On Sun, Apr 27, 2003 at 03:24:18AM -0500, Picdude wrote:
> New project...
>
> So I've gone thru 3 Li-Ion laptop batteries in the past 2 yrs.  Not sure how
> long I'll keep this laptop, and after getting frustrated at prices of
> replacement batteries and not trusting the serial-number-less ones on ebay,
> I've hit upon another idea...  why not go gel-cell.
>
> At the high level, each laptop battery is 10.8V @ 3.2AH, which is a power
> capacity of about 35W.  In contrast, I saw a Yuasa 12V 7AH gel-cell (84W)
> which is not very large or heavy, and is $23, so it's even more appealing.

But the question is the discharge characteristics of each battery as you point
out below. And as Olin stated in another post, you have incorrect conversion
units.

>
> Connection to the laptop, I'd probably use the power-port at the back, but
> the
> external power adapter supplies 16V @ 3.36A.  Since I don't carry the
> pluggable CD drive or floppy with me, I'll bet that the laptop will consume
> less than 3A (and I'll test to be sure).  That value's important to me for a
> reason....
>
> Raising the ~12V to 16V is easy, cause I have the dc-dc converter chip
> already, and even a recently-built test ckt, which I can adjust to 16V by
> changing just a few parts.  So cost is essentially zero there.  However, I
> have a limit of 3A out.  But hopefully that won't be a problem.

Probably not.
>
> Now, all I need is to figure out the charge and discharge characteristics of
> a
> (or this) gel-cell,

And here is your problem. See batteries are magical devices ;-). The faster
you pull power out of them, the less power you get.

Generally the AH rating gel-cell is over a C/20 (1/20 the battery capacity)
discharge rate. But the more current you pull, the less time the battery will
last.

And that's in full effect here. Presuming a 90% DC/DC conversion the input
battery will need to provide ((16V*3A)/12V)/0.9 -> 4A/0.9 -> 4.44A of current.
So instead of a discharge curve of 7A/20 -> 350ma you have a curve of 7A/4.44A
-> C/1.5. I can tell you at that rate the battery won't last a 1/2 hour befure
being totally discharged. And I'll bet that it'll get as hot as hell in the
process.

> and build a ckt to monitor the remaining power and one to
> charge the gel-cell.  I haven't found a suitable gel-cell charger ckt on the
> net yet, but still looking.  And if I find the discharge specs, I'm sure I
> can whip up a quick PIC-based (of course!) power monitor.  I can't even find
> a website for Yuasa.

For charging modes I have a post:

http://tinylink.com/?CY8eaWXWht

>
> Any assistance/pointers appreciated.  And I'll even accept a "what the hell
> are you thinking" before I really get started.

WHAT THE HELL ARE YOU THINKING!!! ;-)

The Li-Ion and other advanced battery types are expensive for a reason.
>
> Cheers,
> -Neil.
>
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