The Good, the Bad, and the Ugly: Classifying SunPower solar cells
If you see me at a trade show and notice that the company shirt I’m wearing does not fit too well around the collar, it is probably because it was bought at low cost from an outlet store by the embroiderer. The label suggests that this shirt is from a highly respected manufacturer and is of high quality, which is all true up to a point, but somehow the buttons and button-holes were mis-aligned during assembly, and so this item was sold off as a “second” or reject item in the manufacturer's outlet store.
Well, guess what? Somewhere out there is an outlet store for low grade, off-spec, SunPower® solar cells.
During manufacture, after solar cells have been tested electrically they are sorted into different “Bins” dependent on how they performed. Some will be over achievers and be put aside for research purposes, while others will simply fail miserably and be destroyed. Those remaining are the Good, the Bad, and the Ugly cells; Good being “Prime”, Bad being “Off-Spec”, while the Ugly cells will have visual or cosmetic defects.
The Prime cells are further graded into High, Premium, and Ultra performance categories, while the Off-Spec cells are
Choosing a Solar Controller - MPPT or PWM
Standard solar panels produce much higher voltages than are safe to feed directly to a battery, so a solar controller or regulator must be connected between them.
Two types of controllers are available; PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).
PWM Controllers - When charging, these controllers feed the power from the panel straight through to the battery until
the battery voltage reaches a predetermined Acceptance level. It will then keep the voltage at that level by pulsing
the panel voltage on and off to keep the battery voltage constant.
During the Bulk stage of charging, the amperage delivered to the battery is slightly less than the amps from the panel due to losses in the electronics.
MPPT Controllers - These controllers take the best mix of amps and volts from the panel to give the maximum power.
They then track this Maximum Power Point as conditions change to ensure the highest power is extracted from the panel
at all times.
The DC panel output is then inverted into very high frequency AC and then converted back down to DC to feed to the battery. The result of that inversion/conversion process is that more amps can be delivered to the battery than were produced from the panel, resulting in reduced charging times.
That is a significant advantage over PWM controllers, and more than justifies the higher cost of the MPPT models.
Marine Solar Planning Guide
Typical marine solar panels are comprised of a number of silicon cells (normally 32+) connected together electrically in a series string. Individual silicon cells produce only around 0.6v to 0.7v, and so enough of them have to be connected together in series to produce a voltage high enough to be able to charge a 12v battery.
A Charge Controller must be connected between the panel and the battery to reduce the panel output to a safe charging voltage. Some panels have less than the normal number of cells and produce less voltage than is required to charge a 12v battery, and these will require either a special boost controller, or for a number of them to be connected in series to produce a higher voltage.