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If you connect a solar panel to a phone battery, but the solar panel provides about 1/4 (or 25%) of charge (mA) compared to the battery's original charger. Will this slow power charge the battery (albeit at a very slow rate)?

If so, what would be side effects of the battery being left on this slow charge all day every day?

EDIT: This question is for charging the battery separately from the phone and so extra precautions as highlighted below are needed to control V and mA, etc.

I want to charge the battery while it's still in the phone. Solution is also below in the answers and comments.

share|improve this question
First of all, what is "1/4 charge"? 1/4th amperage of the original loaders max amperage rating? 1/4th voltage? Charging a cell with a to low voltage will not work. With a low amperage it might, but then you run into problems with Li-Ion cells which need to be charged in a very specific way. – Hennes Jun 8 '14 at 12:39
+1 Question is a useful one. – Russell McMahon Jun 8 '14 at 13:24
Apologies for being vague. I'm not entirely sure how charging works (i.e. all I know is voltage from solar cell would need to be higher than the battery output so that it can actually charge), as for Amps, I'm not sure the output of the solar cells (I'm yet to purchase one). – KickAss Jun 8 '14 at 13:31
+1 Question is useful. – Adi Jun 8 '14 at 14:00
NB: !!! You said "phone battery" and somebody else mentioned charging it IN THE PHONE. My answer is correct if you place the battery in a socket connected to the PV panel - ie make your own solar charger. HOWEVER - if you connect to the 5V USB input on a phone - as long as you do not exceed 5V at the phone input then the internal phone elctronics will stop charging when the battery is fully charged. So they too are correct. Which method you intended I know not. – Russell McMahon Jun 9 '14 at 13:29
up vote 3 down vote accepted

I assume that you mean that the current is about 1/4 of the original current.

Phone batteries will almost invariably be Lithium Ion (LiIon).
LiIon are normally charged at a maximum of "C" = the rate in mA numerically equal to the capacity in mAh.
So, for a 1200 mAh LiIon battery C = 1200 mA.

Assume your battery is 1000 mAh.
Actual values can be scaled if this assumption is wrong.
Normal maximum charge rate = C = 1000 mA.
Solar charge rate = 1000/4 = 250 mAh.
Assume it is a 3.6V nominal battery. This is the voltage of a single LiIon cell.
Max allowed voltage = 4.2V. Charge it with more than 4.2V and it's lifetime will be severely impaired OR the battery will self destruct.

IF the battery voltage when charging is less than 4.2V the 1000 mAh battery can be safely charged at 250 mA with no ill effects.
Once the battery voltage reaches 4.2V on charge the current MIST be controlled to keep Vmax at 4.2V OR the charging must be terminated. IF you continue to charge at too high a rate when Vbat reaches 4.2V so that Vbat keeps rising the battery and perhaps its surrounds and perhaps you will be damaged.

It is not enough to just limit Vmax to 4.2V. If a LiIon battery is connected to 4.2V indefinitely it will be damaged.

In the case of a small solar panel and a LiIon battery three main choices exist.

  1. Feed the PV panel voltage into a LiIon charger and use that to charge the battery. A number of camera chargers have 12 VDC inputs and can be charged from an external voltage source - with the charger electronics looking after charging complexities. or

  2. Charge to 4.2V and then terminate charging completely - do NOT "float" at 4.2V. or

  3. Build your own LiIon charger using one of the many purpose built ICs made to do just that OR 'roll your own' with an opamp, voltage reference, control switch (MOSFET usually) and more.
    Option 1 is easiest.

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That is very detailed and helpful :) I am an absolute beginner on electronics. I needed a way to make my Phone Battery last longer and thought I "could" hook it up to a solar cell on a permanent basis "hoping" it will somewhat charge the battery whatever the amount and extend battery life. But I see this is a LOT more complicated. The solar cell panel, typically 3-5"inch, will not produce enough voltage to charge the battery. – KickAss Jun 8 '14 at 13:37
Also the constant charging damaging the LiOn battery is also very helpful. Using the Charger to deal with the charging complexities is a VERY good suggestion, it will save me a lot of pain. Many thanks :) – KickAss Jun 8 '14 at 13:38
@KickAss, while directly connecting to a solar cell is certainly not a good idea connecting via a charger might not be either, some will continually cycle when supplied with lower voltages / current then they're designed for. – PeterJ Jun 8 '14 at 13:53
@PeterJ Ah I see. Okay, I will bear that in mind. I will look for a higher output PV panel. Thank you – KickAss Jun 8 '14 at 15:24

With regardsto charging li-ion batteries, the above answer is completely correct, however, Most phones control the charging of their batteries within the phone itself, and all we have to do is supply a 5 volt, 500-1000mA power supply to it via a USB connection. The phone then uses a buck converter to lower the voltage to 3.0 - 4.2 volts to appropriately charge the phone.

So if you can find a small solar panel that produces the 5 volts, and you then solder the correct connections to it so you can connect it to the normal USB connecter/charger port to your phone, it MIGHT work. I say might because it will depend on what type of amperage you can supply. Some phones will charge at lower amperages, but some will not even attempt to charge until a minimum charge is obtained (For example, I have played around with solar panels and charging iphones and it seems like the iphone won't start charging until it gets 500mA, but sometimes it will keep charging, although very slowly when that current drops below 500mA)

And you don't have to worry about the "trickle charge". Once your phone's battery is full, it will stop accepting charge from the solar panel whether you unplug it or not.

share|improve this answer
+1 Excellent point. I have added a comment to his question re your answer!. I do not know whether he intended to connect to the battery directly (which was what I assumed) or via a phone USB input - where your answer applies. – Russell McMahon Jun 9 '14 at 13:31
+1. That's very helpful too. Thank you Filek and @Russel McMahon – KickAss Jun 9 '14 at 18:53
@Filek, you wouldn't happen to know if Samsung Galaxy S3 has this inbuilt functionality would you? If it does, all I need is a solar panel to get going. Battery on that phone is a catastrophic disaster. – KickAss Jun 9 '14 at 18:55
@KickAss Essentially any modern cellphone that accepts a 5V "USB" inputfor charging has the charger built in. All Samsung cellphones do. BUT you MUST be certain that the voltage NEVER rises above 5V. How much above 5V is "safe" is uncertain and varies between phobes. In some cases a very small extra amount can badly damage the phone (I've seen charger ICs that are rated at a Vin_max of 5.5 V :-(. Samsung are competent and I'd expect them tolerate 6V in and maybe 7V and just possibly more BUT 5V max is the spec you should stick to. As Filek notes, ... – Russell McMahon Jun 10 '14 at 7:58
... Some phones will not charge on very low currents. Some, such as many or most Apple models, need special combinations of resistors connected from the data leads to V+ and V- to cause them to accept charge. (This is well documented - search for eg "Mintyboost" which provides information on this.) Odds are that the Samsung S3 will be better behaved in respect than most of Apple's fine products.... [Weave a circle round him thrice and close your eyes with holy dread, for he on honey dew hath fed and ..." ] .. huh.. wazzup ... what happened. Oh, yes. Than Apple's fine products. – Russell McMahon Jun 10 '14 at 8:02

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