[ltp] auxiliary battery packs for A/T/X series

[Brian Beattie]

On Mon, 2003-10-20 at 15:27, James P. K. Gilb wrote:
> You may need a voltage regulator to make the DC port of the laptop 
> happy.  In fact, you probably should avoid overvoltage of that port, it 
> could cause problems.
> 
> You can get a project board and variable output linear voltage regulator 
> at Radio Shack (or Frys or Digikey, etc.).

Well, a linear regulator is going to drop a minimum of 2.5V so to get
the 70%, you need a battery pack that puts out 18.5v at 70% discharge or
about 20 cells, which is going to be 30 volts at full charge.  That
means you need to disapate 30-16 or 14v * 4.5A 63W... also that is a
little less that 50% loss, not very efficient.  and boy what a heat
sink...

What I would like to know, is what do you base you recomendation to
"overvoltage of that port".  Is this just ordinary caution, or do you
have some actual knowledge of circuit involved?

> 
> My IBM adapter indicates it it 16 V at 4.5 A, which is pretty sizeable 
> power.  If you drop 2 V at 4.5 A that is 9 W of power.  The regulator I 
> have from RS is 15W, if you tie down the ground to enough metal to 
> spread the heat.
> 
> Also, alkaline batteries have a useable voltage range of about 1.5 V to 
> 0.9 V (in order to get 70% of the power.)  Thus, 12 cells will range 
> from 18 V down to 10.8 V before you really start to run out of power. 
> If you limit the drop to 15 V, (probably too little low), you will have 
> used only a small percentage of the mA-hours of the batteries.
> 
> NiMH, Litium or NiCad cells would work better (in terms of a more stable 
> voltage) than alkalines, but they are more expensive.
> 
> You could try to get a buck/boost regulator (i.e. one that either 
> increases or decreases the voltage depending on the input voltage) that 
> would take 12 to 18 V and regulate it at 16 V, but that will be somewhat 
> more expensive, especially at 4.5 A of current.  If you are going to do 
> that, you may pursue using only a buck regulator and use less cells, say 
> 6 alkalines (i.e. 5.4 V to 7.2 V input).
> 
> If you decide to try, post your results, it sounds like an interesting idea.

well the project you have designed sounds interesting, but that is not
what I had in mind.  I'd really like to hear from somebody who actually
know something specific about the PS circuitry of the Thinkpads if
anybody on this list has that knowledge.

> 
> James Gilb
> 
> Brian Beattie wrote:
> > A while back, there was some discussion of auxiliary battery packs. The
> > little reasearch I have seen indicates that some of this packs may be
> > unregulated, meaning that the output of the battery pack is the raw
> > battery voltage.
> > 
> > This has lead me to wonder, what the requirements for the DC power input
> > of the thinkpads are and if it would be possible to use a battery pack
> > made up of disposable cells (D or C cell alkaline for instance) by using
> > say 12 cells to cover Alkaline and NiMH, 18V - 15V nom.
> > 
> 
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-- 
Brian Beattie            | Experienced kernel hacker/embedded systems
beattie@beattie-home.net | programmer, direct or contract, short or
www.beattie-home.net     | long term, available immediately.

"Honor isn't about making the right choices.
It's about dealing with the consequences." -- Midori Koto