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.
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.
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.
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.
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.
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.
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
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
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.
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.
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.
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