Thinkpads Forum

Certain Thinkpad models (including the X61) have a BIOS quirk that limits the CPU clock speed to 1.2GHz when running the laptop on a 65w AC adapter without a battery installed.

In Linux, this shows up as /sys/devices/system/cpu/cpuN/cpufreq/bios_limit. Note that the N is the CPU core # minus 1, so for a dual-core processor, there would be two, like this:

/sys/devices/system/cpu/cpu0/cpufreq/bios_limit
/sys/devices/system/cpu/cpu1/cpufreq/bios_limit


We do know the reason for this-- it's a protection mechanism in order to prevent drawing more current than the 65w adapter can supply.

However, I encountered a bug, or perhaps a loophole.

If I start the laptop with both charger and battery plugged in, put the system on standby, then remove the battery, the
bios_limit file does not appear (or is not generated) when I resume from suspend.

This allows me to run my X61 at full speed on AC even without the battery. I ran a few 7z benchmarks to confirm its performance.

on AC, without battery at boot --> 1877 MIPs
on AC, with battery installed --> 3522 MIPs
on AC, battery removed during suspend --> 3522 MIPs

I'm running Debian 8 64-bit on an X61 with a Core 2 Duo T7500 processor and 2GB RAM. I have TLP installed, and the processor is undervolted.

I don't know if this is reproducible in other Thinkpads and other Linux distributions. Just reporting my discovery and findings.

Nice find!

Since we are talking about a BIOS feature, it's quite possible that the same quirk can be reproduced on Windows. I bet some would like to test it out. Not that I advocate anyone run their laptop without a battery installed, but I know that some folks do like to do this from time to time, so your discovery will be interesting for them.

Interesting .

But obviously there isn't any advantage in removing the battery. Threshold charging with the X61 and all Thinkpads is a clear advantage.

The primary advantage is not letting the battery heat up due to its proximity to the CPU heatsink.

The real danger IMO, is whether it will cause sudden shutdowns when the system is fully stressed (not just the CPU, but also the USB, graphics, and other subsystems), and also shorten the lifespan (and eventually kill) the 65w AC adapter.

This might be safer to do on the X61s and Tablet, as they have LV processors and are less likely to overload the 65w AC adapter.