# Will This Charging Circuit Work Well.?

Discussion in 'General Electronics Discussion' started by Carlos Belardo, Sep 3, 2014.

1. ### Carlos Belardo

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Sep 3, 2014
Hi Everyone, I am doing this little project, is a solar charger for 2 AAA Batteries 550mah.
The power supply will be a 6V 100mA solar cell.
I found this circuit on the internet and design it on a simulator but i am not sure if it will work.
I will like to know 2 things.
base on the resistor how long will take to full charge the batteries
and i will like to know if the LED on the circuit will turn off when the batteries full charge.

Thanks.

2. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Hi Carlos and welcome to Electronics Point

I don't think I'll ever understand why people draw schematics so strangely. It makes them bigger and much harder to understand. Here's that same circuit drawn up so the subcircuits are easily recognisable.

So clearly D1 protects the circuit against reverse polarity and also prevents the battery from discharging into the solar cell when it's dark. C1 is a smoothing capacitor.

The circuit built around Q1 is a simple constant current sink. When sufficient voltage is available from the solar cell, R2 biases Q1 ON and current flows into the rechargeable cells.

If the voltage between Q1 collector and the negative rail starts to approach around 1.8~2.0V, D1 will start to illuminate. If your solar cell is able to produce about 4.8V or more, D1 will limit Q1's base voltage to about 1.8~2.0V (assuming it's a standard red LED) and this will limit the voltage across R1 to about 1.2V, limiting the charging current to about 25 mA (from I = V / R with V=1.2 and R=47).

If your batteries are rated at 550 mAh and your solar cell is producing at least 4.8V, the batteries should charge from fully flat to fully charged in about 22 hours (calculated from 550 mAh / 25 mA).

The circuit does not detect when the batteries are fully charged. It will continue to charge them at 25 mA while there is sufficient voltage from the solar panel. This is a "trickle charge" and will not damage them since it's only 1/22 of their 1C charge current.

If you want more information, provide the URL of where you got that circuit, and tell us what kind of rechargeable batteries you are using.

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3. ### davennModerator

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Sep 5, 2009
I think you mean D2

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4. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Yes I do, thanks Dave

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5. ### Colin Mitchell

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Aug 31, 2014
But if the solar cells are 100mA you need to have a circuit that uses the available current.
You just need a diode and connect the solar panel directly across the 2x AAA cells. Take them off after a days sunlight.

6. ### Harald KappModeratorModerator

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Nov 17, 2011
No, you don't. A spec of 100m A only means that the solar cell is able to deliver up to 100mA before seriously dropping in voltage. If you use a charger with a 25mA current source (as above) , it will only draw 25mA from the solar cells, the other available 75mA go unused.

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7. ### Colin Mitchell

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Aug 31, 2014
"A spec of 100mA only means that the solar cell is able to deliver up to 100mA before seriously dropping in voltage."
Where did you get this rubbish from ?????

8. ### Colin Mitchell

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Aug 31, 2014
"A spec of 100mA only means that the solar cell is able to deliver up to 100mA before seriously dropping in voltage."
In fact a solar works exactly the opposite to what you are suggesting.
As the sun shines brighter, the voltage and current from a solar panel (solar cell) increases and it will magically adapt to any battery it is charging. It is best to have the battery voltage near to the maximum voltage produced by the panel but the panel needs to have at least 6v extra available to cater for the floating voltage produced by a battery and allow the extra voltage produced by the panel to convert to current.
It is only when the maximum voltage produce by the panel is achieve, will the maximum current flow.

9. ### Harald KappModeratorModerator

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Nov 17, 2011
Colin, your words are harsh and not well pondered.

A solar cell is a voltage source. It will deliver current to a load as long as the rated current of the solar cell is not exceeded. Otherwise without load the would be no way for the current to flow and the voltage would have to increase indefinitely.

To quote your words: This is rubbish. This diagram shows how the output voltage of a solar cell decreases with increasing current.

If you don't believe me, read up on the theory of solar cells.

10. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Harald is quite right, and you are being unnecessarily rude.

A solar cell is a voltage source. A load will draw current from it. If the load tries to draw too much current, the voltage will drop. Just like any voltage source. There is no requirement to draw the maximum rated current from a solar cell.

As the sun shines brighter, the power available from the solar cell increases. If the solar cell is connected directly across a battery, more current will flow. In this case it's unlikely that any current limiting will be needed, because the cell simply cannot produce enough current to damage the battery, but that is not necessarily true. In larger systems where efficiency is very important, power is maximised using a maximum power point tracking (MPPT) circuit, and charging is supervised according to the requirements of the battery.
That's wrong, especially the last statement. I think you need to read up on MPPT and study some more physics. If you are indeed the Colin Mitchell of Talking Electronics fame, certain allowances can be made, but misinformation is misinformation and we are obliged to correct it.

Last edited: Sep 4, 2014
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11. ### Colin Mitchell

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MPPT you are talking about maximum POWER and this is totally different from charging the battery in our case.
The load you are talking about is entirely different to a battery. A Battery is similar to a zener diode.
Why buy a 100mA panel and only draw 25mA???
All the things you have corrected are wrong.

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12. ### Harald KappModeratorModerator

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Nov 17, 2011
Colin,
A battery is in no ways similar to a zener diode. Look at the V/I characteristics of both and you'll see the dissimilarities.

