Why two power ports on a budget laptop??

[moonwalkers]

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.

Haven't tried that specifically on W510 with 90W PSU - unless I load the GPU it just doesn't breach the PSU limit. But, as I already said, I have tried that on the much older R50 that AFAIK doesn't have that resistor and third pin in PSU for wattage sensing, and the result was that it works, but the battery discharges, though slower than without any PSU. I'm not an electrical engineer, but my understanding is if more current is drawn than PSU can supply without voltage dropping below certain limit then battery will be discharging instead of charging, providing the extra current needed for the machine to keep operating. If this is a misconception on my part I would love to see someone correct it. Until someone does - I'm pretty sure there is more to it than simply overestimating the power consumption. R50 on 30W PSU without battery - definitely unstable, unable to reach full performance. But plug the battery in and it chugs along just fine, with battery discharge rate that appears to be several times slower than running on battery alone.

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.

Yes, yes, yes, I read the thread about it, no need to be so pedantic :-) Whatever are the details of the cause, the bottom line - overall power input to the machine is limited by the current battery can use to charge and the machine will not run without battery (the switch on the mobo aside). The whole power system in PBP is (artificially, though probably unintentionally) bottlenecked on the battery charge current, whereas on the same ThinkPads the bottleneck is the PSU itself - if it is not powerful enough the battery will not charge at full speed and may even discharge to supplement PSU, and if PSU has excess power the battery will not charge faster than is safe. But one more observation - it seems to me that unlike the already mentioned R50 on PBP the battery is either charging (however slowly), or just discharging with charger making no difference whether it is connected or not - while I couldn't get any meaningful data from powertop the time measurements on similar loads show little difference between run times with no AC connected and AC connected with red light next to charger port blinking. If this is also a misconception - again, by all means please correct me.