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How Do You Charge a Laptop Battery with a Solar Panel ? - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: How do you charge a laptop battery with a solar panel ? Posted: Thu Jul 16, 2009 1:33 am


Joined: Tue Dec 30, 2008 1:53 pm
Posts: 4
I'm thinking of trying to charge my Lenovo Thinkpad x60s battery with an R-14 Powerfilm flexible solar panel.
I hope this question is not too elementary for this august forum.
Anyway, the idea is to charge the battery with it in the laptop, but with the laptop turned off. I think having it turned On will most likely not work (as follows.)

Someone told me that since the wattage of the solar panel is much less than the draw of the Thinkpad, it won't work. I'm not an electrical engineer, although I did quite well in physics. The only problem is, it's been quite a few years (of disuse.)

Here are the numbers :

R-14 Powerfilm
The Powerfilm is rated at 15.4V and .9A ( ~14 watts)

Lenovo Thinkpad X60s Laptop
The battery is 14.4v / ~5200 mAh
I measured the current draw of the Thinkpad - when OFF (battery charging only) - at .25A and 115v (= 29watts ).
When On, it's something over 50 watts.
I measured this straight out of the wall, before the inline transformer that converts the 110v to ~20v (?). (I'm not sure what the charging voltage is, but the charger says 20v / 3.5A = 70watts - probably what's need to actually power a running computer)

I had thought that the battery charging would charge at a much lower current, like a trickle current, .1C or so, but this is not necessarily the case with Lithion batteries it seems. See Lithium Ion Battery Charging Basics
It's more like .2 to .7C actually.

One idea, if this is not going to work, is to pull the battery off and charge it outside the laptop, but this looks complicated as there are about 8 or 9 contacts on the battery, and I have no idea what is what. It sounds a little risky anyway.

If anyone could be of help that'd be great.



Kirt Blattenberger
Post subject: Re: How do you charge a laptop battery with a solar panel ?Posted: Thu Jul 16, 2009 3:41 pm

Site Admin

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings kbarb:

If you don't mind spending a few bucks, you might get away with buying one of the chargers made to plug into a car cigarette lighter and interfacing it with your solar panel. The voltage specs seem to agree well. Should be a simple 2-wire connection between the two devices.

This one is made for the ThinkPad

- Kirt Blattenberger
RF Cafe Progenitor & Webmaster


Post subject: Re: How do you charge a laptop battery with a solar panel ?Posted: Fri Jul 17, 2009 1:29 am


Joined: Tue Dec 30, 2008 1:53 pm
Posts: 4
Yes, I have one of those bookmarked actually.
In fact a few months ago I bought an inverter to plug into the cigarette lighter, and now I'm thinking I should have just used one of those.
But this time I'm powering off the solar panel.

And it still seems to me that the laptop will try to draw more than the panel will deliver.

That's what I'm trying to figure out actually, if someone can fill me in.

But thanks, nevertheless.


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