Calculating power needed for laptop operating off a 12V battery - Page 1
Forum section: electronics / beginners

author: Circuitous3
author_rank: Contributor
timestamp: February 09, 2023, 02:57:15 pm
content: I am trying to calculate the power and current needed to power my laptop off a 12V battery during astronomy sessions. I would attach a Bistek 300W power inverter to the battery to convert DC to AC and plug the laptop into the AC adapter of the power inverter. The battery is an ExpertPower 12v 7ah Rechargeable Sealed Lead Acid Battery. the laptop is a Dell with a standard power adapter. How would I do the calculation to determine the current draw on the battery? I started by wondering what the current through the power inverter would be, but found my multimeter was not happy getting a resistance reading when I attached the meter probes to the leads of the cigarette lighter adapter of the power inverter (with the inverter unplugged and its switched turned on). Thanks for any help with this.

author: BillyO
author_rank: Super Contributor
timestamp: February 09, 2023, 03:07:14 pm
content: Have you measured the actual power consumption of your laptop? What is the efficiency of the inverter? You need those two numbers.Let's say your laptop consumes 100W and your inverter is 75% efficient, then the power needed from the battery will be 100/0.75 = 133W. 133W @ 12V would be 133/12 = 1.11 amps. So your 7Ah battery will run you laptop for 7/1.11 = 6.3 hours or less (sometimes much less depending on the battery).Edit: It's actually 11.1 amps. so you might get about4030 minutes.

author: BradC
author_rank: Super Contributor
timestamp: February 09, 2023, 03:07:24 pm
content: If you want to do it that way the technical term would be "a crapload". The inefficiency in the chain would be horrific.On the other hand, if you picked up an E-bay DC-DC converter to boost the battery to whatever your laptop requires (probably between 15-19V) you'd be a lot better off.Personally, my laptop uses between 12-20W at anywhere between 15 & 20V. I often run it from a 99Wh USB-C battery bank using a dodgy USB-C to Magsafe2 cable. Your SLA has roughly ~84Wh. You need to figure out when fully charged (and therefore not charging the battery) what your laptop draws and work back from there. Mine tells me. Right now I'm drawing 13.7W from the internal battery, so the external bank would buy me roughly 7 hours, plus the 3 hours I have on my 8 year old and nearly stuffed internal battery.If you insist on using the inverter to power the laptop power supply, factor in the inefficiency plus peukerts law and base it on about half the battery capacity as it will hammer your lead acid battery.

author: BradC
author_rank: Super Contributor
timestamp: February 09, 2023, 03:08:54 pm
content: Quote from: BillyO on February 09, 2023, 03:07:14 pmI'm sorry, you appear to be an order of magnitude out there.

author: BillyO
author_rank: Super Contributor
timestamp: February 09, 2023, 03:16:33 pm
content: Quote from: BradC on February 09, 2023, 03:08:54 pmYou are correct!.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 09, 2023, 03:26:49 pm
content: A 12V 7Ah battery wouldn't power the laptop for very long even with 100% efficiency. I have an HP laptop here the rating is 3.3A @ 19.5V and that's 64.35W and a 12V battery @7Ah is only 84Wh so in the ideal situation it only lasts 1 and a half hour.

