captain387 said:
shawname: I'm confused, I still don't understand what you mean by "shorting". Sounds like you connect the panel positive lead to the panel negative lead, correct? If so, the voltage would immediate drop from 17.5V to almost 0V. Since the panel provides a relatively constant power (watt), dropping the voltage (V) causes the current (A) to rise, which is what you were reporting (2.9A). But that current is not meaningful because the voltage is unknown. So what is the corresponding voltage?shawname said:
the charge controllers were doing their job and reducing the amount of current that they were using, hence my low readings. You're welcome. Looking forward to seeing the measured "shorted" voltage. I'm pretty sure it'll be 0V.shawname said:
SamTexas said:
Not really. If both voltage and current are non-zero numbers then their products is also a non-zero number meaning that energy is actually flowing through the shorted circuit meaning that the wire will get hot, will melt in order to "UNshort" the circuit.shawname said:
It's not my graph, but I think you're right about the sweet spot. I don't have an MPPT controller so I can't say for sure.shawname said:
Not exactly, since charging is about total coulombs transferred and not power. Fastest charge is with the most amps put through the battery, and the most amps are always at the lowest volts that are enough to active the reverse chemistry, but a direct connection will draw the panel down to the battery voltage, always the maximum current but probably not near the maximum power point. A buck converter, controlled by feedback that gives the most current through a shunt, will increase the current beyond that by temporarily storing excess voltage in a magnetic field and then slowly releasing it to augment the current, at KHz rates. Historically the panels are sized around 18 volts to provide enough voltage in summer for 12 volt battery charging; in winter the panel voltage can go up to 21 volts. The excess power used to be wasted, but buck converter charge controllers are nowadays almost as cheap as PWM charge controllers.shawname said:
dak664 said:
dak664 said:
dak664 said:
Light pushes a fixed number of electrons through a nominal 0.5 volt potential barrier in each cell, or 18 volts in a 36 cell panel. Each electron has excess energy depending on the wavelength of the photon causing the push, that excess is dissipated within the cell on the high side of the barrier. With no load the small capacitance of the circuit will charge up to 18 volts, now the electrons have enough energy to fall back through the barrier and the current recirculates in each cell. Connect a 12 volt battery and that capacitance will quickly discharge to 12 volts, and the voltage across each PV cell will drop to 0.33 volts. The original current will begin to flow, but another 0.17 volts of energy is lost on the high side of the barrier (actually the current increases a little because the lower barrier allows longer wavelength photons to be effective).shawname said:
A coil in series will continue to push electrons when the panel is disconnected, the collapsing field going to zero and generating whatever voltage is needed to drive current through the battery. The voltage across a capacitor would be decreasing so it would only provide current for a short time, until the voltage drops to battery equilibrium potential.shawname said:
In series, to measure the current through the battery. For most battery chemistries the voltage always increases with increasing current (not NiMH however) so a feedback loop that maximizes the voltage would work as well. But small changes in voltage would represent large changes in current, and higher precision ADCs would be needed to measure it. For less than 10 amps or so the watts lost across a < 1 ohm shunt are not important; for kWh battery banks it might make sense to measure voltage instead, or use a Hall device to measure the current.shawname said:
dak664 said:
