Power consumption of the Eee PC

The power consumption of the Eee PC is strongly dependent on what you do with it. This page shows how much various uses of the 700 series Eee affect the power consumption, as measured through the AC adaptor. The AC adaptor provides 10.2-10.3 V when idle, and 9.9 V at the maximum current of 2.1 A (or 20.8 watts). The battery provides 8.3 V when full, and about 7 V (check this) when almost empty, with a capacity of 5200 mAh or 4400 mAh, depending on the model.

Power consumption of the Eee PC

It is straightforward to measure the current draw from the AC adaptor when the battery is removed. The numbers below are measured on a 4G model with a 5200 mAh battery. Windows or Linux does not seem to be a big difference.

State Current (mA) Remarks
Off 200  
Standby 250-270  
Battery charging 2090-2110 Off or on does not matter
On, screen off 910 Screen off, wireless off, system fully idle
Screen at lowest brightness +95  
Screen at medium brightness +145  
Screen at full brightness +190  
Wireless +140 – +275 Depends on amount of data transfer. Broadcasting data takes most.
CPU frequency scaling 1:8 -0 System idle. No effect from changing the clock frequency.
CPU usage +250 Heavy use of the CPU and RAM memory, but not the SSD.
Playing H264 movie, loud volume +270  
Speakers on +0 – +100 Depends on what you hear, not on the volume slider.
SSD usage +55 – +65 Heavy reading/writing to the flash storage
Kicker +10 The task bar in the full desktop mode eats power.
CPU and SSD +250 Overall heavy system usage.
SD card +5 – +75 Idle or heavy usage (tested on a slow (2 MB/s SD card).
USB-powered hard disk +160 – +500 Idle – heavy usage.
USB optical mouse (laptop) +10 Small mouse specifically for laptops.
USB optical mouse (desktop) +40 – +60 Larger mouse for desktop usage.
USB flash drive +40  
USB hub +80 Presumably without attached devices
USB bluetooth +40  
overclock 75→85Mhz +30  
overclock 75→90Mhz +40  
External VGA monitor +10  

From these numbers, it appears that it is hard to extend the battery life dramatically. If a system is running with 30 percent CPU usage (for example web surfing, but not movie watching), with the screen set at 50 percent brightness, the total power consumption would be around 1130 mA. Reducing the screen brightness as much as possible would only save 50 mA (4.4%). The biggest built-in power consumer is the wireless adapter; switching it on without using it wastes 140 mA (12%). Very heavy wireless usage (2 MB/s upload) requires 275 mA rather than 140 mA.

Interestingly, using Intel’s SpeedStep technology to change the CPU frequency has very little effect (you can check the control and status files under /sys/devices/system/cpu/cpu0/cpufreq/). When idle, there is no measurable difference in power consumption. At 100% CPU usage, reducing the clock frequency by a factor 8 from 620 to 77 MHz decreases the additional current draw a bit (+250 mA at the normal clock speed, a bit less at lower clock speeds), but also makes the computer proportionally slower. On a system that is mostly idle anyway, it would make very little difference, since the CPU would just spend a bit more time at a lower power, with the same total power consumption as a result.

It is harder to measure the actual current draw during battery usage. The standby mode on a 4G system takes 22 hours to fully drain a new 5200 mAh battery, which suggests 240 mA (1.8 W at about 7.5 V). A powered-on system (idle, screen at full brightness) drains the battery in 3:20 hours (1550 mA, 12 W at 7.5 V). Over the power adapter, the corresponding currents are 260 and 1100 mA (2.6 and 11 W), respectively. When off, there does not seem to be a significant battery drain; the power consumption when on the AC adapter (off or standby) seems therefore to be related to the voltage conversion circuit.

It seems that 2.09 A is the maximum current the 700 series Eee PC will drain. So if you’re watching a movie on your Eee from a USB hard disk, with the wifi on, and at maximum brightness, it might take 15-20 hours to charge the battery.

Replacing the power adapter

As with most laptops, the 700 series Eee PC requires an uncommon voltage from its AC adapter; nominally 9.5 V. The above measurements show that the actual voltage is between 9.9 and 10.2 V. Some people believe that battery-powered equipment is much more lenient (it can handle a 1.5 V battery voltage drop), and that these strange nominal voltages are only there to force people to buy expensive brand-name replacements for the adapters. However, so far there are no reports of people having experimented with 12 V, so it is at your own risk. Ideally, you would use a 2.5 A, 10 V adapter. The polarity of the Eee power plug is positive inside, negative outside. In principle, you could put a few heavy diodes in series with a standard 12 V power adapter, but this is only recommended if you are experienced with electronics.

There is no such thing as spare batteries or chargers available. The original one DC input is 9.5V/2.3A. No one is selling such voltage either. Usual ones are above US$ 60 and start from 12 V up. I found (by mistake) a small switching power-supply 3/5/7/9V, 1A, $35, tweaked it to give about 10 V, disconnected the battery… voila, it worked. Of course, as above current-measurements suggest, brightness has to be lowest, all devices off, otherwise the power supply will switch itself off. Next day, I found another 5/9/12/15V 2.5A, another $35. Did not tweak it, used it directly on 9V. This went OK – as long as the battery is not connected. In order to charge the battery, the eee must be off, connect the power, and just *then* connect the battery. Both battery and eee on does not work – the voltage seems to be at the threshold and eee cannot decide whether to be on battery or on power. Later i may try how it works on 12V…

Using a 12V battery

The power supply inside the EEE is a nicely behaved switching supply. It will run as long as you supply 9.0 VDC to 12.9 VDC … this means that you can use a standard lead acid battery to run it and charge up the interal battery. Just put a single diode (any standard rectifier that can handle 3 or 4 amps will work … actually a bridge rectifier is even better because then you never have to worry about polarity … highly recommended! ). With the internal battery removed, at 9VDC the switcher in the EEE needs about 1.15 to run, at 12.9 VDC that drops to about .85A. Above or below that range the power supply will shut down (obviously going higher that 13VDC is a BAD idea … DON’T DO IT). The numbers given here are based on a display that is fully dimmed, the battery removed, wireless and wired LAN off, audio off, web cam off. Any other use will obviously increase the current, but the running voltages will still be the same.

So you can run this little guy with just a diode in series with any 12V lead acid battery (but use a full bridge circuit and you will never bang it with reversed polarity).

I deliberately did not show any wiring or schematics because anyone that has modest electrical knowledge will know what to do. If you don’t know, just ask a friend with some electrical experience to give you a quick hand … its better than frying your EEE.

<!> NOTE and ATTENTION <!> Only use a 12V lead acid battery or a stable regulated a 12 V power supply that delivers 12 volts! Do not use 12V battery eliminator, they actually deliver 13.8V and are often unregulated so it may even be higher. Also DO NOT use your car’s cigar lighter while the car is running, again it will give you well over 13V. If you must connect your eeepc on such high voltages (like for testing purposes) try to solder some diodes or resistors on the input, or try to use a cigarette lighter with a long cable. Also try to set brightness to full, plug in a SD card, and try to get battery drain as high as possible. Plug in the battery, and turn on the wifi. This way the voltage might lower a bit. The con might be if your notebook fries, everything might be gone!(screen, wifi etc..), if it’s a bet you need to make

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