author: mariush
author_rank: Super Contributor
timestamp: February 09, 2023, 03:27:22 pm
content: Various programs will tell you how much some things consume, if there are sensors for it.For example HWInfo64 will probably be able to tell you the power consumption of the cpu, the video card (if not integrated in cpu), maybe even the monitor power consumption is reported (led driver for the backlight)A laptop would be somewhere in the 30-50 watts.The actual lcd screen anything between 5w or less and maybe 20-30w if you run it at full brightness, ssd is mostly idle (1w or less), processor it depends on how much processing power you need and how you configure it in power managementIf you want extremely accurate results, you could measure it as it comes out the battery (ex solder wires to the battery connector in laptop, solder the wires to the battery contacts, add a multimeter in series to measure current and use another to measure voltage ... then you can multiply voltage with current to get the power consumed at that moment in time.Anyway... a 12v 7Ah battery is quite small, and would most likely keep a laptop consuming 30w working for around 10-15 minutes, maybe even less, because the inverter is not 100% efficient and the adapter of your laptop is also not 100% efficient converting the AC back to DC, so you have two conversions that result in losses.Here's a typical 12v 7Ah datasheet, has some charts on the second page :https://media.digikey.com/pdf/Data%20Sheets/Zeus/PC7-12.pdfYou don't want to let the battery discharge to less than around 10.5v (or around 1.75v per cell) if you want it to charge and discharge for a lot of cycles, so look at those lines (highlighted in the pic below)Look for a value closest to the 30w I went with ... you can see26.41in the 10 minutes column.That's about how much a 7Ah battery would provide 30w constantly under the voltage drops to around 10.5v and you should stop discharging the battery

author: jmelson
author_rank: Super Contributor
timestamp: February 09, 2023, 04:40:46 pm
content: OK you say a Dell laptop. Does that have a 19 V wall supply? I made up an adaptor to run a Dell laptop from a car's ~14V "lighter socket". It worked - sort of - but ran down the internal battery somewhat slower than if just running on the internal battery, but would then give a low battery warning and shut off. So, I got a commercial car adaptor, which worked fine. Most laptops have fairly low power consumption, but the screen backlight and the hard drive are big power consumers.The car adaptors should be pretty efficient, so power in should be close to power out. Maybe add 1-2 W for the adaptor. Do NOT use an inverter to power a 120 V AC power adaptor, the inverters are massively inefficient! Get a car adaptor for the laptop that plugs into the 12 V outlet on a car. Still, your 7 AH battery seems way too small. 7 AH at 12 V is 84 AH, at 50 W the laptop is likely to draw about 4 A from the battery, so you will get quite a bit less than 2 hours of run time.Jon

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 09, 2023, 05:31:14 pm
content: I think if you build a Li-Ion battery pack with 5 cells in series the voltage would be correct without any inverter circuit. Then you can parallel a number of series 5 cell group to get the capacity.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 09, 2023, 05:43:27 pm
content: Quote from: Circuitous3 on February 09, 2023, 02:57:15 pmCheap-ass estimation:Laptop under medium CPU load and medium backlight intensity is maybe 40W. Let's estimate AC adapter efficiency = 80% and power inverter efficiency = 70%. Total consumption from battery will be 40W/(0.8*0.7) = 71W. At 12V, battery current is 71W/12V = 6A.With 7Ah battery, discharge rate is nearly 1C. Lead acid batteries colossally suck at high discharge rates due to Peukert effect, so let's say you have 50% of rated capacity available. Run-time will be thus around 30 minutes.If you must use lead acid batteries, any decrease in load current, or increase in battery capacity pays back more than the numbers suggest. Finally at around C/10 to C/20 discharge rate, you get full rated capacity out.You can reduce the wasted power by replacing the AC adapter - inverter combination by a 12V input laptop adapter, those things exist. I also suggest you measure the actual consumption. Maybe if your CPU load is not that high and you are able to dim down the screen, it might not be nearly that bad.

author: nightfire
author_rank: Frequent Contributor
timestamp: February 09, 2023, 08:21:36 pm
content: As the CPU does have a wide range of power consumption, the only reliable way is to measure the load it impacts running the software it is intended to.Reason: the system throttles it down when there is light load.HP notebooks in the past years with the U-series of CPU typically take less than 30W. The 65 Watt power supplies are usually only used for docking stations etc.Also some tuning can be: remove Wifi module and 2nd memory module, therefore conserving a bit of energy.

