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

I’m considering a solar system to work on the road with my laptop. THe laptop is rated at 65 watts. I know 12v lead acid batteries come in many flavors, but lets just pick a more or less standard 44Ah, 720Amp battery.

How long could I expect such a battery to run my laptop? Assume I have all the correct and necessary inverter, charge controller, solar panel to charge the battery completely, etc.

I essentially would like to run my work computer off of the grid for as long as I can without recharging but don’t know if a standard car battery will suffice.

It depends on what you use it for. Eg if your average power consumption is 50 watts, then (as a rough first approximation) 44 A h x 12 V / 50 W = 10 1/2 hours or so. So you have a good chance of having power until the sun comes back up, especially if you charge your laptop battery as well.

Couple things to keep in mind:

• Inverters tend to have their own power consumption, so you might have to spend 10-20% more wattage to run your 110V laptop via a 12V battery using an inverter;
• If you’re talking about your garden-variety lead-acid car battery, they tend to be optimal for large amperage surges , delivered infrequently, and tend to do worse with low-current draws that slowly discharge the battery. They’re also pretty cranky [NPI] about being fully discharged.

If you’re talking about a car battery that’s just gathering dust anyway, then … no big deal … but if you have to buy one … you may want to look into a deep-cycle battery instead:

How long will your laptop run on it’s internal battery?

just did that experiment yesterday
it ran 3 hours 15 minutes with 15% or 30 min left before I plugged it into a battery pack I have. It took 2 hours to charge back to 97% (all the while I was using it). I currently am using it for the last hour and half on the first recharge and it claims to have just under 2 hours left.

So, I’ve gotten 6.5 hours out of it, and should get another 1.5 hours more before I have to recharge it with my battery pack. I don’t know how many times I can recharge it out of the battery pack before that one goes dead.

I was just thinking that you really want to avoid multiple voltage conversions.
So, if you could just charge your laptop directly off of the Solar panel, that would be significantly more efficient than charging a battery, inverting to AC, and charging the laptop from AC.

What laptop is this? Based on your measurements, it sounds pretty old.

I would prefer to charge the laptop off of the solar panel, but if my laptop lasts 3 hours during normal use, I’d run into problem during the night. So I have to store electricity with a battery.
The plan would be to get enough sun energy during the day, to allow me not to have to plug into an electrical outlet. I can get solar panels sufficient to charge a battery, I just don’t know if a typical lead acid deep cycle battery will last long enough.

I expect I can charge my laptop during the day while I’m using it, then as I lose solar power, it will drag off its battery. But I’m expecting cloudy days and night work will require me to have a battery.

Hence the question, How long will a lead acid battery last? THis will provide insight on how long I can go without sunlight to keep my computer operational.

Its an HP elitebook, I got this past March

@DPRK had the right formula. If your 12V battery is rated at 44 amp-hours then it has the capacity to run a 44 amp device for an hour, a 1 amp device for 44 hours, or any other combination that totals a maximum of 44ah

Your 65 watt laptop will draw about 5.4 amps (65 watts / 12 volts). Assuming 85% efficiency of your inverter, your 12V battery will power your laptop for about 7hrs, at which point your laptop’s internal battery will take over.

RV’ers and van-dwellers sometimes increase their usable battery capacity by buying two deep cycle 6V golf cart batteries and hooking them up in series, or you can buy larger (and higher amp-hour capacity) 12V deep cycle batteries that will give you more time.

Note that devices are rated based on their maximum power consumption, not their average, and computers can vary wildly in how much they use, depending on what you’re doing with them. So your average consumption might be much less than the 65 watts it’s rated for.

One more thing … at the risk of stating the obvious:

Whatever electrical load you place (ie, gizmos you run) on your solar power system during the day comes off the top, meaning: only the solar-generated watts/amps left over (after powering anything running off of your solar system) is available to charge your battery.

Reasonably anticipating your usage requirements (and charging hours (ie, sun) per day) will help you size all the components of this kind of system.

Best of luck !

Just to keep track of data here are my results:
I ran off my laptop until it got to about 15% (30 min left)
recharged it for 2 hours with my battery pack while running the computer.
Used the computer for another 3 hours, then plugged back into the battery pack.
after 1 hour of charging the battery pack gave out (laptop charged to 83%)
Ran the computer for 2 more hours and could probably get one more hour out of it.

So, with my current setup using my battery pack, I can go 11 hours and probably 12 hours easily before I run completely out. It does take 24 hours to recharge my battery pack so I think I do need a solar system and deep cycle battery.

Some thoughts…

• GoalZero makes a line of “Yeti” battery packs for just this purpose. I think some of the older ones use lead-acid car batteries, but the newer ones use lithium ion, which has a higher depth-of-discharge (you can drain them much more, even more than deep-cycles, without causing permanent damage) but possibly fewer lifetime cycles (they would only last a couple years, maybe 500 complete discharges, or so before needing to be replaced). Another advantage is that lithium ion is much more energy-dense, so you can get the same capacity in a more compact package, if by “on the road” you mean “in a car/RV with limited space”. And while you can definitely DIY this cheaper, GoalZero puts the battery pack, charge controller, inverter, USB ports, etc. all in one easy package so you’re not left with a rat’s nest of cables.

• If you don’t NEED a laptop, you might consider an iPad with keyboard instead. The battery should last much longer, and it’s a lot easier to charge (you can go straight DC to DC from sun to battery to laptop), not losing efficiency to AC and back in between. You can achieve similar outcomes with USB-C laptops, or with special 12V car adapters that are available for some laptops. But usually tablets have more power-efficient processors, at least until Apple releases their “ARM” Macbooks soon.

• You’ll probably need to oversize your solar panels by quite a bit. Even the slightest shade will drastically reduce output (to like 15-20% of max) because of the way most solar cells are wired – the decrease in output is NOT proportional to the amount of shade, and one tiny passing cloud could mean the difference between having a charged battery pack that day or not.

• As a matter of usability, you probably wouldn’t want to charge the laptop directly from the sun.

• One, your laptop’s battery capacity is typically much smaller than that of a car battery (maybe 25Wh for some Elitebooks, up to 60 or so for bigger laptops, whereas your car battery is 528 Wh), so your solar panels would charge it to full pretty quickly and then the rest of the day’s output is wasted. Having a bigger battery pack than your load (your laptop, in this case) means you can take advantage of sunny days and store excess power for extra days of autonomy when the sun isn’t great out.

• Two, lithium ion batteries tend to perform more poorly under high heat, and if it’s an especially hot day, it may not charge at all; you don’t really just want to leave the laptop out in direct sun all day to charge it.

• Three, unless you’re nocturnal, you probably want to work during the day yourself… having a separate battery means the sun can charge that battery wherever, while you’re working inside, and you can charge the laptop from the battery overnight while you sleep.

• Four, most laptops use more power when they’re plugged in, because they think you’re connected to the wall outlet, and so become faster/more powerful while draining more power; in your situation that would be counterproductive.

For what it’s worth, I used to work for a solar company and wanted a similar setup for myself, but after doing the math a few ways, realized that it was simpler to just get an more power-efficient laptop plus a bigger lithium ion battery pack, charging it from the wall (or a car inverter) every night instead of trying to deal with the solar setup under changing conditions every day. In theory it’s nice to work off-grid like this, but in practice it’s hard to generate enough consistent power to be able to work without disruption day after day without a house-sized solar setup. Those portable panels that you might bring with you camping are too small for laptops; in general you’ll need something about a square meter or larger to provide enough power, which either means a big fold-out panel (flexible panels, or briefcase-style foldouts, or RV roof-mounted). If you’re camping anywhere with trees, you will get terrible performance because of the shading mentioned above. So all of that is a lot of hassle vs just plugging the battery pack into your cigarette lighter and letting your gasoline engine charge it; doing so is a small drain on your gas mileage but so much more consistent than solar on the road.

Sounds like a fun project, though. Good luck!

Really good info, really well presented.

Jackery is another name for those self-contained (and can be charged by 12V, 110V, or solar panels) power sources … should you want to go that way.

thanks for this, good thoughts to ponder

I like the idea of charging a battery pack off my vehicle, but there will be days when I don’t drive much at all. I don’t expect it would be worthwhile to run my car for a few hours just to charge the battery pack.

I do like the idea of the Goal Zero route, and maybe I can get a couple of them for the cost of a solar kit and charge them all when I’m driving.

Thanks!

And Jackery looks like a good brand too. Some of them seem a better value than the Goal Zero ones, I think?

Yeah, and there’s nothing stopping ya from getting a solar panel too, to top off whenever you’re not driving that much. Or even microhydro if you stay near streams. The trick is just sizing your battery pack big enough for several days without power, and then bulk filling it up whenever you do have reliable power, be it a long drive, a wall outlet, or a lazy day in the sun. It’s kinda like water or gas cans, you just fill up between excursions instead of trying to refill every single day.

Sounds like a fun adventure One of my favorite wilderness offices:

And then the AC is converted back into DC before going to the computer.

Sounds logical, but that’s not the way lead acid battery manufacturers specify amp-hours

For typical car battery it’s measured over 10 hours. For deep cycle it’s measured over 20 hours.

So if a battery is rated 44 amp-hr over 20 hrs, it’s amp-hr will be significantly less if it is discharged over lesser time.

On a related topic: does anybody know if charging your laptop (or your phone, in my concrete case) slower over the car (phone takes almost two hours until full, as opposite to about one hour on the OEM charger on the socket at home) has any effect on how long the battery lasts? Will slow charging ruin my battery or will it last longer?

I’d add a couple more points.

• If you’re only charging DC items, such as a laptop, no need for an inverter. It is much more efficient to stay in DC and not convert to AC then back to DC again.

• If you use an inverter, you’ll want a pure sine inverter as electronics such as laptops may experience difficulties with cheap modified sine wave inverters.

• The voltage of your laptop battery and system is a very important factor. Remember W=V*A. If it’s higher than 12v, say 18v, it will draw 1.5 times the amperage maintain the proper voltage.

• If you are running the computer while charging the battery, your power consumption may be significantly higher.

How can I stay in DC to run/charge my laptop? Lets assume I have a full lead/acid car battery; the easy way is to connect an inverter to it, then plug my laptop in. Clearly it would be more efficient to run and charge my laptop battery directly in DC from the car battery…
I assume there is an adapter? that runs off the battery to the laptop. Is there? where could I find such a device?