Charging a Anker 20,000mAH battery pack with solar question?

...The most basic approach for maintaining peak power point voltage (VMP) sets panel voltage to a constant voltage level, based on the manufacturer-specified open-circuit voltage (VOC). The rationale for this approach is based on the fundamental electrical characteristics of a solar panel. A typical solar panel comprises a series of cells that, electrically, are simply forward-biased p-n junctions. Consequently, engineers can approach solar cells as common p-n diodes -- with similar electrical characteristics including VOC and temperature coefficient (typically about 2 mV/°C).

For a solar panel, VMP can be approximated as a fixed voltage below VOC, with a temperature coefficient at VMP essentially the same as that at VOC (and considered linear across normal environmental temperature ranges). Designing an efficient solar battery charger then becomes a matter of augmenting the battery protection capabilities mentioned above with a simple temperature-compensated resistor network designed to set panel voltage at VMP.

The Texas Instruments bq24650 and Linear Technology LT3652 each offer MPPT based on this constant-voltage approach.

For the TI bq24650, engineers can set the solar panel voltage to the peak power point using a resistor network across the device's VCC and MPPSET pins. The device's input voltage regulation circuitry responds when the input voltage drops as the solar panel loses the ability to provide the total power of the system. When the device senses that voltage on the MPPSET pin drops below 1.2 V, the IC maintains the input voltage by reducing the charge current. For MPPSET pin voltage below 1.2 V, the bq24650 stays in its input voltage regulation loop while the output current is zero. Engineers can also disable charging completely by pulling MPPSET below 75 mV.

Along with a full range of battery protection capabilities, the bq24650 supports diverse battery chemistries including Li-Ion/Polymer, Lithium Phosphate, and lead-acid batteries. Based on the TI bq24650, the TI bq24650EVM Evaluation Module offers a complete solar power battery charger with multiple test points and jumpers for experimenting with this device.

For the Linear Technology LT3652, engineers can program the peak power voltage for a solar panel by setting the required values in a resistor divider across the VIN and VIN_REG pins. Designed specifically for solar power battery charging, the device features multiple battery protection modes and supports diverse battery chemistries including Li-ion/Polymer, Lithium Phosphate, and NiMH/NiCd. The Linear DC1568A is a demonstration kit that provides a complete solar power battery charger based on the Linear LT3652.

With their use of resistor networks for setting the fixed voltage for MPPT, both the TI and Linear devices enable engineers to employ a relatively simple approach for implementing temperature-compensated MPPT. Because the temperature coefficient for a typical solar panel is essentially linear, engineers can implement basic temperature compensation by augmenting the resistor network with a 3-terminal temperature sensor such as the National LM234...