# Solar Powered Li-Ion Battery Charger

• posted

Hey all,

I'm going to be building a solar powered Li-Ion battery charger for a class project, and I just wanted to ask a few questions that I hope some of you may be able to help me out with.

First off, here are the specs for the battery: Camera: Minolta DiIMAGE G500 Battery: Lithium Ion NP-500, 3.7V, 820 mAh

I think I found a good match for a solar cell and IC - the links are below:

Unfortunately the only specs given for the solar cell are Voc = 8V, Isc = 310 mA. I also estimated the power dissipated in the IC (as per the equation in the datasheet) and it is nearly negligible - about 50 mW.

Some assumptions that can be made are the device will be used only during peak sun hours and will be angled perpendicular to the sun for maximum insolation on the cell. I don't necessarily need the battery to fully charge in one shot, but merely keep the battery from dying over the course of a week or so.

So with that said, is there a way to determine (or estimate) the operating point of the solar cell without an IV curve? Obviously the output of the chip will be around 4V (battery voltage), so does that imply the solar cell will be operating at that voltage? The pulse charger requires a current limited source (which a solar cell is inherently), so will that mean the solar cell operating point will be the lowest Vcc at which the IC will operate, in order to maintain the highest current? I'm just confused as to how determine the operating point of a device that uses a pulsed current, where the high current value is basically the maximum the solar cell can provide.

I'd also like to do a load analysis. What do you think would be the best way to do this? I know most are simply a static voltage and current value multiplied to give you the power (and also time the device will be used to yield an energy figure). Again, though, the IC is a pulse charger and has two main sections in the charge cycle - constant current and constant voltage (where the current pulses decrease until it reaches full charge). Should the load analysis follow the complexity of the device, or would a simple estimate of output voltage and charging current suffice?

Please do not reply with comments such as 'this project is pointless, just use a car battery adapter for power', or 'you can buy a solar powered charger at this website'. I'm doing this project and want some helpful input! Any suggestions or help in understanding the solar cell operation with this chip and battery are greatly appreciated! Thanks so much.

• posted

"Ray D."

** That charger IC is probably *quite unsuitable* for use with a small solar panel - due to the total unpredictability of available voltage and current.

Quote:

" I don't necessarily need the battery to fully charge in one shot, but merely keep the battery from dying over the course of a week or so. "

Since you only require to perform top ups on the battery - simple, direct connection is all you need. That solar panel will likely never deliver more than 200 mA anyhow.

Rest of your tortured, over-analysing deleted.

..... Phil

• posted

Sorry about that - the chip I was looking at was the LTC4052-4.2. The link you have there is for the LTC4062 which is a linear charger - not suitable since the power dissipation is much greater.

Are you referring to the unpredictable operating point of the cell, or the unpredictability of the sun? If it's the latter, I already said I would only use it on a perfectly clear day where the solar insolation is relatively constant for the time the battery is charging (anywhere from 1 -3 hrs at around 12PM). If conditions were hazy or cloudy, I wouldn't be using the charger. Also, the IC accepts a voltage range from 4.5V-10V, so as long as the current is somewhat constant (which it is over a wide range of voltages on the IV curve, and I think it would be as long as the insolation does not greatly vary) I don't think the voltage changes would affect the system too much, right?

I don't think directly connecting it would be a good idea because there would be no means for over-charge protection. Also, a Li-Ion battery needs to charge in two steps where the second step does not demand a constant current like the solar cell would most likely provide (and does require constant voltage of ~4.2 volts which I doubt the solar cell would provide). And for a battery with a capacity of

820 mAh, 200 mA does not seem insignificant. Obviously it would take a few hours to fully charge, but like I said, if I built this and the solar cell provided 200 mA for an hour or two I think that would suffice to keep the battery from dying. I really want to know, with this setup, what would determine the operating point of the solar cell (more specifically the voltage). As long as the voltage isn't close to Voc (where the current on the IV curve severely drops off), I think the current would be high enough to provide sufficient charging.
• posted

"Ray Dope" "Phil Allison"

** Yaaawnnnn .....

** The sun is quite predictable - been that way for ages....

Shame about the weather, sun angle and assorted light obstructions etc.

** I will puke if you post this utter TRIPE again.

Tip:

Load it with a 5.1 volt, 5W zener and insert a current meter is series.

Learn something.

** Snip more spew inducing drivel....

** Your pathetic, wimpy panel is all the protection you need.

Snip more spew inducing drivel ....

...... Phil

• posted

=A0I really want to know, with

the easiest way may be graphically..

draw an IV curve of the solar cell at some illumiunation...

see where they intersect...

the usual object is to operate near the peak power point of the solar cell.

Mark

• posted