches (see Fig. 2b).

Fig. 2: (a) Schematic of USB-based charger showing the power switch. (b) Solar-panel-based charger with back-to-back power switches.

Charge termination

Charging Li-ion batteries requires precise control of applied current and voltage across the battery to ensure it is

charged to its full capacity, to prevent reduction of cycle life, and to prevent hazardous conditions. A common

process for charging Li-ion batteries (see Fig. 3) is broken down into three phases: preconditioning, constant-current


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charging, and constant-voltage charging.

Fig. 3: Plot of battery v oltage and current during the phases of charging a Li-ion battery.

Charge profile

During the preconditioning phase, the battery is charged with a constant current of 0.1C (typically) to slowly raise

the battery voltage to approximately 2.5 V. This phase is used only for deeply discharged batteries.

After this phase constant-current charging is used. During the constant-current charging phase, a 1C (typically)

current is applied until the battery voltage reaches ~4.2 V. At that point a constant-voltage source of 4.2 V is used.

In this phase, once the current into the battery drops to 0.1C, charging is terminated. Once the current falls below

0.1C, the charging source must be completely disconnected from the supply. If this is not done, the battery can

become unstable as plating of metallic lithium starts occurring, which can lead to a hazardous condition. Li-ion

battery charging termination must be based on the current into the battery to ensure that the battery will be charged

to its full capacity.

Problems with maintaining this profile mainly occur during the constant-voltage phase when the current into the

battery is being monitored. It is possible that the current into the battery has been reduced, not because of the

increase in capacity of the cell but because of a reduction in solar-panel output due to change in ambient conditions.

In such case the battery never reaches its full capacity and the solar panel is indefinitely connected to the battery. To

address this concern, a timer can be used to disconnect the solar panel from the charger, irrespective of the battery

capacity, to prevent damage.

Preventing panel output collapse

In traditional chargers, the current and voltage capability of the source is known in advance and the charger can be

designed to operate within the bounds of the source. In the case of a solar-panel output, the current capability and

open circuit voltage is dynamic and dependent on ambient conditions, making design of the control loop more

challenging.

The charger design must not inadvertently collapse the solar panel while trying to maintain the required charging

profile. I f the panel voltage collapses, useful energy cannot be extracted from the panel. The possibility of collapsing

the panel occurs mainly during the constant-current phase when the solar panel may not provide the current drawn

by the battery. When this occurs, the panel voltage starts to collapse rapidly. The charger design must detect the

voltage decrease and immediately reduce the current draw.

A number of battery charge control ICs are available. One such chip is the bq24210 (see Fig. 4) from Texas

Instruments, a highly integrated Li-ion linear charger that works with low-cost unregulated adapters, USB inputs, or

solar power.


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Fig. 4: The bq24210 charge controller has an input voltage regulation loop with programmable regulation threshold, suiting it for charging from alternativepower sources, such as a solar panel or inductive charging pad.

The device has a programmable maximum charging current of 800 mA and a 1% voltage regulation accuracy. To

maximize the charge rate from solar panels, the IC has a selectable battery-tracking mode that is equivalent to

maximum power point tracking. And, it provides battery protection from overvoltage, overtemperature, and short

circuits.


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Solar Chargers for Lithium-ion Batteries - Electronic Products

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