Because that's the way the current source is designed. One could use 4 of these chargers in parallel, or use some of the power not used by the charger to power other things. Or maybe the 100mA panel jaust came in handy?

Why do you come to think that Kris and I are wrong and you are right? Both of us do have some experience, I can assure you. This is not your first post on this forum with incorrect or useless "information". I ask you to read the linked information I gave you and re-consider your statements.
Even if we were wrong, which in this case we aren't, you could employ more friendly wording to express your opinion. Rude language is notthe way we communicate on this forum. If you continue in this way, there may be consequences which will give you time to think.

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13. ### Carlos Belardo

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Sep 3, 2014
HI all you guys and thanks for your answers I really apreciate them,
Ok here i am going to show the adress of the schematics http://www.circuitdiagram.org/nicd-nimh-battery-charger-circuit.html
The Batteries are Ni-MH Panasonic 1.2v 550 mAh AAA
KrisBluENZ Thanks for your explanation i understand much better yur schematics now.
So as you can see on the adress, theres a chart on the bottom of the page explaining how changing the R2 will change the charge current
Colin MitChell
I think that way will charge the batteries to fast and reduce the battery life.

I Will like to know if theres a way to make a timer with a resistor and capacitor to charge the capacitor very slow lets say in 22hours and then when it gets 63% or fully charge iluminate another LED.

KrisBluENZ i will design your schematic now and watch the current flow etc, thanks so mouch.

14. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011

Well, at least they're consistent - they have a poor description to go with the poor design.
OK. You want to charge them in series, right?
I doubt it. The cells have a capacity of 550 mAh, so even if the solar cell is in direct, bright sunlight, and even if its specifications are not completely invented by the marketing department of the company that you bought it from, and it does actually deliver 100 mA, that is only a charge rate of 0.18C which is bordering on trickle charging anyway.
Reliably determining end-of-charge for NiMH cells is not possible with such a low charge current. End-of-charge is best detected through rate of temperature rise, at a charge current of at least 0.5C (i.e. 275 mA in your case). Also, properly charging NiMH cells to optimise their capacity and lifetime involves several charging phases and is best controlled by a microcontroller. In your case I think your best option is to charge the cells directly from the solar panel with just a diode in between them. Initially, connect a milliammeter in the circuit as well, and find out what the maximum achievable current actually is. If it's less than about 55 mA (0.1C) you don't need to worry about a timer.

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15. ### Carlos Belardo

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Sep 3, 2014
Thanks
Batteries will be in series.
I will be doing some test and i will post back the results.

16. ### Colin Mitchell

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Aug 31, 2014
In fact you don't even need the diode. If it is very sunny day, the cells will be charged in 6 hours (if they are fully discharged) and removing them from the charger will mean you don't need a diode.

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17. ### Gryd3

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Jun 25, 2014
Diode sounds like a requirement to prevent the battery from feeding back into the panel...

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18. ### Colin Mitchell

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Aug 31, 2014
The cells are already removed from the charger. The only time when there will be a small leakage into the panel is at night time and this will be less than 1mA.

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19. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Let me offer another opinion.

The panel will only likely be capable of 100mA into the batteries on a very sunny day. During the day as the level of solar irradiation goes up and down, and as the angle between the sun and the panels changes, the charge current will likewise go up and down.

The constant current source set for 25mA provides a method of ensuring a certain (known) charge is delivered to the batteries per unit time in the majority of cases.

The end result is that the charger allows a relatively simple "place battery in charger for X hours" (during daylight obviously) to charge.

Initially I though that using another transistor's Vbe to regulate the current would be better, but the use of a LED provides an indication that charging is happening.

The transistor's BE junction acts as a diode to prevent the battery discharging through the panel at night, however I would retain the diode because a higher voltage battery pack (and therefore, panel) could be enough to exceed the maximum Veb for the transistor (or indeed the maximum reverse voltage for the LED). Whilst the current may again be low, the price of a single diode is not great.

Another advantage of the constant current source is that it allows the charger to be connected to another voltage source (car battery, power supply, or larger panel) with little risk of damage tot he batteries under charge. In this case the series diode provides important protection against the power source being connected backwards.

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20. ### Carlos Belardo

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Sep 3, 2014
Thanks, colin, steve and gryd3 for the suggestion.
I was doing some test. This were my results.
Ok first the 2 AAA belong to a house phone and i used the phone to test the actual charge on the batteries and the multimeter as well.

I connect everithing as you guys recomend me. Solar cell, diode and batteries.

The cell without the charge was supplying 6.28v and 0.05ma. If my calculations are not wrong only 50ma
With the batteries connected to start charging volts drop to 3.25v and 0.01 or 0.03 depends the sun light.

Batteries were fully discharge and in a period of 3 to 3.5 hours batteries are 1.25v and 1.35v but amps on each batterie 0.06 0.08....

When i put them on the phone it turns on and says low bat turns off..

I will say with that solar cell i will need to leave them a hole day charging on sun light. 550/30 arround 18.5 hours.? Correct me if i am wrong.

So now is a question for you guys.
What will you guys recomend to charge those batteries with solar source on a desent amount of time. Lets say you use them on might and whants to charge them the next day to be available to use them on the night.

Better solar cell.?
Biger batteries (more mah)
Thanks everyone.