[Lancaster] Re: Laptop supply from 12v DC

[Ken Hough]

max wrote:
> On Wednesday 01 Feb 2006 15:33, you wrote:
> 
>>Max,
>>
>>Since last posting on this subject, I've bought a 12v to laptop supply
>>converter from Maplin. This is Stock code number L40BB. Cost £32.49 incl
>>P&P. It's a 120W unit. A neat unit which works well to allow use of my
>>IBM Thinkpad T21 via the car battery.
> 
> bit pricey. what's it actually eat up though? 

What does it eat up??? Do you mean how much power does it draw from the 
12v source?

These units use switch mode power supplies which are typically around 
90% efficient. That means that if, for example, your laptop nedds 16v at 
4 amps (which is 16 x 4 = 64 watts), then you would draw something like 
64 x 100/90 = 71 watts from the 12v source, which in turn translates to 
5.9 amps.

Hope that this helps.

> 
> 
>>The reason for this stems from my budding interest in astronomy and need
>>to use the laptop outdoors, but usually with the car at hand. During
>>cold nights (like now) I discovered that the laptop will run for maybe
>>only 1/4 of an hour. At room temerature it will operate for > 2hours so
>>battery seems OK.
> 
> yes - my ipod lasts for 2-3 hrs instead of over 10!
> 
> apparantly the battery still has the energy, but can't get it out in the cold 
> weather. whatever that means!

Whatever it means, it a problem. The old lead/acid batteries are far 
better in this respect. Just as well or our cars would have problems 
starting in cold weather. Also, lead/acid batteries hold their charge 
MUCH better than NiCads, Ni metal hydrides or rechargable lithium cells.


Ken