Solar Powered Circuit

[carbonzit]

Also, where am I wasting .4 volts?, I could use 4 batteries and have a higher R value if its cutting it too fine with too little voltage? Ideally I think I want 20mA through the LED's I would like them to be quite bright.

You're "wasting" the difference between the supply voltage (3.6V) and the LED voltage (3.2V). It's not much to worry about in this case (0.008 W per LED).

There are other ways of powering LEDs that are more efficient, involving step-up power supplies. Fascinating stuff. But of course, they're a lot more complicated.

So, are we getting close to the point where you're ready to give this a try? If you have all the parts, you're pretty close to having a working model here.

 

[gravymister]

Ah right I see what you mean, so you think the voltage of 3.6 is fine? and I could use a 22 Ohm resistor? Only problem there is, if the NiCd's run at a higher voltage than 1.2v each it'll raise the current quite high to around 60mA!

 

[gravymister]

So, are we getting close to the point where you're ready to give this a try? If you have all the parts, you're pretty close to having a working model here.

Yeah I think so, I'm already part way to completing the radio schematic I posted earlier! Once I've finalised everything on this schematic I'll start soldering!

 

[audioguru]

The sun is not bright at night, in the morning and in the evening. So the solar panels will charge mostly near noon on a sunny day.
We don't know how much current they produce when loaded by the battery but it looks like they might need a week of sunny days to fully charge a dead battery.

CarbonZit: A solar cell produces only 0.5V. These solar panels are made with 4 to 5 cells in series. The schematic shows four solar panels in series.

EDIT: Then he changed it to only three solar panels in series.

 

[gravymister]

They don't need to be fully charged though, in total there is 1.5Ah of batteries in the circuit at 3.6V. The panels in the schematic give off around 150mA in good sunlight so even if they only give off light for 4 hours thats enough to power the LED's for 3 hours which is fine really.

 

[audioguru]

The datasheet for your LEDs will list their maximum allowed current which is probably 30mA if you keep them cool. With 60mA then make and post a video of them burning out.

 

[gravymister]

So you're saying the maximum voltage from the NiCds will be 1.5v each? I doubt they'll even get charged that much surely? As long as the voltage doesn't go above 3.9 so 1.3v per battery there won't be a problem

 

[audioguru]

According to Energizer, when a Ni-Cad or Ni-MH cell is charged to only 1.3V then it is almost dead.

 

[edman222]

Are you sure??? I've always been told that 0.8V per cell is dead, and 1.2 volts is full charge for NiMH and NICAD... otherwise how would we get 6 cells batteries (7.2V) that are sold everywhere, if 1.3V means discharged???

 

[audioguru]

The nominal voltage of a Ni-Cad or Ni-MH cell is 1.2V when it has a heavy load and is half-discharged. I showed that a battery manufacturer lists its fully charged voltage as 1.4V when trickle-charged and is 1.5V when strongly charged. When the battery cell's voltage is as low as 1.3V then its capacity is almost nothing.

From the same Battery Applications Manual here is a graph of the discharging voltage vs remaining capacity:

 

[gravymister]

From that diagram looks as though I should use resistors of 33 or 36 ohmas the voltage will mainly be around 1.3v, I doubt very much they'll be fully charged ever by the solar panels so 1.4v won't be an issue

 

[audioguru]

You don't know how high the battery cells will charge and you don't know the minimum forward voltage of the LEDs. Put the two together to see that the LEDs might burn out. So increase the value of the current-limiting resistors.

 

[gravymister]

So you think I should revert to the original plan of 50 ohm resistors?

That could limit the current too much.

 

[audioguru]

You could connect the solar panels to your battery (and its series diode) and leave it in bright sun for a few days then measure how high the battery voltage gets.

You could connect each LED to a series resistor set for 20mA and measure the actual forward voltage of each LED.

Then you will know the details and can simply calculate the value of the LED current-limiting resistors.

 

[gravymister]

Yeah Ill do that. Thanks audioguru!