Dell M6700 much slower when connected to external battery pack (Energizer XP18000)

Question

A while back, I bought some external battery packs to extend the battery life of my laptop. They're Energizer XP18000s. But I got the feeling that my laptop was going much slower with the battery connected. I just ran a build process as a rough benchmark to see how bad it is.

Plugged into external battery:

time make clean common
real    8m7.359s
user    46m23.586s
sys     4m37.153s

Unplugged from everything: time make clean

common
real    1m28.118s
user    8m15.467s
sys     0m55.539s

Any ideas why this might be the case, or how to improve the situation? If it matters, I'm running Windows 7 (time, make, etc. are running on an Ubuntu VM).

Answer 1

Some Dell laptops limit CPU performance if connected to a power source that is below some specific level. For example if I connect a 65W Power supply to an E6400 laptop, that need a 90W supply, it will give a warning about it as it is not enough power to both charge the internal battry and run the computer at full speed. This could be what is happening to you when connecting the external batteries. As your M6700 is delivered with a very powerful AC adapter I doubt that external batteries can provide it with enough power.

Answer 2

I would check the power plan as well, and change it to "High performance" if not already set. Default for M6500 is "Balanced", so probably for M6700 it is the same.

Maybe, despite your connection to the Energizer, laptop is using the "On battery" scheme, and in this case power plan change should help.