Solar Cells Charger

[Transistorman]

Hello,
I have been planing on making a solar cells powered charger lately. My plan is that I will be using two 10V 30mA solar cells in a parallel circuit, I will be connecting a Schottky diode at the output and it will directly be connected to my cellphone battery which has the specification of 3.7V 1500mA through a usb connector.

My main question is, do I have to lower/regulate the voltage of the output so that it will not "burn" my battery and but if so, what kind of regulator should I use for my plan (please suggest me an simple method as I might not be able to provide all the necessary needs for a more complicated project)

I would consider myself as a beginner in solar cells, so I hope the best from you guys!

P.S.
The battery type is a Li-Polymer battery.

 

[Boncuk]

Hi T-man,

look into the LM317L.

It can supply an adjustable output voltage and current up to 100mA.

The package is a TO-92.

Boncuk

 

[ronsimpson]

My cell phone and battery also has a USB connector for charging the battery. The USB supplies 5 volts. Inside the phone is the battery charger circuit that makes the 3.7 volts.

I believe the 10 volts from the solar cells must be reduced to 4.5 to 5.5 volts before powering the USB. A linear regulator like the LM317L or a LM78L05 will work but will waste 50% of the power in heat. Using a switching power supply will only waste 5% of the power.

I have some concern that the phone will try to draw more power than the solar cells can give.

 

[Transistorman]

Thanks for the reply Boncuk, I will have a look on it.

If I have to regulate the voltage of the output, what range of voltage would you suggest?

 

[Transistorman]

My cell phone and battery also has a USB connector for charging the battery. The USB supplies 5 volts. Inside the phone is the battery charger circuit that makes the 3.7 volts.

I believe the 10 volts from the solar cells must be reduced to 4.5 to 5.5 volts before powering the USB. A linear regulator like the LM317L or a LM78L05 will work but will waste 50% of the power in heat. Using a switching power supply will only waste 5% of the power.

I have some concern that the phone will try to draw more power than the solar cells can give.

I have noticed that quite early before I started on planning the project, I do realize that it will take a tremendous amount of time in order to charge the battery fully, but that didn't really bother me alot as it is unlike I will be using it for my formal activity in my daily life.

And as for the project, I think I will be taking the 7805 and LM317L into my consideration .

 

[ronsimpson]

I have a fear the phone thinks it is on a USB power supply and will try to draw 400mA. The cells can not supply that and their voltage will drop to 4 volts and reset the charger. I think you need to add a large capacitor on the 5 volts or 10 volts lines to store energy.

Good project. I think you need to build one and see how it works.

 

[Transistorman]

Hello again,

I have done further experiment with my plans, the photographs below are taken for proof of my progress. I connected the solar panels directly to the LEDs and got a pretty bright light even with a desk lamp as I was busy in daytime.

Hope you enjoy .

**broken link removed****broken link removed**

 

[nickelflippr]

I do not know the charging circuits of a cell phone, or how they would be affected by external voltages. If the circuits are reverse voltage protected, then:

I would use a window comparator circuit (lm393) for this to regulate the max and min voltages of the battery. Use a couple (or more) of diode voltage drops to provide a constant voltage reference, then scale the voltage dividers accordingly to set the max/min window.

For a Li-ion battery you would set the max voltage of 4.2V, and over discharge voltage of slightly less than 3V. Take a look at this link for a protected rechargeable li-ion cell. Using a linear regulator is just not required for the solar circuit being used, and would be a waste of valuable power.

 

[colin55]

Just use an ordinary diode and 120R resistor on each panel. They are such low output that they are hardly worth using.

 

[Transistorman]

This is much of just a electricity test, in order to confirm if the solar cells are generating any electricity. As I have mentioned in my previous post, I haven't done with my plan, I don't even have a fully finished schematic yet . I am not an expert in electricity but I am trying, I am not expecting you guys to help me throughout my entire project, I am just asking for advices and I shall put them into my consideration . peace.

I shall put a protector as what you guys have mentioned. I should be looking for additional informations about my handphone internal circuit. I really do appreciate the helping that you guys have posted.

