How Long Can You Power a Laptop from a Car Battery

radderfire · 24 Jul 2010 at 18:33

I'm thinking of buying a power adapter so I can power my laptop from my car's cigarette lighter. How long could you power a laptop from the car battery before risking flattening the battery?

Thanks

PhillyDee · 24 Jul 2010 at 18:37

Depends on the laptop, the power draw on the adapter, the size of the inverter / adapter you will use, if the battery on the laptop is charged, and what you are doing at the time.

I would not risk it on my cars battery! Only if the engine was running. Or you had a backup.

radderfire · 24 Jul 2010 at 18:42

I have a Dell Inspiron 1545 laptop. The power adapter for it is Model LA65NS2-00, which says output is 19.5V (then two lines the top one solid, the bottom one dashed, not sure what that means) then says 3.34A.

So, you reckon it's just a matter of hours before I drain the car battery?

Rgds

PhillyDee · 24 Jul 2010 at 19:14

By rights, a 12v source of 1 amp will use 1ah of the battery. So a 50ah battery would last 50 hours at 1amp.

But, there is a big but, losses are incurred of at least 20% every time you pass it through a transformer or change the state of the power.

How low can you run a battery before the car will not start? I have no idea!

Anything I can do, would be guesswork

. Theres bound to be a site out there that works it out for you.

radderfire · 24 Jul 2010 at 19:19

OK, thanks for your reply. If you were going to use your laptop in your car regularly, say for three to four hours a day, then do you think just buying a second high performance battery would be a better solution? I'd probably rather do that than risk a flat car battery.

Rgds

PhillyDee · 24 Jul 2010 at 20:18

You would want a lesiure battery in that case, and it would need charging every night.

Tbh, I can still get around 3 hours or so out of my Delll Inspiron 6400 laptop and that battery is over 2 years old now! You might be better off getting a high capacity battery for the laptop.

vsmora · 27 Jul 2010 at 08:24

For occasional use, a 2nd battery is the way to go. If you're going to be doing this regularly, then charging a 2nd battery every day may become quite a chore, especially if you have to charge both in the laptop one after the other.

As an alternative that does use your car battery, I'd suggest starting with a cheapo £5 car battery monitor which will let you keep an eye on the voltage drop, which correlates to the charge level (12.6V = full, 11.8V = empty). I'd expect a midrange 60Ah car battery to run a 65W laptop for approx 5hrs safely but your erm, mileage

may vary. You could then consider uprating your car battery if necessary, because deeply discharging a lead-acid battery will shorten its lifespan. If you don't want to have to keep an eye on a battery meter, checkout http://www.batterybrain.co.uk/index.html which cuts power before the battery gets too low.

Whatever the case make sure you are using a decent car laptop charger, as using your mains one with an inverter will drain things much faster because they are quite inefficent.

If it's for work and your employer is paying, then the best solution is to simply get a laptop that can run for as long as you need from a single charge. Either an N450 netbook or a more powerful biz grade laptop (e.g. Dell Latitude) with dual simultaneous batteries.

Meaker · 27 Jul 2010 at 10:11

Car battieries are not designed to be fully discharged.

You'll wreck it pretty quick.

manisinghb · 27 Jul 2010 at 11:12

I'm a part-time mechanic, and I'll tell you right now, that car battery is going to dieeeeeeee! You'll be able to get something out of it, but it's a quick sure fire way to kill that battery, and the ironic thing will be that with your car not starting, at least you'll have the laptop with you to call for help

But seriously, PhillyDee makes a good point, you're better off getting a 2nd higher performance laptop battery, sure it will become a chore having to charge it every night, but better than a flat car battery by far I'm sure!

Stickman · 27 Jul 2010 at 11:54

Providing you have a decent alternator you should be able to keep the car battery topped up by starting it up for about 15 mins per hour so the alternator can put some juice back in.

I used to run a 50W ice box on my car's battery (so about 4 amps) and it used to work for about 3-4 hours before it had trouble starting. To make it last longer I bought a 40W solar panel (were on special offer) and mounted that in one of the rear windows... just connected it straight to the battery to keep it trickle-charged, and the hotter the sun got, the cooler my ice-creams got.

If you're worried about how much power is left in your battery just connect a multimeter to a cigaratte lighter attachnmeent and watch that it doesn't drop too low.

edit: then again it depends on a lot of things: how much welly it takes to start the engine, how good the battery is, etc... so these times will be different for your car. Mine was a small new car with a big alternator.

mast3r · 27 Jul 2010 at 12:02

will the car be running?

Stickman · 27 Jul 2010 at 12:12

Just noticed that you mentioned that the laptop needs 19.5v 3.34 amps (which is over 60 watts) so... probably best to go the spare battery route after all. The line and dashed line on your adaptor usually just mean it gives out Direct Current (DC) electricity, and your car battery will only put out up to 13 volts or so when full... so you'd need to convert, and then come the afforementioned losses... yeah, ignore my previous post... get a spare battery or two.

radderfire · 27 Jul 2010 at 23:10

mast3r said:
will the car be running?

No, it won't be.

Thanks for the comments guys, I've ordered Dell's 9 cell battery which is a better spec than the current laptop battery and should last 6 to 7 hours in theory. It wasn't cheap though (£126!!) ... maybe I should have bought a little netbook instead but it's too late now!!!

Rgds