author: amyk
author_rank: Super Contributor
timestamp: February 10, 2023, 01:19:16 am
content: Quote from: nightfire on February 09, 2023, 08:21:36 pmAverywide range. The CPU may be drawing <1W when idle, and >50W at full load. If you don't need absolute full-throttle performance when using it, e.g. if you only need it to be displaying some document, you can even use software to lock the CPU to lower speeds. All the calculations above seem to be assuming a pretty high power usage from the laptop, as would be expected if you were playing video or perhaps even gaming....but for a more realistic estimate of how long it'll last, I took a look at random Dell laptop batteries and they ranged from around 20Wh to 92Wh, whereas your SLA has 84Wh. Thus you should get roughly the same battery life with the external battery as you did with the laptop's internal one, including any conversion losses.

author: IanB
author_rank: Super Contributor
timestamp: February 10, 2023, 02:34:13 am
content: The simple and obvious answer that nobody has mentioned is to plug the laptop power supply into a Kill A Watt and observe the power consumption under various laptop usages and states of battery charge. This is the power output that will be required from an inverter if you use one. Then assume the inverter has an efficiency of about 85% and ratio it back to the 12 V battery.But the power requirement is complicated, because the laptop has an internal battery, and it will try to charge that battery if you plug in the power when the battery is not fully charged. So the best option is probably to start out with the internal battery at 100%, run from external power until the external battery runs out, and then run from the laptop battery until that runs out also.Many laptops require a DC input voltage of about 19 V, which happens to be exactly the voltage of a power tool 18-20 V battery pack. So a very efficient option is to dump the lead acid battery and inverter altogether and use a lithium battery pack instead with a direct DC-DC cable. That way you avoid any inefficiencies from voltage conversion.

author: IanB
author_rank: Super Contributor
timestamp: February 10, 2023, 02:37:04 am
content: By the way, how can you do astronomy with a laptop? Astronomers I have been around were aggressively bothered about any ambient light at all, because it would upset their night vision. Any light from a laptop screen would have made them go crazy.

author: Whales
author_rank: Super Contributor
timestamp: February 10, 2023, 03:38:40 am
content: A no-special-equipment needed solution:1. Find the laptop's existing battery capacity in joules or watt-hours (many batteries are labelled with Wh to aid aeroplane travel, others only Ah and you have to multiple by their nominal voltage to get Wh)2. Find how long the laptop lasts on this battery with your usage patterns3. Calculate the average power draw (in watts) using these two numbers.Now you can tell how long your external battery will increase the runtime of your laptop by. For rough estimation purposes: reduce that number by 25% if you are using a DC-DC converter (eg 12V->19V), or reduce it by 50% if you're using an inverter+power brick. Also keep in mind that 100% draining common lead acid batteries is a bad idea.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 04:38:11 am
content: Quote from: BillyO on February 09, 2023, 03:07:14 pm

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 04:53:26 am
content: Quote from: BradC on February 09, 2023, 03:07:24 pmDoes this mean that a DC-DC converter is more efficient than a power inverter? I don't have to use a power inverter. It's just that I have one for those times when I need to use my laptop for work on long drives (I'm not the driver at these times!). I will look into a DC-DC converter.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 05:25:09 am
content: Quote from: mariush on February 09, 2023, 03:27:22 pmNo, I don't have to be that accurate that I would need to solder to the laptop or battery contacts. I could get an estimate for the laptop power consumption from the laptop specs or the manufacturer (and for a rough estimate I would use 30 W). But that raises the question of what an easier way would be to actually measure the power consumption of the laptop. Couldn't I just take current and resistance readings with a multimeter across the laptop battery (fully charged) terminals while the laptop was on?Thanks for the data sheet. Yes, I take your point and am realizing that a 7Ah battery is underpowered for the job at hand. Even more so because the main reason I was planning to use the laptop was to attach a digital camera to it (Svbony 250) which will raise the power requirements a little higher. I see now that I'll have to go with a powerbank or larger capacity battery.

author: mariush
author_rank: Super Contributor
timestamp: February 10, 2023, 05:27:06 am
content: Or look into one of those power banks which have inverter and/or 12v / 24v outputsIf you can find a travel adapter or car adapter that powers the laptop from the cigarette lighter for example, then you could use that with such battery bank to get better efficiency and much longer lifetimeI'm talking about devices like these:https://www.amazon.com/Jackery-Portable-Explorer-Generator-Optional/dp/B082TMBYR6