 

[Diver300]

I suggest that you have a look at this:-

https://www.triunesystems.com/docs/ds/TS52001.pdf

I don't know how easy those ICs are to obtain in Indonesia. They are designed to be connected between solar cells and a Li-Ion battery. They regulate the charging so that the battery won't be overcharged. When the battery is not at its maximum voltage they adjust the loading so that they get the maximum power out of the solar cells to give the fastest possible charging.

 

[colin55]

You are wasting your time trying to regulate the current. Just connect the two together.

 

[Diver300]

You are wasting your time trying to regulate the current. Just connect the two together.

If you don't limit the voltage to 4.2 V per cell on the Li-Ion batteries, you will eventually damage them, although I realise that many Li-Ion batteries contain protection circuits so that may not matter.

Also, the maximum power point tracker can give you more current than connecting the solar cells directly to the battery. The buck regulator will give out more current than you put in.

 

[colin55]

You will never push the voltage of a 1500mAH cell above 4.2v with 30mA charge-current.

 

[Evalon]

Hi

I would say that your solar cells are of such a low power that quite simple measures may be adequate. The specifications of solar cells are so that e.g. your 10V 30 mA is the absolute maximum they can output and normally the output can be much lower. Depends on whether you can place them in direct sun, and if they are mono, polycrystalline, or something else. Also if you can point them towards the sun.

My guess is - as other posters have noted - that the mobile phone will not be able to work with a fluctuating supply voltage and current. Probably there's a switch-mode supply inside the mobile phone and my guess would be that it is not designed for this kind of supply.

Also, solar cell voltages drop quite rapidly when they have to deliver power so for a first start I'd suggest a zener diode with a ~0.5 volts higher value than the voltage you need for the mobile phone. The zener should be placed in parallel with the output of the solar cells (which should be connected in parallel as well). Remember to rotate the zener correctly! 0.5 watts or 1 watt zener will do.

In this way the solar cells will not overload the input of the mobile phone and you may cheaply and in a simple way check if the solar cells can power the mobile phone's charge circuitry.

Regards,

Jesper

 

[Transistorman]

Thank you guys for the advices.

I predicted to get around 40 mA from the solar cells under direct sunlight although I did get 10 V constantly under the desk lamp, I will not be putting any protector as I am sure the mobile phone itself already has one internally, just in case, I will hold my plan with the 5 V regulator and place a big capacitor by it.

Anyway, I am just trying to regulate the voltage, I would have bought 5 V solar cells if I had a bigger budget and make things simpler, but they produce a tremendously lower current somehow.

I suggest that you have a look at this:-

https://www.triunesystems.com/docs/ds/TS52001.pdf

I don't know how easy those ICs are to obtain in Indonesia. They are designed to be connected between solar cells and a Li-Ion battery. They regulate the charging so that the battery won't be overcharged. When the battery is not at its maximum voltage they adjust the loading so that they get the maximum power out of the solar cells to give the fastest possible charging.

I wasn't able to find the IC that you recommended, even in the biggest store from Indonesia I could think of, but I appreciate it though .

Thanks,

Transistorman

 

[Transistorman]

Hello again everyone,

I have successfully finished and tested the prototype for the charger, it was quite slow in charging but as long as it is charging, it should be alright for me . It worked pretty well on my unused phone which has a Li-on battery of 850 mAh and below are the photographs taken by myself for the quick view of the prototype.

**broken link removed** **broken link removed**​

On the second photograph, please take note to look at the battery status on the top right , we can see that it was in the charging status.

Thanks guys for the support anyway .

Regards,

Wawan

 

[wandeha]

Hi T-Man (teman), I am intersted in your project because, as my opinion, it can be used for other power sources... so if you have done this, would you please share here?
thanks

 

[Transistorman]

Hi T-Man (teman), I am intersted in your project because, as my opinion, it can be used for other power sources... so if you have done this, would you please share here?
thanks

Sure, I have actually done the final version long ago, but you can see me explaining about the prototype version right on my Youtube account, by clicking the 'My Video' hyperlink below and look for 'Solar Cells Charger'.

Thanks,

Wawan