solar panel DC current

[audioguru]

Your solar panel produces its rated current (100mA) only when it is on the equator of earth at noon with it pointing directly at the sun. At other locations and times the current is less.

In your location it might produce only 80mA at noon, and 20mA in the morning and evening and 40mA to 60mA for hours in between. So for 10 hours it might produce:
8:00AM to 9:00AM= 20mA
9:00AM to 10:00AM= 40mA
10:00AM to 11:00AM= 60mA
11:00AM to 12:00AM= 80MA
12:00PM to 1:00PM= 80mA
2:00PM to 3:00PM= 60mA
3:00PM to 4:00PM= 50mA
4:00PM to 5:00PM= 40mA
5:00PM to 6:00PM= 30mA
6:00PM to 7:00PM= 20mA
The total is only 480mAh. It might be less. On cloudy days the current is almost zero.
A 800mah battery needs at least 1000mAh for a full charge.

 

[ravaka]

Thanks for the break-down Uncle Scrooge...
I have two panels hooked up which put out 200mA (under perfect conditions). But, the battery is still not being charged... at all! Is it likely because my voltage is not stable enough for a lithium battery? (Hence the need for an IC).
I just found out the OEM charger for the device puts out 5V (to charge a 3.7V lithium battery)

 

[audioguru]

A cell phone has a charger IC inside. It has an input of about 5V and it regulates the voltage to exactly 4.2V so that the lithium battery does not catch on fire.

The charger IC also shuts off the battery when its discharging voltage drops to about 3.2V because a fully discharged lithium battery might catch on fire and will not charge anymore.

The lithium battery might have the charger iC inside. It might not charge when the voltage is less than 5.0V.

 

[Ubergeek63]

With solar cells having a high internal resistance you might get a constant voltage, more or less, across the battery. At low currents the battery may act as its own Zener.

Solar cells do not have a constant voltage or resistance. Voltage goes down with temperature as the resistance and leakage current go up.

Peak output is only gained through Maximum Power Point Tracking, and then only if the power levels are high enough to overcome the circuit drain.

 

[Willbe]

Solar cells do not have a constant voltage or resistance. Voltage goes down with temperature as the resistance and leakage current go up

With all the interest in these cells on this and other forums, do you have a link to model the V and R vs. illumination and/or cell temp? It'll save a lot of experiment time.
It's got to work on paper first. . .

 

[ravaka]

Ubergeek,

Can you break that down into simpler terms? My DC knowledge is limited. Thx,

Willbe,

I have put together a solar array that puts out 5V at 100mAh. Since the OEM charger puts out 5.1V, I may actually get some charge from this set-up. The old set-up put out 4.8V at 200mAh (again, like you said, under "optimal light conditions")

 

[ravaka]

Ok, I hooked up the lithium battery to a 5.2V 100mAh solar panel array and left it under an indoor lightbulb for about 2 hours. After checking the battery strength, it actually LOST charge... Any reason for this? Could the solar array be drawing power out of the battery? Why doesn't this work!?!

Thx

 

[Nigel Goodwin]

Solar panels need to be outside in the sun - under an indoor light will provide almost no power at all. You should connect the panel to the battery via a diode, this will prevent the battery discharging through the panel.

 

[Ubergeek63]

Ubergeek,

Can you break that down into simpler terms? My DC knowledge is limited. Thx,

Well that is a fair basic description. I do not recall if it goes into the fact that the max power point changes with temperature and illumination.

What the circuits do instead of plugging in all sorts of specifics about the solar panel and sensors for the environment is to simply keep changing the load while monitoring the output power. If the output power goes down you start changing the load in the other direction.

 

[audioguru]

Hello Ravaka,
You should read about rechargeable lithium battery cells.
If they are charged with a voltage that is slightly higher than 4.20V then they overcharge and CATCH ON FIRE!

 

[Ubergeek63]

Hello Ravaka,
You should read about rechargeable lithium battery cells.
If they are charged with a voltage that is slightly higher than 4.20V then they overcharge and CATCH ON FIRE!

You mean like this?

Overcharging Lithium Battery - Video

 

[Willbe]

You mean like this?

Overcharging Lithium Battery - Video

So, first it outgassed and then it burned. What can be the flammable material? Lithium? Electrolyte?

I'm wondering if a lithium battery would not catch fire if it were charged continuously with a current source (or solar cell approximating a current source) putting out some low value of current.
The battery voltage may rise above 4.2v but there just isn't the energy there to overheat the lithium cell. . . ?

I guess my reasoning is that an ideal voltage source could deliver infinite power, but an ideal current source would have to have the battery voltage rise to infinity to get that infinite power. Maybe the batt. voltage will eventually rise very high with a low value current source.
Perhaps there is a trigger temperature after which the combustion energy comes from the battery and not from the charger. I'm wondering if, once the battery caught fire, they turned off the power.

Dunno'.

 

[audioguru]

Lithium is an extremely active metal. Magnesium is similar and is used in fireworks for a very hot white flame. Remember the British ships that caught on fire in the Falkland Islands war? They were aluminum or magnesium (not wood).

If the voltage of a lithium battery cell becomes higher than 4.20V then a chemical reaction sets it on fire, not heat from overcharging.

A lithium battery cell is also unstable if its voltage becomes less than 3.0V. Then its charging current must be reduced until its voltage gradually increases to a higher amount.

 

[Willbe]

Lithium is an extremely active metal.
If the voltage of a lithium battery cell becomes higher than 4.20V then a chemical reaction sets it on fire, not heat from overcharging.
A lithium battery cell is also unstable if its voltage becomes less than 3.0V. Then its charging current must be reduced until its voltage gradually increases to a higher amount.

Cranky little buggers, aren't they?

 

[audioguru]

Cranky little buggers, aren't they?

But Lithium batteries are very nice when they are properly cared for.