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 05:51:10 am
content: Quote from: jmelson on February 09, 2023, 04:40:46 pmYes, the laptop adapter specifies a 19.5 V output. The Bistek power inverter is rated at ~83% efficiency. I was originally going to solder a female cigarette lighter adapter to the external battery terminals and plug the male cigarette lighter connector of the power inverter into it. But now I realize I need to use a powerbank and some of them come with a built-in cigarette lighter adapter. The 7 Ah battery powers a Celestron C8 telescope, requiring only about 1 amp, for many hours.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 05:53:13 am
content: Quote from: BeBuLamar on February 09, 2023, 05:31:14 pmThat's a nice idea! I'm not sure how I would go about the build, but maybe I'll look into that.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 06:06:02 am
content: Quote from: Siwastaja on February 09, 2023, 05:43:27 pmAre you suggesting a 12 V input laptop adapter like this?https://www.ebay.com/itm/Nippon-America-DVCS-3500-12VDC-to-15-24VDC-3500mA-Universal-Laptop-Power-Adapter-/362681873610

author: BradC
author_rank: Super Contributor
timestamp: February 10, 2023, 06:10:24 am
content: Ditch the concept of leveraging the SLA. Grab something like this and go go go.https://www.auspowerbanks.com.au/product/dell-87w-pd-laptop-power-bank/Disclaimer : I have 2.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 06:14:32 am
content: Quote from: IanB on February 10, 2023, 02:34:13 amThank you for mentioning the Kill A Watt. I just looked it up. That's a great idea! I assume the lithium battery pack is similar to some of the powerbanks and would be rated for higher amp hours.

author: Berni
author_rank: Super Contributor
timestamp: February 10, 2023, 06:21:53 am
content: Quote from: Circuitous3 on February 10, 2023, 06:06:02 amYep this is exactly what you should be using.This way you can either plug into a vehicle or just bring a 12V battery and directly connect it to the terminals by wrapping wire around the contacts.As for estimating the power use of the laptop, that depends a lot on the laptop and even more on what you are going to be doing with it. Just sitting idle on a desktop and baclkight turned down way low you can get down to only 10W. On the other extreme a high performance laptop actively doing heavy number crunching on all cores simultaneously might use >150W of power.The better way to calculate this is in terms of energy in Wh. You can check the specs for your laptop to see how many Wh of every your laptops battery can hold. You can then see how many hours the laptop lasts on its own battery at the workload you want. If you divide the Wh by the hours you ran you will get the average Watts that the laptop used. At the same time you can calculate the Wh of your battery. If its 12V at 7Ah that is 84Wh. This is about twice the size of a typical laptop batery so it should add about +200% to your total battery life.

