How long will 12 Volt 18 AH battery run a laptop?

Hi,

It seems the more I read, the more confused I get.

I have a Fujitsu Lifebook C 6581 laptop and a Kendrick 12 volt DC 18 AH battery. The laptop AC adapter says the output is 3.75 amps. An inverter store rep says I should allow an additional amp for their unit when doing my calculations to find out how long my battery will run the laptop.

A poster on Google's sci.astro.amateur said that his P4 laptop requires 18 v at 4.2 amps when charging and 18 v at 1.4 amps when running it. He also said to rely on a battery delivering about 60% of its rated power as the listed rating is for the '10 hour' discharge rate.

The used laptop seller suggested a 150 watt inverter and the inverter store rep suggested a 450 watt one. Fujitsu could not give me a recommendation.

So, is there anyone who can tell me how long my battery will run the Fujitsu? What wattage inverter should I get? How were these amounts calculated?

Here's the link to the Google posting:

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm=ivwec.15671%244N3.3792%40newsfe1-win

Thanks for your replies.

Clear Skies,

Darlene

To Dan, James, Floyd and Larry,

Thank you for your replies!

OK, let's see if I understand things.

While the Google poster's laptop requires 18 v at 4.2 amps when charging and 1.4 amps when running, my laptop requires 16 v at 3.75 amps when charging. Per the AC adapter, the output is 16 v up to 3.75 amps. Input 100v-240v ~ 1.4 amps. Does this mean that the laptop requires 1.4 amps to run?

So, if I'm using a DC/DC converter with an efficiency of 85% and allowing for a 5% 'headroom,' my 12 v 18 AH battery will power the laptop for about 8.04 hours. Correct?

Here's how I got the answer: 16 v X 1.4 a = 22.4 watts. 120% X 22.4 watts = 26.88 watts. 26.88 w / 12 v = 2.24 amps. 18 ah / 2.24 a = 8.04 hrs.

But another question pops up. What happens to the laptop's own li-ion battery while I'm running the Fujitsu with the 12 volt 18 AH battery? Is the Li-ion battery being drained and charged to some degree too? And if so, are these amounts negligible? Does this also affect how long the 12 v 18 AH battery can power the laptop significantly?

Thanks for your comments.

Darlene