external battery pack for laptop, current limiter - Page 1
Forum section: electronics / beginners

author: purezerg
author_rank: Contributor
timestamp: September 30, 2019, 04:35:38 pm
content: Hi all,I have a external power back that is rated to output 20v @ 7A peakI am trying to figure out a way to limit the laptop from drawing too much currentI have a HP zbook workstation that is rated 200w on the power adapter.however I managed to figure out that the middle pin is 220k ohms to trick it into believing that the DC power coming in is from a 120w power adapter.I have successfully used the 120w adapter on the laptop for hours without problems.the laptop knows it's power limit and adjust accordingly using extra power from the battery when the AC doesnt provide enough when it needs to boost.the question is it's not full proof. I have blew the power bank's 20v output.so the question is how to I limit it to 6a@20vit doesnt always draw 6a. its usually doing 1-2a when idle and 4-6a when average loaded.so how do I build a 20v @6a current limiter.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 30, 2019, 06:22:08 pm
content: Don't brown out your laptop unless you want bad things to happen.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 30, 2019, 06:39:25 pm
content: Change the maximum processor power state in energy management.My Dell does this, very aggressively, when I plug in a power supply that isn't 240 Watt.It goes to around 500 MHz on one core, locks the screen light to almost off and shuts down WiFi. It's very effective in reducing power.Also, when you starve the chipset from power just the slightest, it turns off, no warnings.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: October 01, 2019, 05:07:55 am
content: Quote from: Jeroen3 on September 30, 2019, 06:39:25 pmI had a Dell that did this too well. It couldn't tell what it's own power supply was as it aged. The CPU stayed at min speed and the battery wouldn't charge. Thankfully I don't think they make these types of power supplies anymore.

author: purezerg
author_rank: Contributor
timestamp: October 01, 2019, 11:45:00 am
content: my zbook 15 comes with a 150w adapter. I have tried 120w adapter and 90w adapter.plugging in 150w, the CPU can sustain 4ghz.120w it does 3.6ghz.90w it does 3.6ghz but doesnt charge the battery as fast.150w with full charge battery and idle. the laptop consumes 15w.when running prime95. it does 65wcharging an empty battery when the laptop is shut down, consumes 70wi believe the quaddro on my laptop is 45w itself also.that said. there is a middle pin in the chargers that help the zbook identify200w (untested)150w 120k ohms120w 220k ohms90w 300k ohms65w 380k ohmsI have made a cable with 220k ohms. the power bank is rated for 135-140wso how do i limit the amps to 6a? 20v@6a.

author: purezerg
author_rank: Contributor
timestamp: October 01, 2019, 11:48:26 am
content: Quote from: Jeroen3 on September 30, 2019, 06:39:25 pmyeah i know what you mean. in the bios there is a option that allows the system to use battery as a buffer to turbo boost if required.

author: Zero999
author_rank: Super Contributor
timestamp: October 01, 2019, 02:08:10 pm
content: Quote from: purezerg on October 01, 2019, 11:45:00 amBy applying Ohm's law.P = VIV = 20I = 6P = 20*6 = 120WGoing from your list, you're right in using a 220k resistor.

author: purezerg
author_rank: Contributor
timestamp: October 02, 2019, 08:16:12 am
content: Quotethe problem is that the laptop still draw in excess of 120w even thou it identify it's a "120w"I thought that tricking the laptop to only draw according to 120w was sufficient. but I was wrong. I still need to limit the current draw.so. any efficient ideas on how to go about doing so? maybe even a diagram?

author: Jeroen3
author_rank: Super Contributor
timestamp: October 02, 2019, 08:34:52 am
content: If you limit the current externally, the voltage will plummet. Causing the charger to be disconnected by the BMS.You have to limit the power usage within the laptop itself.

author: purezerg
author_rank: Contributor
timestamp: October 03, 2019, 02:05:57 pm
content: any idea how much the voltage will drop? the powerbank outputs 20v. but the 2 official HP charger i tested was 19.5 and 19.2v

author: ledtester
author_rank: Super Contributor
timestamp: October 03, 2019, 03:01:22 pm
content: A current limiting circuit will drop the voltage all the way to zero if necessary.Current limiting isn't going to work. The laptop draws more current because it needs more power, but current limiting will reduce the output voltage which reduces the power supplied to the laptop. The laptop will respond by trying to draw more current resulting in even a further drop in voltage.As Jeroen3 said you need to limit the power within the laptop itself.

author: soldar
author_rank: Super Contributor
timestamp: October 03, 2019, 05:54:11 pm
content: It is probable that it is not just the value of the resistor but that communication is going on through that pin. Have you checked with a scope to see if there is any signalling going on? Specially at startup?https://kakopa.webcindario.com/Dell_PS/index.htmlhttp://www.laptop-junction.com/toast/content/dell-ac-power-adapter-not-recognizedhttp://www.laptop-junction.com/toast/content/inside-dell-ac-power-adapter-mystery-revealedhttp://www.laptop-junction.com/toast/content/dell-ac-power-adapter-id-chip-diedhttp://www.laptop-junction.com/toast/content/dell-battery-charging-system-d5xx-and-d6xx-laptops

author: purezerg
author_rank: Contributor
timestamp: October 03, 2019, 06:56:55 pm
content: Quote from: ledtester on October 03, 2019, 03:01:22 pmoh i didnt know that.that said. i wonder how a 120w power adapter copes with 150w demand. how does it prevent itself from blowing up.

author: purezerg
author_rank: Contributor
timestamp: October 03, 2019, 06:58:14 pm
content: Quote from: soldar on October 03, 2019, 05:54:11 pmfrom what i know HP adapters are resister based. the dell are digital chip id based. nope i dont have a scope to test. any other means to tell if it's not just plain resistor?

author: Zero999
author_rank: Super Contributor
timestamp: October 03, 2019, 07:52:58 pm
content: Quote from: purezerg on October 03, 2019, 06:56:55 pmIt wouldn't surprise me if it can cope with more than 120W, especially if it's for short periods of time.