author: MrAl
author_rank: Super Contributor
timestamp: February 10, 2023, 06:23:02 am
content: Quote from: Circuitous3 on February 09, 2023, 02:57:15 pmHi,Some good advice already in this thread i'll just add a little more...First, nothing has been said about the life of the lead acid battery so i'll mention that first.For these kinds of batteries you have to be careful how much you discharge it. If you discharge it too low it will become useless quite fast. That's the way many lead acid batteries work. For applications like this maybe a deep discharge battery would fair better.That said, i feel you are really following in the footsteps of the proverbial, "beating a dead horse with a stick". That's because there will be more modern solutions now. For one thing, there are probably lower power laptops available that will run much longer with a full charge. That will mean longer run time right out of the box. Second, there are mini computers out now that only use about 10 watts during the running time even with some software that uses some cpu power. This is the basic box only (about 6 x 6 x 4 inches total size, plus wall wart). You'd also have to get a small HDMI display panel and keyboard. Hard disk would be from 64GB up to around 512GB depending on the model. Total power could be under 20 watts, and some can run DIRECTLY from a 12 volt DC power supply such as a battery.Now with that said too, back to the run time with the 12v 7 AHr battery on the laptop...As another cute little proverb goes, "You're going to need a bigger battery".In fact, you should probably go for a Li-ion battery not lead acid. Lead acid is too temperamental. Li-ion is much more modern and can be recharged a LOT of times. They come in 12v versions too that look just like a 12v lead acid battery. To charge you get a charger that can handle those kinds of batteries. What is also nice is you can get higher capacity with lower weight. You can start at 20AHr and go all the way up to 100AHr or more depending on your weight limitations. They are more expensive, but will last much, much longer than the lead acid counterpart. You could check Amazon to get an idea how much it would cost.Note that with a 20AHr Li-ion battery and the 12 volt mini computer solution you could get a run time of something like 10 hours, and part of the reason for that is there would be NO converter circuits between your battery and your computer.Good luck with whatever you decide to do, and maybe you can get back here and tell us what you ended up doing and how well it is working out.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 06:29:27 am
content: Quote from: IanB on February 10, 2023, 02:37:04 amI am mainly a visual observer; so far, no astrophotography for me. But recently an amateur astronomer friend gave me a digital camera (Svbony 250, an electronic eyepiece) to use to take short exposure pictures of bright objects (she's tryting to convert me over to the dark side!). The camera plugs into the laptop for capturing the images and seeing them in real time. You are quite right about astronomers shunning ambient light. I'm one of them. However, some use imaging devices which project the images onto the laptop screen. They keep the brightness settings down and keep their distance from other observers. They can observe their images in real time and save them to the laptop. But because they are not looking through the eyepiece (their cameras are) they don't have to worry about their dark adaptation. Technology is changing everything, and astronomy is no exception.

author: IanB
author_rank: Super Contributor
timestamp: February 10, 2023, 07:17:41 am
content: Quote from: Circuitous3 on February 10, 2023, 06:14:32 amI experimented with a Ryobi tool battery, since I have some Ryobi cordless tools. A 4 Ah battery is about 75 Wh if you assume 18.5 V x 4 Ah. You can get adapters that plug on the top of the battery to use them for other purposes. For example, I looked at running a low voltage lamp in case of power outages, like this:The cable is feeding the ~20 V DC from the battery direct to the bulb without any converters in the chain. The bulb has some internal electronics to drive the LEDs.

author: james_s
author_rank: Super Contributor
timestamp: February 10, 2023, 07:25:52 am
content: You mention trying to take a reading of resistance, that doesn't make any sense, did you mean current? I can't think of anything related to this where you'd be measuring resistance.Keep in mind that the 7Ah rating of those SLA batteries is at a discharge rate of something like 1A, if you draw more current the Ah capacity will decrease pretty significantly. Your best bet is probably to use something like a Kill A Watt to measure what the laptop draws through the charger and then figure around 80% efficient for the inverter. It wouldn't surprise me if the battery pack in the laptop has a higher Wh capacity than a 12V 7Ah SLA battery (84Wh).

