Andrew Cornwall
Home Page

Electrical Requirements of Portable Computing
Update 2003

by

Andrew Cornwall

Copyright December 2003

Why Would an RVer Be Concerned With Electricity Use? I worked with computers in various capacities for more than 30 years, and without a computer nearby to use I feel somewhat incapacitated. While RVing we use a notebook computer to plan our travels ( plot routes and research places to see), to browse the Internet and send /receive e-mail with an Internet- enabled cell phone, and to word process correspondence and ideas. Also, the notebook is invaluable for storing, editing, and cataloguing pictures taken with a digital camera. On occasion the time spent on the notebook computer can add up to more than a couple of hours a day. Conserving use of RV electricity is important in loactions which do not have a 'shore power' hook-up and the rig's electric power is provided by 12 volt 'house' battery. Such locations include undeveloped campsites in state (provincial) and national parks, overnight parking at truck stops and shopping center parking lots, and boondocking on improvised sites. On travel days when the trailer is being towed, the tow vehicle's alternator usually keeps the house battery topped up. Managing battery power becomes of special interest when the trailer is stopped for several days, or more, without a hook-up to shore power. In the past couple of years my wife and I have become fond of snow-birding on the open deserts of Arizona and California. Our power source there is the sun with the aid of a solar panel that on sunny days can deliver as many as 4 amps to charge the trailer's 12 volt house battery and to run various electrical devices. A notebook computer system has the potential for hogging available electricity during sunny days and draining the battery at other times. As part of effectively using the trailer's self-reliant electrical power resources, it is worthwhile to know how many amps (at 12 volts) all of the trailer's electrical devices draw. Without taking care to budget electrical use it is likely that the house battery will become depleted, disabling lights, water pump, refrigerator, and other vital 12 volt appliances.

This article is an update of one that I posted in 1998 describing the electricity usage of my then new Compaq Presario notebook computer and Lexmark inkjet printer. My purchase in 2003 of a new HP Pavilion notebook computer caused me to wonder how its electricity usage compares with its 1998 counterpart. At issue is the extent that using the notebook computer system would drain the 12 volt �house battery� of my 5th wheel travel trailer. While RVing and not connected to �shore power�, I rely mostly on the trailer�s house battery with a 115 volt AC inverter to run my notebook computer system. The notebook�s internal battery provides operating power for a total of only one to two hours before needing to be recharged. Further, the printer, which does not have an internal battery, requires regular AC power at all times. The 2003 notebook computer is considerably more powerful than its 1998 forerunner, but would it have the disadvantage of needing more electrical power? The results of my comparative measurements are described in the present article. The 1998 measurements of printer electricity use are repeated in this article. Although the results herein refer to specific notebook computers and printer, they do reflect generally on how amperage requirements of notebook computer systems are changing. RVers contemplating buying a notebook computer may find these results interesting.

The 1998 Compaq Presario and the 2003 HP Pavilion are typical of the 'value' notebooks in their respective eras: competent computers but not the most powerful currently available. In the intervening 5 years notebook computers, and personal computers generally, have become much more capable while their prices have declined sharply. Indeed, the HP Pavilion in 2003 cost one-third less than its predecessor Presario in 1998. Here is how the specifications of the two notebooks compare:

- - -

Amperage measurement for this article simulated 12 volt battery operation by substituting a 13.8 volt DC power suppy for a battery. A 150 watt inverter was connected to the power supply. The purpose of an inverter is to tranform nominally 12 volt DC voltage to 115 volt AC (modified sine wave). In turn, the notebook's AC power module was plugged into the inverter's AC socket. With this setup the notebook runs as if it were plugged into an ordinary AC wall outlet. The amperage measured was that entering the inverter from the DC power supply. Several observations apply to this technique:

• the measured amperage is about 8% lower than that expected from an RV battery. The reason is the higher voltage of the DC power supply, 13.8 volts, versus that of a fully charged deep cycle battery, at 12.6 volts. The inverter apparently compensates for higher voltage by reducing current draw. Further, in a real life situation as the voltage of a battery declines below the full-charge level, the inverter will draw more amperage to maintain 115 volts AC ouput.
  
  

• A notebook's amperage depends on what it is doing. Several sets of measurements were taken based on common computing tasks. Each task was run separately. High-processing intensive applications, for example graphics-intensive games, and mulitasking of applications would likely increase the power required by the notebook.
  
