Solar panel battery charger
Solar powered devices rely on a highly variable energy source, so for a device to be useful at all times, energy from solar cells must be stored in a rechargeable battery. Solar panels have a maximum power point, a relatively fixed voltage at which the panel can produce the most power. Maximum power point tracking (MPPT) is usually achieved by limiting a converter’s output current to keep the panel voltage from straying from this value. The LT3796’s unique combination of current and voltage loops make it an ideal MPPT battery charger solution.
shows a solar panel to sealed lead acid (SLA) battery charger driven by the LT3796. The charger uses a three-stage charging scheme. The first stage is a constant current charge. Once the battery is charged up to 14.35V, the charging current begins to decrease. Finally, when the required battery charge current falls below 100mA, the built-in C/10 termination disables the charge circuit by pulling down VMODE, and the charger enters float charge stage with VFLOAT
= 13.5V to compensate for the loss caused by self-discharge.
Figure 10: A solar panel battery charger maximum power point tracking (MPPT)
Click on image to enlarge
The charging current is programmed by the resistor network at the CSP and CSOUT (CTRL) pins as follows,
Maximum power point tracking is implemented by controlling the maximum output charge current. Charge current is reduced as the voltage on the solar panel output falls toward 28V, which corresponds to 1.1V on the CTRL pin and full charging current, as shown in Figure 11
. This servo loop thus acts to dynamically reduce the power requirements of the charger system to the maximum power that the panel can provide, maintaining solar panel power utilization close to 100 percent.
Figure 11: ICHARGE vs VIN for the solar charger in Figure 10