Why two power ports on a budget laptop??
#42
(08-10-2020, 10:09 PM)moonwalkers Wrote: I'm not entirely convinced that is necessarily the case - e.g. that same W510 I mentioned running off 90W PSU thinking it is a 170W PSU. The PSU in ThinkPad has three lines - ground, power, and "signal". The simple resistor in the PSU connector between ground and "signal" is what tells the laptop how much power the PSU is supposed to be able to provide. When I shorted that pin on the motherboard that told the machine the PSU is a 170W model, but the machine still wasn't drawing all 170W to power the machine and charge the battery, it was drawing only as much as the PSU was actually able to provide. AFAIK the older PSUs in ThinkPads from before Lenovo acquisition do not have third signal pin, it's only power and ground, and they did come in varying power yet they were (mostly) perfectly interchangeable - e.g. I am able to use my old R50 with a 30W PSU from my 701C instead of the proper 72W, and while it doesn't charge the battery unless the laptop is pretty much turned off it does slow down the battery discharge. Something I'm not noticing with PBP (not to bash - just stating the fact).

And to be clear, when I was talking about 3A charging limit I was not talking about battery's theoretical capability, I was talking about the actual circuitry in PBP.

Did you try to run the CPU and GPU at full tilt and charge an empty battery while using a 90 W charger disguised as a 170 W unit?  I'm pretty sure that wouldn't work well.

I forgot the exact way the ThinkPad chargers identify themselves to ThinkPads, so this was a good reminder.  Here's detailed information, if anyone wishes to check it out.  However, sensing a resistor is also a way of communication between the charger and laptop; sure, not nearly as complicated as the communication required for establishing high-power USB charging, for example.

Why can a 90 W charger work in place of a 170 W charger?  Well, ThinkPad designers overestimate the power cosumption, while the charger designers undervalue the power outputs of their products.  Plain simple safety margins, but that surely doesn't imply that a 90 W charger is going to happily chug along for years disguised as a 170 W charger. Also, it doesn't imply that a disguised 90 W charger is going to be able to both charge the battery and power the laptop running at 100% of CPU and GPU.

The sad thing about PBP is that the internal charging circuitry isn't maxed out at around 3 A.  It can go up to 4 A, but it is simply limited to 3 A through the configuration of the BQ24171 charger IC.  By the way, guess what, the charger IC is configured using resistors. Smile


Messages In This Thread
RE: Why two power ports on a budget laptop?? - by dsimic - 08-10-2020, 11:20 PM

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