Just a quick note...

Your laptop may well pull much less than 3A.  In general a laptop uses a
lot of current only when charging its own battery.  Since you won't have
the 'other' battery in the laptop, then it should consume less current.
But it's very easy to measure - cut the cord on your AC adaptor (or
build a through-adaptor) and measure the current it uses during your
normal activities.

The flip side is that the laptop will think it's on AC power and not be
as conservative as it could be.

Another problem with Gell Cells is their intense dislike of deep
discharge, or fast discharge.

Alternately you should think about using NiCd or NiMH.  For my last bike
race I needed low weight, low cost, high capacity batteries and found
that one could get 4AH (four amp hour!) 4/3 A NiMH cells for several
dollars apiece.  More recently I found a retailer selling 4.5AH cells
for $3 each.  A 5 cell pack gives you 27WH of power, and the cells are
designed for high loads, and high charge rates.  I use an RC charger and
can charge one in under an hour (my light runs for over 2 hours with one
pack).

They are the next best thing to Li-Ion, at a much lower cost and similar
weight per WH,  not to mention the ability to charge them more than once
a day without boiling the electrolyte off.

-Adam

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.
>
>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.
>
>Now, all I need is to figure out the charge and discharge characteristics of a
>(or this) gel-cell, 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.
>
>Any assistance/pointers appreciated.  And I'll even accept a "what the hell
>are you thinking" before I really get started.
>
>Cheers,
>-Neil.
>
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>
>
>
>

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