Calculating power needed for laptop operating off a 12V battery

[Circuitous3]

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.

[BillyO]

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 about 40 30 minutes.

[BradC]

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.

[BradC]

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.

I'm sorry, you appear to be an order of magnitude out there.

[BillyO]

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.

I'm sorry, you appear to be an order of magnitude out there.

You are correct!.

[BeBuLamar]

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.

[mariush]

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 management

If 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.pdf

You 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 see 26.41 in 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

[jmelson]

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

[BeBuLamar]

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.

[Siwastaja]

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.

Cheap-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.

[nightfire]

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.

[amyk]

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.

A very wide 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.

[IanB]

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.

[IanB]

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.

[Whales]

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 patterns
3. 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.

[Circuitous3]

Have you measured the actual power consumption of your laptop?  What is the efficiency of the inverter?  You need those two numbers.

No, I haven't yet measured the actual power consumption of the laptop, but based on the values given by others, 30 W seems to be a reasonable estimate. The power inverter efficiency is listed at about 83%.  Thank you, your calculation makes sense and gives me an idea of the battery time I would have.

[Circuitous3]

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.

Does 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.

[Circuitous3]

If 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.pdf

You 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 see 26.41 in 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

No, 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. 

[mariush]

Or look into one of those power banks which have inverter and/or  12v / 24v outputs

If 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 lifetime
I'm talking about devices like these: https://www.amazon.com/Jackery-Portable-Explorer-Generator-Optional/dp/B082TMBYR6

[Circuitous3]

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

Yes, 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.

[Circuitous3]

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.

That's a nice idea! I'm not sure how I would go about the build, but maybe I'll look into that.

[Circuitous3]

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.

Are 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

[BradC]

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.

[Circuitous3]

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.

Thank 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.

[Berni]

Are 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

Yep 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.