  

• The power-save features of the notebooks were turned off. This is usual for AC powered operation. The hard drive was spinning throughout and the screen was a dimmed (except as part of a test). Presumably, some savings in electricity use could be achieved by activating the power-saving features.
  
  

• The precision of the measurements was on the order of 2 digits. Because the amperage fluctuated somewhat even while the notebook was doing a specific task, stable measurements could not be obtained with the digital multimeter at hand. Instead an analogue meter was used that allowed for visual reckoning of amperage, but without the higher level of resolution available with a digital readout.
  
  

• there is a temptation to use the measurement of amperage going into the inverter as an approximation of that entering the notebook's power pack. Such a use is not unreasonable. However, the inverter's specifications claim 90% efficiency, which implies that the amperage going into the inverter is about 11% higher than that actually entering the power module. Moreover, there is the factor of the inverter's 'no-load' current draw (i.e. when nothing is plugged into it) which is specified as 0.1 amp.

The results of the measurements are shown below. The notebooks' cooling fans were not running and, except for the measurement of battery charge, the batteries were removed. In some instances maximum (and minimum) values are also shown to indicate the transitory effects of disk access and background processes, or to reflect other parameters such as speaker volume and screen brightness settings.

AMPERAGE DRAW (AT 13.8 VOLTS) OF INVERTER POWERING NOTEBOOK COMPUTERS
	1998 Compaq Presario - Amps	2003 HP Pavilion - Amps
Computer Off	0.3	0.2
Booting (max.)	2.1 (2.2 )	3.0 (3.8 )
Idle (no application)	1.8	1.8
Copying Files from HD to: - Floppy Drive (max.) - CD-R (max.)	2.0 (2.1) n/a	2.2 (2.4) 2.4 (3.2)
Copying Files from CD to HD (max.)	2.1 (2.2)	2.8 (3.8)
Word Processing (typing)	1.9	1.9
Playing Music CD (Volume: min / max.)	2.1 (2.1 / 2.2)	2.4 (2.3 / 2.5 )
Displaying Picture Image (Brightness: min / max.)	1.8 (1.7 / 1.8)	1.8 (1.6 / 2.0)
Showing Slide Show (IrfanView) - From HD (max.) - From CD (max.)	1.8 (2.0) 2.1 (2.2)	2.4 (2.8) 2.5 (3.1)
Playing DVD Movie (max.)	n/a	2.6 (4.0)
Internet Modem Access (max.)	1.9 (2.1)	2.2 (3.6)
Charging Battery (50% depleted) - Notebook Off - Notebook Idle	2.8 3.7	3.0 4.8
Note: With the notebooks' fans running the amperage of the HP Pavilion increased by 0.1 to 0.2 amps, but there was negligable increase with the Compaq Presario. An optical mouse added to the HP Pavilion utilized 0.2 amps, while adding a 'ball' mouse to the Compaq Presario had no noticeable effect on its amperage.

- - -

The amount of electricity used by the 2003 HP Pavilion is not substantially higher than that of the 1998 Compaq Presario. In some tasks, i.e. wordprocessing and displaying picture image, the HP Pavilion uses essentially the same amount of electricity. These results are suprising given the markedly greater capability of the HP Pavilion. Transient maximum amperages, usually involving HD and CD disk activity, are higher for the HP Pavilion, but due to their short duration they would not have a significant impact on overall electricity usage. Charging the notebooks' batteries adds considerably to the electricity use of both notebooks. In this regard the HP Pavilion draws more amperage than the Compaq Presario. With either notebook, though, it is advisable to charge the battery only when shore power is available. The battery should be removed from the notebook when power is provided by the RV's house battery.

Previous measurements of the current drawn by the Lexmark printer are as follows:

Amperage Draw of Printer
Printer Idle Printing	Amps 0.3 0.5 (2.0 during page feed)

The notebook computer system, including printer, has the potential of drawing considerable amperage for short durations. For example, when the notebook is charging its battery and word processing at the same time that the printer is performing a page feed, the combined current drawn by the Compaq Presario system would approach 6 amps and that of the HP Pavilion system would approach 7 amps. In addition to the momentary higher drain on the RV's house battery, there is the further consideration that the electrical cabling (and connector) from the battery to the inverter needs to be sufficiently robust to carry this peak amperage without significantly reducing the voltage available to the inverter. This is even more critical as the battery's charge becomes depleted and the battery's voltage declines. The inverter used for these measurements, which is typical of many inverters, shuts down if the supply goes below 10 volts.

Hosted by www.Geocities.ws