The question is how long will it run for assuming full charge in the 12V battery

laptop with a 100-240V/ 1.5A/50-60Hz 90W power supply plugged into a 300W inverter attached to a 12V 40Ah battery

thanks for your help 🙂

Get a 12v adaptor for the laptop and cut out a whole chunk of inefficiency in the inverter.

Quite. Why convert 12V DC > 230V AC > 15V DC (or whatever the laptop takes)? Far simpler to convert the DC voltage.

How long it will run for isn’t an absolute figure; the power rating of the PSU is a maximum, and no indication of usage. Driving the CPU hard, lots of disk activity, using the optical drive, turning up the screen brightness etc etc will all have a negative effect on the runtime from battery.

Best course of action, of course, is probably a second laptop battery. (-:

As Jam bo says, do away with the DC-AC-DC converter and it will be a lot more effecient, just put an inline fuse in for protection.

It would be about 5-1/3 hours if it is running @ 90w, without an inverter (i think!).

I’m in a similar situation, running 2 wireless routers in a field, done away with the 12v PSU’s and running directly from a battery.

A couple of variables missing. The lappy charger is rated at 90W, but the laptop will probably not take 90W, if the internal battery is fully charged, or the laptop power settings are medium/low. Also, the 12V battery capacity is rated for a 20 hour discharge cycle(assuming lead acid or similar) Also, the inverter will obviously have losses, which may be expressed either as a % of the running load, or a fixed number of watts (or a combination of both)
At best, the Batt can supply 12 X 40 = 480W
Worst case the laptop takes a full 90W
Assume the inverter to be (say)75%efficient,

480/90 = 5.3 hrs X .75 = 4hrs

then derate the battery AH for 4 hr discharge cycle, I’d guess you’d be lucky to get 3.5 hrs (assuming the lappy takes a full 90W, which I doubt). All a bit guessworky I’m afraid.

Plus what the internal battery gives you.
HTH

Get a 12v adaptor for the laptop and cut out a whole chunk of inefficiency in the inverter.

not easily done with the shonky old laptops being used, additionally they aren’t cheap either (if one can be found) and I would still need the battery

Plus what the internal battery gives you.

unfortunately the internal battery is knackered

cheap as chips

eg
http://www.laikeet.com/catalog/product_info.php?language=en&currency=GBP&products_id=1114

If the internal battery is not fitted, then the power taken by the machine will be reduced-maybe considerably. You may be able to assume the power taken is reduced by as much as 50%(from the 90W stated, which should be enough to run the machine and charge the battery from flat)

not easily done with the shonky old laptops being used, additionally they aren’t cheap either (if one can be found) and I would still need the battery

Universal adapters are available at maplin too (cheap as chips again).

have you actually tried it yet? i tried to charge a dewalt 18v battery the same way once and the inverter just beeped and wouldn’t have it.
i’m not sure you can charge 18v from 12volts
i could be wrong

Chuck the laptop away and ride your bike dude 🙂

Enjoy natures vibes man …. 🙂

Chuck the laptop away and ride your bike dude

Oooh, now you’re onto something. Old bike, take the rear wheel off, hook up a belt to a generator, and then into the 12V battery. Or use a dynamo.

i’m not sure you can charge 18v from 12volts

you can but you need to step the voltage up which has all the inefficiencies of an inverter IME.
The one linked above would charge my laptop [80 watt one] but not allow me to use it whilst plugged into that device as it blew the fuse in the charger. I would probably go down the inverter route as both are inefficient but the inverter more reliable. Not all inverters will power a laptop the rated power is for a motor such as a drill. They are much poorer with loads not associated with motors
As a battery is considered flat at half charge [ and the amp drainage increases as the volts reduce]. I would reckon on circa 3 hours but cant be bothered showing my working.
I would get a larger battery as well tbh.
I have lived of 12 volts FWIW.