Practical advice about a car battery 12VDC to 110VAC inverter typical length of time to last without electricity

Forget your car battery idea. Get a UPS:

Typical UPS is designed to run half rated power output for up to 20 minutes - and many will NOT start without having power input (they are made to KEEP the computer running untill you finish a task and shut it down properly. Pretty useless for what the OP wants - - --

On Tue, 3 Jan 2023 16:17:26 -0700, John Robertson snipped-for-privacy@flippers.com wrote as underneath :

The uPS idea is rubbish unless you spent a fortune on a UPS with a massive battery - not practical! The car battery would run down quite quickly - inverters take a large current - you could get an inverter capable of n laptops but you would have to run the car engine to charge itsself by measurement, guess at every 30 mins for 15 mins while charging (until V back up to full charge say 13.?V. Otherwise a more versatile option is to have a small petrol generator which has enough power to run essentials, CH. router. a few LED lamps, and use it sensibly, not on all the time etc. in 3 days you might freeze, depending! C+

A car or suv battery has about 40 to 60 amp hours. To charge the above 3.8 amp PC for an hour, which probably gets it to near full, would take 3.8 amp hours, but that's at 20V, not 12. So adjusting for that, you'd need 5.5 amp hours at 12V. Then the inverter and PC charger have losses too, so figure maybe 7 amp hours as an estimate. So you could charge that PC about 6 to 8 times. That's my math. Next problem is that car batteries are for starting, not deep cycling, so a car battery isn't going to survive many cycles. If you want the right battery, look for a deep cycle battery. As to the inverter size, the larger one from above, output is 20 x 3.8 = 76W, input is 1.9 x 100 = 190W,. That doesn't really add up, but I suppose it might draw the higher when starting. But I'm sure you could charge one at a time with a 200W. Another option would be to get a car charger for the PC and use that instead of an inverter, though with the inverter you could use it for other small AC loads.

I'm an old man. Everything with a battery will die. In about five years. Worse, everything with a rechargeable battery needs to be recharged.

I always tell my friends who buy those "jumper" batteries that in five years they'll be calling me to drive over to their house to jump them.

Invariably the thing is dead when you need it five years from now.

The beauty of something that alligator clamps onto a car battery is you always have a car in your garage. And that car always has a battery.

That car battery is always recharged. And it's always replaced about every five years or so.

Even better, for the price of the core charge, the next time you replace that car battery, you can just not bring the old one back.

Now you have a spare car battery (albeit it's a bit worn out). While it may be too worn out to reliably start a car, it's not too work out to run my CPAP machine.

And, I hope, it's not too worn out to run the inverter. If it is too worn out to run the inverter, then I still have the battery in the car in the garage.

Which is why I'm asking whether or not a car battery will charge laptops.

1. What inverter size is needed to adequately charge a typical laptop?
2. How long will it take for a typical car battery to die from doing that?

If that's what you want it's called a 'converter' to change

An 'inverter' changes current between AC and DC which is not ideally what you need (multiplies transformer losses).

RV sales/service outlets have 12vDC to 110vAC inverters if you want to do it that way.

I never knew that a "converter" existed until this very moment!

I agree with you that a converter plus the special plug should work.

One problem with any given converter is you never know how much DC voltage you'll need for any given laptop as it changes from laptop to laptop (I'd rather make a long term investment instead of a short terms purchase).

If I can find a converter which is adjustable to any voltage below around

Here's someplace to start:

If the battery isn't worthy of being in the car, I sure would not be relying on it to run a CPAP machine in a power outage. Once they start to go, it's an accelerating process. Plus auto batteries are for starting and not designed for deep cycling, which you now want an old battery to do. Even a new auto battery won't last long deep cycling it and you won't find out how bad it is until you need it to work and it doesn't. If you're going to do this, I would buy a deep cycle battery.

CPAP is 12VDC at 1amp. Starting a car is something like 80 amps. Big difference.

Runs CPAP for multiple nights.

I doubt 1 amp over 12 hours is "deep cycling" the battery, is it?

It's only 1 amp. Anyway, this isn't about CPAP.

It's about how much power an inverter needs from a car battery to charge a laptop.

They've been around a long time although you might not realize it. The old tube type car radios had a vibrator, a transformer, and a rectifier to get the 6 or 12 volts up to a level required by the tubes.

What is the current requirement of the laptop charger? Mine is a 45 watt charger. It is rated at 1.4 amps at 120 volts. If it takes 8 hours to charge the battery that is 11.2 amp hours - maximum. Since the current tapers with the state of charge, more likely closer to 8 amp hours (the equivalent of a small UPS battery capacity) Current higher end inverters are something approaching 80%? efficiency - the cheaper stiff better than 70%? but you are still farher ahead to get the auto/airplane adapter for the laptop so you are only converting once instead of twice.

see

Typical laptop charger is 45 watts - some older ones 60 to 65.

They make universal laptop power supplies.

Mr Bowman's right:

Make up your mind. You started off saying you wanted this setup to charge a couple of laptops. Then you threw the CPAP into the mix. So it sure looks like you're going to try to do more than just charge a CPAP with an old battery that came out of a car because it's dying. Since you already have experience with how long things can run on a battery, why are you asking?

While the battery can run CPAP for days, the question isn't about CPAP but about how much juice is needed to charge a laptop & can a battery handle it.

I only mentioned CPAP when I said a worn out battery might not reliably start a car but it reliably runs a 12VDC 1 amp-hour CPAP so I was asking if the 150W/200W/300W/500W inverters (which I do NOT have) can ALSO be reliably run to charge typical laptop batteries.

While I have experience with CPAP (which I have), I'm asking if an inverter (that I don't have), will charge laptops on a car battery and if so, which inverter is the right wattage to charge typical laptops.

Yup, all the rage back in the 50s, just don't use one too long when the car is parked with the engine off. They drag down a heap of current. Can still recall the background hum of the things, back when I could hear such things. ;-)

Ya ain't listn'n lad. You've had all the answers spoon fed to you but your brain isn't engaged.