The previous page dealt with an exercise to calculate the capacity of battery and solar panel given requirements of the client and how long it will run. In practice, we need to do some R&D and build a simple model to understand how the system really works. Let just done that.

Suppose we plan to design a system:

• using 20Wp solar panel, optimal working voltage 18V
• lithium battery in 3S setup, the nominal voltage is 12.8V (3.7V*3S)
• solar charging with Maximum Power Point Tracking (MPPT)
• measure current and conversion efficiency

Let start from the panel

Panel specification:

#	Name	Value
1	Maxium Power (at Pmax)	20Wp
2	Open Circuit Voltage (Voc)	20.1V
3	Optimum Operation Voltage (Vmp)	18.1V
4	Short-Circuit Current(Isc)	1.19A
5	Optimum Operation Current(Imp)	1.19A
6	Power Tolerance (%)	+3%
7	Dimension	430x350x23
8	Made	Hames Mono Solar Panels

From the graph above and give the specification on the panel, the panel should be operated at 18.1V and expected to produce 1.1A (at standard condition, usually 1000W/m² (1Sun) irradiation).

Solar charger

The name itself is a big topic. An ideal charger should efficiently convert all electrons collected by the panel and charge into the battery. To maintain the maxium power of the panel, the charger keeps the uptream voltage at V_mp and keeping the voltage downtream suitable for the battery. In addition, the charger should have a smart mode to charge the battery when it is low in power, in mid range and when the battery is getting full. Here is another nice post about PWM and MPPT.

PWM: converting voltage of solar panel to a slight higher than the battery's and using a MOSFET to turn on/off current to the battery

MPPT: holding voltage of the solar panel to it's maximum power point while reduce the voltage to charge the battery

For MPPT charging, the type and voltage of the battery is another factor. Consonance Electric produces a series of chips for solar charging based on chemistry and voltage of the battery. For example, with a single-cell (1S) battery, CN3791 is a popular choice. For multiple-cell battery, CN3722 is one option, and the complete charger is avaiable for $7. There is a newer version that both MPPT voltage and charging voltage can be adjusted by resistors.

Battery

Lead-acid battery has been (and still is) a mainstream choice. It is cheap, reliable, and low maintenance. The drawbacks are lower energy density per volumn, not as environmental-friendly, and less efficient [see more]. For lithium battery, 1S is the most simple setup like in most portable power bank to charge the mobile phone. To provide 5V from 3.7V, a boost-up DC converter is used. In compare with buck-down DC converter, boost-up method is less efficient.

Configuration battery in 1S setup is a better choice if you have batteries with different in resistance or capacity. In this setup, cells are similar to each other wired in parallel and protected by one (or more) protection board. Each layer of cells are isolated by protection board is a must. Multiple cell wired in series (like 3S) requires cell are similar to each other. The capacity of each layer is similar to others, and cell capacity in each layer is similar so that minimizing charging-discharging between cells.