how much juice would a laptop need?

Submitted: Monday, Mar 01, 2004 at 23:40
ThreadID: 10930 Views:3563 Replies:7 FollowUps:3
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considering mounting a laptop in the car and just leaving it run permanently....

if you have to ask why i would wanna do this you've missed the point...

my question is -

how much juice would a laptop suck out of a battery in 24hours?
i.e. if i don't start the car for a few days will it still be alive when i get back? (i know i could use a voltage cutout but that's not the issue - i want the thing ALWAYS running)

is this possible with just one additional deep cycle batt? or am i dreaming?

any electrical savvy ppl wanna give me a formula or something given that i know the lappie wants 15volts, 3amps? (ignoring any power saving stuff the lappie may have)


Are you stoned or stupid?
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Reply By: silvia - Tuesday, Mar 02, 2004 at 00:12

Tuesday, Mar 02, 2004 at 00:12
Laptops can be very touchy when it comes to power surges and unless the powerage you give it is very stable and even you will run into problems of the burnt electrical sort. Using a surge protector may do the trick, though.

The other thing you would need to look out for is vibration as not all laptops take kindly to dust and movement. The Toughbooks are the only exception I can think of there, with protection built into their permanent cases. Also take care as a hot car will kill a laptop, again the Toughbook is designed for this but many aren't.

As for power needs, look on the adaptor of the laptop you have in mind and you will see the specifications there.
AnswerID: 48828

Reply By: Bilbo - Tuesday, Mar 02, 2004 at 00:39

Tuesday, Mar 02, 2004 at 00:39
Roughly speaking, if the laptop draws 3 amps at 15 volts, then it will need to draw 3.6 amps at 12 volts. That being the case, a 90 amp hour battery will be totally flat in about 25 hours. At which point the battery, if totally flat is ruined. In practical terms, the car won't start after about 12 hours and the laptop will probably chuck it in at around 20 hours after getting wobbly for about 3 hours.

AnswerID: 48831

Follow Up By: V8troopie - Tuesday, Mar 02, 2004 at 01:37

Tuesday, Mar 02, 2004 at 01:37
Yes but.... Keep in mind that a laptop is designed to run on its internal battery. If you plug in the charger/ powerpack then it keeps the internal battery fully charged while you are using it.
There is no fixed current draw with a laptop that is 'doing' something. It all depends wether the hard drive runs, the CD drive runs, the fan runs (and at what speed). How much number crunching it is doing, etc.
The only time you get a fixed current is when the laptop goes to 'sleep' and the screen goes blank.
I have connected my laptop's power pack via an ampmeter and watched the current draw while I used it. It's up and down like a jojo. Wanted to know how long I could run it on my boat. Definitely not full time :-)

If you want to run a laptop from a 12V battery for a long time, check how long it runs on its internal battery. The longer that says, the less total current draw the machine has. The better ones (read expensive) run 5 hours or more on the internal battery. Mine only goes 2 hours :-(
FollowupID: 310644

Reply By: Gerry - Tuesday, Mar 02, 2004 at 09:32

Tuesday, Mar 02, 2004 at 09:32
Tried leaving mine on overnight a couple of times - touch and go. First time all seemed OK by next morning, however, the next night I tried it and by morning the thing had shut itself down due to low battery power. Perhaps you could try putting it in hibernation mode overnight so that power use is minimised to almost nothing, with a quick startup to where things were in the morning.
AnswerID: 48845

Reply By: Phil G - Tuesday, Mar 02, 2004 at 10:26

Tuesday, Mar 02, 2004 at 10:26
I left mine running overnight once, and it took heaps of power out of the battery.
AnswerID: 48852

Reply By: Member - Frank - Tuesday, Mar 02, 2004 at 17:41

Tuesday, Mar 02, 2004 at 17:41
There are a lot of factors that have to be considered

what and how many times it does harddisk rquire access

will the screen shut down in power save

will the hard drive shut down in power save

have you brought one with power save options

a bench mark for internal batteries is about 3 hours if you cant get that look in your power options a reset some of the varibles, if you accessing it all the time forget it you will be lucky to get 2 hours (accessing maps)

at your next 4 wheel drive show go to the 12 volt stand and they will have options I did not look to much this year but tere has been a thred in the last two days look back and they may be able to steer you in the right direction

Cant Bl**dy Sitstill
AnswerID: 48898

Reply By: joc45 - Tuesday, Mar 02, 2004 at 17:47

Tuesday, Mar 02, 2004 at 17:47
You can change the power saving settings, usually under the Control Panel, "Display". There are usually two groups of settings, one for internal battery and the other for external power (which is what you will be running off). Things like display and hard drive can be set to shut down after a certain time, just leaving the processor running. Depending on the type of laptop, you can also set the processor to go into hibernation, which draws minimal power. Check on your laptop user manual.
Under these conditions, you should be able to get the laptop to drop from (say) 3A down to less than 1A. A lot will depend on what sort of processor the laptop uses. Suggest using a DC converter rather than a 240v inverter, as the 240v kind can have a higher standing current and less overall effiency end-to-end. Dick Smiths and Jaycar sell these converters, output selectable from about 12v to 24v.
AnswerID: 48899

Follow Up By: joc45 - Tuesday, Mar 02, 2004 at 20:57

Tuesday, Mar 02, 2004 at 20:57
OK, here goes; Just did some checks on my old HP Inspiron 2100 (Pentium 233MMx). This computer uses a 19v 3.1A mains power pack.
In the vehicle, I run it off a DC-DC converter putting out about 19.5v, capable of delivering 3.5A.
With no load, the converter draws 45mA from the 12.7v vehicle supply. Charging the computer battery, computer running, the draw from the 12.7v supply is 4A. Once charged, the results are:
- Hibernate mode: 0.2A
- Running, screen shut down: 0.7A
- Running, screen active: 1.2A
- Running, accessing hard drive: 2.0A
These are all current draw at 12.7v. Different laptops will give different current draw, but this should give you an idea. A modern Pentium-M/Centrino laptop will give much better results.
You'll note that the converter draws bugger-all with no load. Running a combined 12/240v inverter and a power pack will not come within cooee of this efficiency. (I could run some tests, but I don't see the point)
Hope this info is of use to you.
FollowupID: 310752

Reply By: Member - KG (QLD) - Tuesday, Mar 02, 2004 at 21:11

Tuesday, Mar 02, 2004 at 21:11
thanks for the replies guys...

The lappie is an old toshie piii (hence why im not so worried about leaving it in the heat, etc)
BAsed on joc45's measurements... I can guesstimate at about 2amps running... so does that mean a 90amp hour deep cycle batt will give me approx 45hours running?
i know conventional starting batts have a percentage of charge that you shouldn't use... do deep cycle batts have a similar percentage? if so, what is it?

if i bit the bullet and went 2 x deep cycle batts would my little hilux alternator cope?


Are you stoned or stupid?
AnswerID: 48936

Follow Up By: V8troopie - Wednesday, Mar 03, 2004 at 01:18

Wednesday, Mar 03, 2004 at 01:18
you said:
"i know conventional starting batts have a percentage of charge that you shouldn't use... do deep cycle batts have a similar percentage? if so, what is it?"

My answer is yes, deep cycle should, as a rule, never be discharged below 50%, they last much longer still if not discharged below 75% or so.
Having said that, if you do discharge them much lower occasionally and IMMEDIATELY recharge them you might get away with it, depending on circumstances and battery brand/ type/ etc.
They are too expensive not to take care of 'em, flattening them too often shortens their useful life drastically.
FollowupID: 310797

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