Laptop Power Requirements

[Blue5]

Like many others I use a laptop as a chartplotter and the earlier thread got me thinking about power requirements.

At present I use a 12 V variable adapter. In the laptop destruction book it lists power output as 70W but on the ac adapter it lists DC output as 18.6v-----4.9A now I calculate that as 91W so my question is what figures do I use to establish ah required and If I were were using no other power on board at all and a fully charged 85ah battery at what point as a percentage would the fully charged battery appear flat.

 

[Danny]

Although the PC is quoted as 70W it won't be constantly taking that much power. Check to see if you can set up the power management scheme on the laptop so that the screen switches off after a few minutes inactivity, and similarly the hard drive. This will reduce the power consumption. You could also check to see if you can run without the laptop battery installed - this works with some laptops and saves power lost due to the inefficiency of charging the laptop battery. If you do this you lose the UPS benefit that comes from the laptop battery.

As far as judging the amount of AH consumed you need to take into account that the 12V power converter won't be 100% efficient. Maybe you could use a meter to measure the input current it consumes with the laptop connected and running. You'll probably need to do this over a long period to get a reliable average though.

Sticking my wet finger in the air (and from my own experiences) I'd say that in practice you'll probably find you can run for about 10 to 12 hours with a reasonable power management scheme set up. After that you'll probably want/need to charge your battery.

 

[Anonymous]

I'd say seven hours until battery is half charged (you don't want to discharge more than this or the battery will tend to sulphate). To help other with different sized batteries and loads, I will do the worked calculations here. There are several ways to work this out but this method is probably the most suitable.

NOTE: You will need to check the actual current drawn and refine the calculation when you try it out.

Calculated from:-

Average terminal volts of battery during discharge 12.20
Average Amps drawn = 70Watts/12.20Volts = 5.7A
Inefficiency of invertor is ignored as that is assumed to be within the 70W rating
85 Ah divided by 2 = 43 Ah (i.e. to half charged state and not fully discharged)

Therefore the total number of hours you can run is the Ampere hours (Ah) divided by the Amperes i.e. 43/5.7 = 7.5 hours. Say 7 hours to round down, which is always the safest thing to do as batteries are not made to close tolerances anyway.

As a second check, put a digital voltmeter on the battery and stop using it if the Volts drops to 12.00 or below, on your load. As a general rule, with the battery delivering 10% of its 'one hour rate' of current the terminal Volts should be higher than 11.50 V for long life. In your case, the 'one hour rate' is 85A so 10% of that is 8.5A. Don't let the Volts drop below 11.50 when drawing 8.50A or say 12.0V when drawing around 6A.

Running from a cheap lead acid battery is a good idea - saves the life of your expensive laptop batteries and some laptops run faster on mains. If you are using a 230V invertor I strongly recommend that you use a pure sine wave and NOT a modified sine wave. Modified sine invertors sometimes work perfectly - no problems - but sometimes they blow up the equipment (in this case, probably your laptop mains adaptor).

 

[Blue5]

Thanks Danny and Lemain both very helpful and the worked calcs will be a value to many

 

[shmoo]

Figures for my X60 thinkpad: measured with old fashioned analog meter, which is important since it averages the highly pulsed current draw of a laptops switching power supply. Done with Targus airline-type 12V power supply.

Backlight, disk and processor about 3A
disk and processor about 2A
processor alone about 1A

Your millage may vary!

Tips:
Set very aggressive power management settings. Dim the back light (worth about 0.5A). Turn off your charting software's "tracking" feature since it keeps waking up the disk. Max out the RAM and use a big disk cache to allow the disk to time out and stop rotating. Use "hibernate" rather than turning off and on: is quicker and lets disk time out sooner. Turn off wireless, modem, LAN etc.

 

[Blue5]

Thanks will give the power settings a try and see how it behaves, once the programme is running as the laptop is continuously receiving data from GPS, AIS but I am not touching keyboard etc presumably every time I want to look at position I will have to wake it up, anybody got first hand experience or personal preferences for power settings in this situation

 

[shmoo]

Don't actually put it to sleep mode. Set the display to time-out after say 30s. That will kill the backlight but keep everything processing. The moment you touch the mouse or a key the backlight comes on. You can set the disk to time-out too. I use 1m. You don't notice any difference apart from a sec or two delay while the disk spins up. Trouble is there are lots of background progs that use the disk and keep it awake (virus - turn it off: how you going to get virus on a boat!, windows file search indexing etc)

I use never use actual sleep mode: since I want the AIS collision alarm to work. The hibernate trick is just instead of turning off once you come alongside. Its just quicker.

You can often create custom power mode with all these settings then name and save it. Mine is called "boat_power"

 

[shmoo]

Thought I better go and check: here is screen shot of my Boat Power scheme.

The left hand column is the interesting one, since the pc thinks it is powered. Not quite the settings I gave in previous mail (1m and 3m, actually). Note it doesn't standby (sleep) or hibernate on its own accord, at least while the boat is supplying power. (If we click off the big switch by mistake with pooter running, in eagerness to get to pub, it reverts to right-hand column and safely hibernates)