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Not Just The Numbers

Recently I was working on a high visibility end user problem with computer performance that ended up having a somewhat-unexpected cause: the laptop’s external power supply.

For months this person had been complaining of serious performance issues with a JavaScript-heavy websites that he needed to get his job done, and despite numerous technicians taking a look at the machine, none were able to fix it. In attempts to resolve the issue he’d been given new hardware, switched from a 32-bit to 64-bit OS, and had his user profile (all settings) reset to defaults more than once, to no avail. By the time the case got to me he was quite frustrated as fault was now being assigned to his data and the websites he was accessing, so I set up a meeting so he could show me the problem and talk through what’s going on. I hoped to get a better understanding of what he was doing and what was occurring to see what I could do for him.

After some cursory remote poking to check the laptop’s capabilities and be sure the it seemed stable we sat down and talked. He showed me what was clearly unacceptable performance, explained how the issue only seems to occur when he’s in the office at his desk, sometimes when at remote sites, and never when he’s connected via VPN. Seeing a nicely bundled set of cables behind his the monitor to which his laptop was connected I asked if he had a another power supply that he used when traveling, and if the one on his desk stays there.

That was it; the one on his desk was the cause. Newer Dell and HP business-class machines both use the same physical power connector and they’ll often charge each other’s devices, but depending on the laptop model, power supply model, and BIOS differences sometimes the  laptop will significantly scale back its performance. This is to save battery, allow charging on a limited supply, or (if you are conspiracy minded) steer people away from the use of third party power supplies. When in the office or at a borrowed desk at a remote site he was using a mismatched power supply, so the laptop would scale back its performance and the job-critical website would be unusable slow. Working from outside of company facilities (via VPN) he’d use the power supply that he carried with him — the one which shipped with the laptop –and performance was as expected.

When troubleshooting complicated problems like this it’s easy to fall into the trap of blaming user behavior, providence (the kind of data being stored), or the big mysterious technical places: bad hardware / software. The numbers. Sometimes one has to step back, sit down, talk to those involved, and look over the whole of the problem. Sometimes it’s as simple tab A being plugged into an incompatible slot C, but without stepping back and taking the user and his/her report into account this can be very hard to find.

Power supply model will even cause power scaling issues within the same brand if a given laptop requires, for example, a 90W supply and it is connected to a 65W supply. There is a POST prompt which warns the user of this, but sometimes users or technicians will see the laptop charging anyway (albeit at a lower rate) and disable it without realizing the consequences.


  1. Ryan McGavock
    Ryan McGavock August 16, 2013

    My work laptop does this with an older P/S model that I acquired. Fortunately this HP shows a dialogue when you plug it into a different model of P/S, so at least it gave me a fair warning.

  2. MajikSpoon
    MajikSpoon August 16, 2013

    I’ve ran into this personally, but not with using 2 different power supplies. I’ve seen it when a power supply begins to fail for what ever reason, whether the cable is stressed, the laptop connector is damaged or internal failure. Although, with my Dells that I have owned that do this power stepping for BIOS unidentified power supplies, I have always gotten a message with the Dell power management software pre-installed or at boot-up from the BIOS stating that it was unrecognized and that the laptop will run at reduced performance and sometimes it has said it won’t even attempt to charge the battery.

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