author: Siwastaja
author_rank: Super Contributor
timestamp: February 10, 2023, 08:28:07 am
content: Quote from: Circuitous3 on February 10, 2023, 06:06:02 amYes, something like that; I don't know about the quality of that particular brand. It's a simple boost converter and even if it's crap, it's still at very least 80% efficient, possibly more. Way better than the combination of inverter + ac power supply.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 05:27:52 pm
content: Quote from: MrAl on February 10, 2023, 06:23:02 amYes, thanks for the good advice and information. I'm actually due for a laptop upgrade anyway. And the lithium-ion battery sounds like a good way to go. I only started thinking about how to power a laptop in the field because of the digital camera I was given for the telescope. All the good information in the thread will help me figure out a good way to go. I may just go with a temporary solution for now, to see how much use of the camera I will get. But I've also been wanting to learn more about electronics and studying it recently. I'm finally making a dew heater system for my telescopes, something I've been thinking about for a number of years. The heater controller has 4 pulse width modulators controlling the voltage to each of 4 output channels. The heater elements plug into each channel and warm different accessories on the telescope to prevent dew formation (on the eyepiece or finder for instance). The current for each channel is only about 0.2 amp, so that 7 Ah SLA battery should power the elements for many hours.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 05:30:18 pm
content: Very nice Ian! You'll be all set for the next power outage.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 05:41:57 pm
content: Quote from: james_s on February 10, 2023, 07:25:52 amWell, I was thinking that to get a sense of the power requirement of the power inverter-laptop system I should try to get a measure of resistance. Then I could easily calculate current give a 12 V supply. Then I wondered what the resistance of the power inverter would be, so tried to get a measure of that. But, I realize I would need to know the resistance of the inverter-laptop combination. I suppose I could connect the inverter-laptop to the 12 V battery and then measured the current across the battery terminals, but I wasn't set up to make those connections, so was only thinking in the abstract. Was I thinking about this the wrong way? Having followed the thread, I realize now there are easier ways of meaasuring the power, such as with the Kill A Watt (which I would need to get).

author: wasedadoc
author_rank: Super Contributor
timestamp: February 10, 2023, 06:25:58 pm
content: Quote from: Circuitous3 on February 10, 2023, 05:41:57 pmMeasuring the resistance of an inverter input using only an ordinary multimeter does not give any meaningful information about how much power it takes when in normal operation.

author: james_s
author_rank: Super Contributor
timestamp: February 10, 2023, 06:36:22 pm
content: Quote from: Circuitous3 on February 10, 2023, 05:41:57 pmMeasuring resistance only works for a purely resistive load such as a resistor or heating element. The inverter is an active circuit, the power it draws will have no relation to whatever resistance you measure across the input circuit. It doesn't even make sense to talk about the "resistance" of an active circuit. You have to measure the current it draws while it is operating, which will vary depending on the load you apply to the output of the inverter. The power the laptop draws will vary widely too depending on state of charge of the internal battery and how hard the CPU is working.

author: Circuitous3
author_rank: Contributor
timestamp: February 10, 2023, 07:26:25 pm
content: Quote from: james_s on February 10, 2023, 06:36:22 pmI'm confused about why the power drawn by an active circuit has no relation to the resistance of that circuit. Just as I can measure the current through a circuit (say the inverter-laptop system) to get a measure of the power it uses, can't I also get the resistance of that circuit (since we know the voltage, say 12 V)? And can't I also measure the resistance of that circuit to get the current that it should draw?

author: james_s
author_rank: Super Contributor
timestamp: February 10, 2023, 10:39:37 pm
content: Quote from: Circuitous3 on February 10, 2023, 07:26:25 pmBecause when you measure resistance, that is done by passing a very small current through the circuit from a low voltage. When you do this with an active circuit, the circuit will not be operating, when it is not operating what are you going to measure? Resistance is for measuring resistors, a switching power supply is not a resistor, it's as simple as that. If you measure the power draw you can work backwards and come up with an equivalent resistance that will draw the same amount of current but it still doesn't make sense to talk about the resistance of an active circuit. Something else you could read up on is impedance, with DC impedance and resistance are the same thing, but with AC they are wildly different, the concept is related.For a very simple example take the humble incandescent light bulb. Technically it's a purely resistive load, however the properties of a tungsten filament are such that the resistance when cold is dramatically lower than at operating temperature. If you measure the resistance of a bulb you will be measuring it under conditions far outside that in which it operates so the resistance value you measure will be perhaps 10% of what it will be at operating temperature and your calculated power will be off by a factor of 10. To take a meaningful measurement of resistance, you have to do so under operating conditions.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 10, 2023, 11:01:39 pm
content: Quote from: james_s on February 10, 2023, 10:39:37 pmI addition to what you said. If you measure the voltage supplying to an inverter and the current that it's drawing you will have a wildely varying current draw depending on the load connected to the inverter. So there is no way you can figure out how much current the inverter will draw by trying to measure its resistance.