I have a power source that outputs 3.3v open circuit and 2ma in short.
How can I charge a 3.7v lithium battery rated @ 100mah with such current?
I tried a step up converter but it itself consumes more than this.
Is there anything that can operate at such low power + provide enough to push to the battery?
Google "micro power boost converter"
Plenty out there that run on fumes.
Probably not easy to get those parts.
Leo..
How much power does the power source produce?
Short circuit current doesnt mean anything useful.
Its going to take an very long time to charge the battery regardless.
Charge a supercap. When it has has enough energy use it to power a boost converter.
mikb55:
Charge a supercap. When it has has enough energy use it to power a boost converter.
I was initially thinking that I could charge a 1F @ 5.5v rated supercap at around the 3volts and then dump it's energy to a battery charger with the help of a small booster to at least 4v.
But this option concerns me about current loss since I'm full OCD on efficiency. Regardless, pleas provide some info regarding this matter of using super caps to dump low power to a battery. Links and opinions on how to proceed are welcome.
Why a super cap.
It's internal resistance is worse than a normal cap (e.g. 3300uF/6.3volt).
If you haven't got the skills to design a switching boost converter with inductor, try a Cmos 555 with a schottky diode voltage doubler or tripler. That should work from <=2volts.
Leo..
Wawa:
Why a super cap.
It's internal resistance is worse than a normal cap (e.g. 3300uF/6.3volt).
If you haven't got the skills to design a switching boost converter with inductor, try a Cmos 555 with a schottky diode voltage doubler or tripler. That should work from <=2volts.
Leo..
A switching boost converter with an inductor? Like a Joule Thief? Feeding from the cap pushing whatever it can to the battery? How would I trigger the booster circuit when cap is almost full?
Also, can a 555 work with such current?
Thank you!!!!
Is this project just for fun, or is there a compelling reason for it?
jremington:
Is this project just for fun, or is there a compelling reason for it?
There a compelling reason for it.
There a compelling reason for it.
Good to know!
That would put you in the "energy harvesting" regime. Linear Technology has several solutions.
Of particular interest might be this energy harvesting battery charger.
OP still hasnt responded as to the power output from the power source, whatever it is.
Simply an open circuit voltage and a short circuit current tells us nothing.
mauried:
How much power does the power source produce?
Short circuit current doesnt mean anything useful.
Its going to take an very long time to charge the battery regardless.
I don't know how to answer your first question. When I measure the volts on my multimeter, I get around 3.3v and if I put my multimeter in the mah setting and connect the leads across, I get around 2ma.
The battery will probably take a long time to charge, but also will take a long time to discharge.
mauried:
OP still hasnt responded as to the power output from the power source, whatever it is.
Simply an open circuit voltage and a short circuit current tells us nothing.
Sorry @mauried, I've been busy. I had a couple of minutes to play with it today.
Didn't progressed much but, I did connected a blue LED to it and took some measures.
LED off, 3.5v @ 1ma. <- the ma I measure shorting the power source with mulimeter
LED on, 2.6v @ 0.68ma. <- the ma I measure in series with multimeter
How can I get the data that you need?
Thanks!
jremington:
Good to know!
That would put you in the "energy harvesting" regime. Linear Technology has several solutions.
Of particular interest might be this energy harvesting battery charger.
Great link, upon more reading, I found a lot of compelling IC's among the one you provided.
Would you agree that THIS is a more suitable one for my requirements/power source?
I skimmed through the datasheet and everything looks so pretty but they only mention voltages in and out and I'm concerned that if I connect my 3.3v power source to it, the voltage might just drop due to low ma.
Thank you so much for your reply!
Wawa:
Google "micro power boost converter"Plenty out there that run on fumes.
Probably not easy to get those parts.
Leo..
I found out a lot of good info.
They do run on fumes.
You're right! Not easy to get, specially if you are broke.
Wawa:
Why a super cap.
It's internal resistance is worse than a normal cap (e.g. 3300uF/6.3volt).
I tried a super cap and now I see it is a bad idea. Yes, the internal resistance is no bueno. I tried it with a 3200uF/6.3volt cap and it was way better. I manually pulsed a 5v boost converter and it lid up and charge the battery for around 500ms. Waited another 5 to 10 seconds to do another pulse and so on. It was a fun experiment, possibly a solution if all else fails.
Wawa:
If you haven't got the skills to design a switching boost converter with inductor, try a Cmos 555 with a schottky diode voltage doubler or tripler. That should work from <=2volts.Pringles:
A switching boost converter with an inductor? Like a Joule Thief? Feeding from the cap pushing whatever it can to the battery? How would I trigger the booster circuit when cap is almost full?
Also, can a 555 work with such current?
I made a comment about this but I'm 99% sure I know so little to know that I do not know enough about it.
Would you be so kind to provide more information about it and everything else? Sounds promising if it operates under 2.01v 
Thank you for taking time to respond!
A CMOS 555 can work at 2volt, a "normal" 555 can't.
Plenty of 555 voltage doubler diagrams online.
A joule thief will also work.
Every solar garden light has one.
The LED used in those lights is 3.3volt, while the battery is only 1.2volt.
Plenty of things around you to experiment with.
Leo..
OP still hasnt responded as to the power output from the power source, whatever it is.
Simply an open circuit voltage and a short circuit current tells us nothing.
Actually, those measurements are all you need.
The Thevenin equivalent circuit of the power source is an ideal voltage source of 3.3 V in series with a resistance of 1650 ohms, so that the short circuit current is 2 mA.
Maximum power transfer takes place when the load has resistance equal to the source impedance, so the optimal load will also be 1650 ohms.
In that case, the voltage across the load is 1.65 volts, and the power supplied to the load is 1.6 mW.
The power source is roughly the equivalent of 5-6 very small, inefficient solar cells in series, like those found in light powered pocket calculators.
My apologies if this is obvious, but, if on any of this IC's LTC3105 or the SPV1040 if they get an input of 2.5v @ 1ma to 5v @ 20ma, what sort of output would one get?
Counting that we need to charge a Lipo battery. Let's say 4.1v output set to be safe, what would the ma output be at that voltage and will any of the IC's even function with such low ma?
EDIT: I mean for instance, in the LTC3105 website in a single photovoltaic cell setup, they say from 225mV to 5v, first Fig, what kind of milliamps do they excpect us to provide at that voltage from a single cell to output 4.1v?
Thanks!
input of 2.5v @ 1ma to 5v @ 20ma
Utterly impossible. That would be 2.5 mW power input providing 100 mW power output.
The input current depends on the load. Study the data sheet "typical performance characteristics" section.
jremington:
Utterly impossible. That would be 2.5 mW power input providing 100 mW power output.The input current depends on the load. Study the data sheet "typical performance characteristics" section.
By "input of 2.5v @ 1ma to 5v @ 20ma" I meant a voltage and current range.
I believe you interpreted that as in and outputs, but, I could be wrong.
I have read the datasheet more carefully, and every time I understand a bit more. I'll get there.
Thanks!
Pringles, may I offer that is you start of by telling us what you have, in some details, and what you want to do, with some details, that it would be easier to offer a solution.
you have a "power source" if it is a constant, never varying one, then some solutions would be possible.
if it is transient like a PV cell, then the solutions would be different.