On Tuesday 01 June 2010 12:49:59 Frank D. wrote: > Does it matter how efficient it is?
The question of efficiency wouldn't even come up in this context unless it related to powering something off a car battery when the car /isn't/ running. Does it matter then? Well... yes. Car batteries are optimized for cold cranking amps, not the amount they can be discharged. Deep cycle batteries can be discharged about 50% before permanent damage is done to the battery, but with car batteries it's much lower -- more like 20%. And no car battery can ever be fully discharged for very long, otherwise the sulfur in the acid comes out of solution and falls as a solid precipitate to the bottom of the battery, i.e. the battery "sulfates". After that happens there is no other option than to replace the battery. So you can go through a calculation of how man Ampere-Hours your car battery has, take approximately 20% of that, and that's about how much you can discharge it without causing it any harm. Off the top of my head I'd guess a typical car battery has about 50 Ah -- so by that math only 10 Ah is usable without harm. The battery in my own laptop is 6600 mAh, or 6.6 Ah. This means that if I'm using I use some system that gives me an 66% overall energy efficiency, I get one single recharge off the car battery before doing harm. Many converters (both DC-to-DC and DC-to-AC) can get about 80% efficiency or better. However in the case of DC-to-AC, you also then have to take into account the efficiency of the laptop power adapter. I'm unable to find an efficiency rating for my Thinkpad T61p's power adapter, but one thing I've also noticed is that they have about a 66% power factor associated with them (measured with a "Kill-A-Watt" meter). This has an effect on their actual efficiency. If you want to try to understand what "power factor" means, see the chapter on "Power Factor", Chapter 11, of this pretty cool freely distributable book: http://www.allaboutcircuits.com/pdf/AC.pdf But as I said, just using the DC-to-AC converter is easier -- I'm posting the info above more because it's educational rather than actually useful for this case. ... > Why an older laptop? They tend to be pretty inefficient. Probably because IT'S WHAT HE HAS. Duh. -- Chris -- Chris Knadle [email protected] _______________________________________________ Mid-Hudson Valley Linux Users Group http://mhvlug.org http://mhvlug.org/cgi-bin/mailman/listinfo/mhvlug Upcoming Meetings (6pm - 8pm) MHVLS Auditorium Jun 2 - Android Jul 7 - Patent Absurdity - The Movie Aug 4 - Samba
