Solar Charging

**mgb_phys** · Nov29-09, 10:16 PM

Originally Posted by Draven

I see. Is amperage proportional to voltage with solar cells? Is it possible to find one, or create an array of several wired in series, that are ~4.4 volts but at the same time ~250mA?

Wired in series increases the voltage, wired in parallel increases the current.
So if you have 4.4V cells that are 18mA each you would need 250/18 = 15 of them in parralel to give 250mA.
That's the problem with these garden lights, to recharge a 2500mA battery in a day would need a large and expensive panel to give 250mA.

By the way, are there any other more reliable power sources for this sort of thing besides NiCad?

NiMH are a bit better and about as cheap. Lead acid are easy to charge but are large for the capacity
You also need to take into account the temperature when they are used outside.

**vk6kro** · Nov29-09, 10:30 PM

Solar cells also come in various sizes and the current capability of these is roughly proportional to the area of the cells. Big cells give more current than small cells.

Each cell delivers only about 0.6 volts regardless of its area, though, so you have to stack them in series to get useful voltages from them, even if they are large cells.

**Draven** · Nov29-09, 11:02 PM

Each cell delivers only about 0.6 volts regardless of its area, though, so you have to stack them in series to get useful voltages from them, even if they are large cells.

Then why does this solar cell advertise to give 6.7 volts?

Wired in series increases the voltage, wired in parallel increases the current.
So if you have 4.4V cells that are 18mA each you would need 250/18 = 15 of them in parralel to give 250mA.
That's the problem with these garden lights, to recharge a 2500mA battery in a day would need a large and expensive panel to give 250mA.

That's good to know!

NiMH are a bit better and about as cheap. Lead acid are easy to charge but are large for the capacity
You also need to take into account the temperature when they are used outside.

In what was are NiMH better? I am looking for a battery that is reliable; that is, it lasts a long time, and is not picky and how much it is charged before it is used.

Temperature will be of no concern for this project.

**vk6kro** · Nov29-09, 11:42 PM

Each cell delivers only about 0.6 volts regardless of its area, though, so you have to stack them in series to get useful voltages from them, even if they are large cells.

Then why does this solar cell advertise to give 6.7 volts?

That would be because the panel has 11 cells wired in series in it. (11 times 0.6 is 6.6 volts)

NiCd and NiMH cells both lose charge with time, even if they are not being used.
There is a new generation of NiMH cells (Eneloop is one kind) that are much better at holding a charge for longer.

But these batteries are all very fussy about charging and you can easily destroy them by overcharging. Not that this is a problem with the small solar cells.

They all also do not behave well if they are fully discharged. They should not be discharged to less than 1 volt (from 1.2 volts) or else they tend to grow conductive whiskers internally and these short out the battery so you can't recharge it.

I don't know of anything better though.

**Draven** · Nov30-09, 02:50 PM

That would be because the panel has 11 cells wired in series in it. (11 times 0.6 is 6.6 volts)

Oh!

They all also do not behave well if they are fully discharged. They should not be discharged to less than 1 volt (from 1.2 volts) or else they tend to grow conductive whiskers internally and these short out the battery so you can't recharge it.

Oh, I see. How long (rough estimate assuming they are starting off fully charged) do you think it would take 4 NiCads to discharge to the point of damage when running 3 LEDs? More than 12 hours, or less?

**vk6kro** · Nov30-09, 06:47 PM

If the NiCd batteries are in series and the white LEDs are each in series with a resistor, the LEDs will actually protect the NiCds.

This is because they need about 3.5 volts to turn on, so the NiCds will not continue to pour current into the LEDs once the total NiCd voltage drops below 3.5 volts.

That would mean each NiCd would have a voltage of 0.875 volts, if they split the voltage equally. This is below the 1 volt which is ideal, but avoids the total discharge situation which is most damaging.

**Draven** · Nov30-09, 07:47 PM

Ah, that is a good idea. Sorry for such a rudimentary question, but would the resistor be inserted on the positive or negative end of the LED-wired-in-series array (for lack of a better name)?

Oh, and at the 0.875 volts cutoff point, how long do you think it would take for the batteries to be damaged to the point of no longer functioning, if there were about 1 charge/discharge per day with that mechanism implemented?

**vk6kro** · Nov30-09, 09:07 PM

Originally Posted by Draven

Ah, that is a good idea. Sorry for such a rudimentary question, but would the resistor be inserted on the positive or negative end of the LED-wired-in-series array (for lack of a better name)?

Oh, and at the 0.875 volts cutoff point, how long do you think it would take for the batteries to be damaged to the point of no longer functioning, if there were about 1 charge/discharge per day with that mechanism implemented?

It wouldn't matter which end of the LED / resistor string was which. It seems traditional to put it in the + lead, but it is only there to limit the current, so it doesn't matter where it is.

I doubt if the batteries would be damaged by being discharged to 0.875 volts. I have a battery tester that discharges batteries to 0.6 volts and they seem to accept that OK.

Incidentally, a battery with a terminal voltage of 0.875 volts has very little charge left in it. NiCds and NiMH batteries keep a fairly constant 1.2 volts during discharge and then rapidly drop voltage as they reach the end of their stored charge.
So, 0.875 volts is just one point on a steep discharge voltage decline.

**Draven** · Nov30-09, 10:07 PM

Oh, excellent! I'm gonna ask another newbie question (but I promise I don't ask questions twice

); what kind of resistor should be used (for example, a 2.2k)?

**vk6kro** · Dec1-09, 02:46 AM

Assume the input voltage to each LED and resistor is 4.8 volts (4 NiCd cells at 1.2 volts each)
The current in each LED is 20 mA (this is a guess, ask when you buy it.)
The LEDs are white LEDs, and so they need 3.5 volts on them.

The voltage drop across the resistor is 4.8 minus 3.5 Volts or 1.3 volts.
So the resistance is 1.3 volts / 0.02 amps (ie 20 mA) or 65 ohms.
You would buy a 68 ohm resistor.

You would need to know how much power the resistor needed to dissipate.
Power = 1.3 times 0.02 = 26 mW.

**Draven** · Dec1-09, 10:32 AM

Oh, I see how that works. Thank you!