Can a small photocell panel be connected to a flashlight type rechargable battery?

oldschool wrote

Some charge controller chip would be in place. Do not use AA, use lipos. What is the short circuit current that panel gives in direct bright sunlight? And the open voltage? Your controller needs to be able to handle that. That also determines if you need a switcher or not to go the battery voltage level.

See how much current the panel puts out in bright sun. Also check its voltage unloaded. Then we'll know how runnable this is.

NT

Nope in more sunlight it will put out more current. The voltage is mostly fixed. You can put panels in series for more voltage.

George H.

If it's showing 1 volt "open circuit" in room light, when measured with a high-impedance voltmeter, then I think it's unlikely to put out much more voltage under direct sunlight. More current, yes; more voltage, no.

You _can_ connect it directly to one or two batteries in series.

It is unlikely to charge them at all, based on what you've written.

Ideally, what you would have is a voltage-boost circuit which is capable of raising the voltage enough to charge the batteries, _and_ to do so in a way which operates the panel at its point of maximum power delivery. Professional solar charge-control circuits do this (although they usually _drop_ a higher panel voltage via a buck regulator).

You might want to look into the simple "joule thief" circuit. This is a simple blocking oscillator voltage booster, which is often used in small LED flashlights to boost a single AA cell (1.5 volts nominal) to a high enough voltage to turn on a white LED (roughly 5 volts I believe). You might be able to build one which can boost a 1-volt panel's output up above 1.4 volts (enough to charge a NiCd or NiMH).

Most cheap cell phones use 5.0V DC to the charger connector. My guess(tm) is that it's either an amorphous silicon or thin film panel delivering about 6V DC. Amorphous are nice because they're cheap and produce usable power in fairly dim lighting, while polycrystalline cells are more expensive and don't work well in dim lighting. The problem with amorphous cells is that the efficiency sucks. Approximately: Amorphous 9% Thin film 16% Polycrystalline 18%

Lots of guesswork. Are you sure it's a cell phone charger? Perhaps it's solar powered garden light?

Duz the panel look something like this polycrystalline panel?

Wrong. You didn't have enough light to properly determine if the circuitry was functional. A minimum light level will be needed before the electronics will delivery any output. Indoor room testing isn't bright enough, unless you put the panel under a desk lamp. Put the device back together and try again outside in sunlight or with a desk lamp.

Well, try it. You should measure the open circuit voltage and the short circuit current. Don't leave the ammeter across the panel for very long or the panel might overheat.

I know about NiCd, NiMH, lead acid, LiIon, and other chemistries, but somehow, I've never seen a "common rechargeable" AA cell. Is that a brand name of something?

Buy a small 3.6V LiPo cell. You can buy charge controllers that will charge it if your solar panel can deliver enough power. I'm not going to try and find one until you supply some numbers.

They don't change colour in response to a load, open-circuit and short circuit produce almost exactly the same self-heating

Change color? No, I didn't say that.

I use a short circuit test to check for defective cells in a panel. Short the panel, full sunlight, and use an IR camera or IR thermometer to measure the temperature rise of each cell. A defective cell will usually be warmer than the other normal cells: "Hotspots Detection in Photovoltaic Modules Using Infrared Thermography" For example, when someone shorts a 300 watt panel, that generated power has to be dissipated somewhere, which means dissipated in the solar cells. With 60 cells, that's 5 watts per cell at the MPP (maximum power point), which is not a trivial amount.

The instructions for such tests often include some manner of warning not to leave the panels shorted for too long or they might overheat. Sometimes, they show a hot cell with an open circuit condition (usually when the shading protection diode goes open circuit).

The tiny panel the OP is using will probably do nothing dangerous if the panel is shorted, but I consider it good practice to add the overheating warning since there's no clear boundary between what size panel is safe and unsafe to short. In other words, if he burns his house down shorting a power source, I would want to at least claim that I warned him.

You can make a crude maximum power point tracker out of one of those and an 8 pin AVR uP. The uP power pin gets hooked up to the boost output and its ADC can be used to measure its own supply voltage, which should vary with light intensity. The battery can be charged with a PWM output from a pin and output current into the charging battery can be sensed indirectly via sensing die temperature with the on-chip temperature sensor

No, but assuming that conservation of energy is a law of the universe you implied it.

A shorted panel produces no power on the terminals. An open circuit panel receives the same amout of energy from the sun as a shorted one (otherwise it would change colour), so same temperature.

Sitting on the side line listening to this the space between what you said and what he said became apparent. You are both right. You are talking abo ut the panel and he is talking about individual cells. Since the panel doe s not have to be uniform unless it is a single cell, the problem cells can be hotter than the fully functioning cells because they are passing the ful l current when the output is shorted. Functioning cells are generating ele ctrical power which leaves the cells so they can be cooler. So there shoul d also be a difference in voltage across the cells when some are bad. The bad cells will have a lower voltage (since this is power leaving the cell) or even reversed.

Rick C.

If the panel is shorted, it is not at maximum power point.

Jeff Liebermann wrote

Do you have any reason to assume that a piece of metal of say 1 square inch exposed to direct full sunlight will set things on fire? Now how about silly con and glass?

Much more dangerous is all the magnifying stuff on my desk, I deliberately move it out of the suns way.

Hmm just to be a contrarian, if I bent the piece of shinny metal around part of a sphere, I might be able to start a fire... (solar cells are not nearly as efficient :^)

GH

George Herold attempted:

I would start by using a connector with a leading ground pin in your power supply.

And not just be a contrarian and do get a clue.

Color me confused.

A polycrystaline solar cell has an internal resistance of about 2 ohms which is easily measured. When I short the terminals, the current produced is dissipated by this internal resistance. P=I^2*R. However, with an open circuit, there is no current path through this internal resistance, therefore no self heating. Most solar cells and panels have a Low-E coating on the glass cover designed to reflect IR and prevent heating of the cells by the sun which reduces the solar cell's efficiency.

Drivel: Conservation of energy doesn't work with utility power. The more I conserve on my electric power use, the more PG&E charges me for the power. See graphs at: Moral: It doesn't pay to conserve.

That's not relevant. MPP refers to maximum output power. That was simply used as a data point for power handled by the cell. With a short on the output the internally dissipated power would be even more since the current will be higher.

Rick C.

ch exposed

You don't seem to understand the flow of energy in a solar panel. If one c ell becomes a resistor, it can absorb all the power put out by the other ce lls when the output is shorted. I think that has some potential for being dangerous depending on the size of the panel... at least in theory. I have no idea how often cells fail in a way that would make them largely resisti ve.

Rick C.

True. However, it is the maximum current output (ignoring internal heating effects).

Maximum power point is where the solar cell or panel internal resistance is equal to the load resistance: "According to the maximum power transfer theory, the power delivered to the load is a maximum when the source internal impedance matches the load impedance". and "In the ICM approach, the output resistance of the PV panel is equal to the load resistance as expected from the celebrated max power transfer theorem..."

Since the internal resistance and the load resistance are in series and equal at the MPP, the solar panel should dissipate the same amount of power as the load when they are matched. This is in contrast to utility grid power, where the internal impedance of the generator is much lower than the load impedance. Otherwise, the generation station would burn to the ground dissipating gigawatts of power.

So, why don't the all the solar panels catch fire due to self-heating when operated at the MPP?

This may be one of your tongue in cheek posts.

I looked at the data and found a slow rise in cost up to Aug 2016 where there was a rather significant drop from nearly 20 cents per kWHr to about 12 cents per kWHr. Virtually none of the features in the usage curve show up in the unit cost curve.

I certainly wouldn't draw the conclusion you did.

Rick C.