How long will a 12v lead battery run a laptop?

Slow charging is better for the battery, but it’s not worth worrying about.
Li-Po batteries start degrading the moment they are made. After 5 years or so, they are at the end of their life, regardless of how well you treated them.

You can use an appropriately rated DC boost converter. Of course they are not 100% efficient.

Lots of modern laptops can charge off their USB-C port. Since you got this laptop recently, you may be able to go from your 12-volt --> Cigarette lighter port --> USB converter. I don’t know if there are any 12-V->USB converters that can output enough to power the laptop, though.

Aside from studying Electrical Engineering in college, decades ago, I’ve been looking at this crap for a long, long time. I have to admit … I had no idea that this was the case !

I wonder how many others – ‘well-schooled’ in this stuff – also had no idea.

Many thanks for enlightening me. Seriously.

This gadget says it will deliver 60W from its USB-C port, which doesn’t sound too bad, I doubt she will be using even that much with this laptop. Maybe you can find one rated for 100 W.

Thanks for the information, very clear and understandable!
Only, er… do you really use Lithium-Polonium batteries in the USA? I am impressed. They would scare the hell out of me, I must admit.

Tons of energy though.

But, no:

Thanks again, that is very reassuring :wink:

I didn’t know this either. Thanks for sharing!

Does this mean, then, that:

  1. For the purposes of charging a laptop, relative to car’s battery max capacity, you can expect even more amp-hours than rated because it would discharge the car battery very slowly – over several days, just a few hours at a time?

  2. That Ah is still a useful comparison between car batteries of the same type? If so, is there a clear way to identify the “type” of car battery you’re looking at, in terms of whether it’s a “typical car battery” vs “deep cycle” or anything else? Are there regulations/ratings standards for this sort of thing if your overall goal is to compare apples to apples in terms of usable watt-hours for non-automotive applications?

I also wonder how big a difference this makes. Is it a 5-10% thing, just one of many small system losses in efficiency? Or can be a much bigger issue?

You can sometimes also find aftermarket DC to DC adapters for your laptop. It’s a cigarette lighter penis on one end and whatever kinda port goes to your laptop on the other end. Something like this (never tried that particular one).

USB-C just makes this easier since it’s a widespread standard, but otherwise you just gotta match the voltage requirements and tip of your laptop (look at the DC output specs of your AC adapter, or look on the bottom of the laptop). If the amperage is a little lower it might still charge, would just take longer. Try not to pull more than 80W or so out of your cigarette lighter or you risk tripping the fuse (no big deal, just annoying to have to replace during a drive, especially if it takes out your radio too).

If you need to charge a laptop or bigger item on the regular, they also make these direct-to-battery DC chargers that have terminals that hook up directly to the car battery, bypassing the cigarette lighter altogether. One would hope it has some sort of built in intelligence for safety and such, but who knows…

The rated amps are about the best a lead acid battery can do.

Look at this Typical battery datasheet for example (and it answers your second question too, I hope): https://www.powerstream.com/bb/bp4.5-12.pdf

Nominal Capacity(AH)
20 hour rate F.V.(1.75V/cell) (225mA to 10.50volts) …4.5A.H.
10 hour rate F.V.(1.75V/cell) (428mA to 10.50volts) …4.28 A.H.
5 hour rate F.V.(1.75V/cell) (765mA to 10.50volts) …3.83 A.H.
1 hour rate F.V.(1.55V/cell) (2700mA to 9.30volts)…2.7 A.H.

Now look at the curve at the top left corner on page 2, you’ll see that the A.H. Asymptomatically approach the rated value as the discharge rate is reduced

Update
A friend offered as a christmas present a solar kit, SO I have this one coming to me
https://www.northerntool.com/shop/tools/product_200773337_200773337

Maybe, I should make a new thread…but I’d like suggestions for a battery and a cigarette lighter type plug that will work with this set up. I just bought a cigarette lighter plug for my computer to charge it off of DC.

hmm…maybe I’ll start a IMHO thread

That kit already comes with a charge controller, if you want to use it with a car battery. It also comes with an inverter so you can plug the laptop AC adapter into that if you want. If you want DC to DC, you’ll need some sort of regulated voltage converter to go from 12V to ~19.5V.

Or if you want to get one of those integrated Jackery or Goal Zero units, I BELIEVE (not 100% sure) you can use one of these SAE to male 8mm adapters, paying attention to the polarity to ensure power flows the right way from panel to power pack. According to one review on that page, you might have to shave off a bit of plastic to get the 8mm end to fit into a Goal Zero, but electrically it seems compatible.

The Goal Zero and Jackery units have built-in charge controllers and regulated outputs, so you won’t need to use anything else in that kit that you got. It would go solar panel --> (optional polarity reverser) --> SAE to 8mm adapter --> power pack --> laptop via DC cigarette lighter adapter.