Thinkpads Forum

I am looking for ThinkPad L540 expert and users, to answer this question.

I have had some trouble with my L540, If I moved it or left it off charger for some hours, I could not turn it on. Only if I Switched the RAM positions(2x8GB), then it would start.

I thought it were the RAM, even though they tested fine in Memtest86, but it also happened with the original 4GB RAM, so I took the laptop apart and cleaned and repasted the thermal paste. Still happened.

I then yesterday removed the CR2032 RTC cmos battery and tried to start the laptop without it, it didn't start(it should, right? Or did my brain have a meltdown), I then replaced it with one from an older laptop, and laptop started. Last night I shut it down as usual.

This morning there were no problem starting it, but it has been connected to charger overnight, I will try later to see if it happens again after it has been off charger for a while.

Have you ever experienced that a faulty CR2032 RTC Cmos battery prevented the laptop from Starting up(even before post)?
Just sitting there with a blinking Power button LED?

Thank you in advance for your assistance



Ps. Additional question, what is the exact specifications of the Cmos battery for L540, here I mean, I know it CR2032 3V, but what is the mAh?

I ask because I have seen some listed with 120mAh and some with 220mAh and one on ebay listed with 1050mAh, which I find weird, it should be the same.

https://data.energizer.com/pdfs/cr2032.pdf

The original CMOS is about 10 years old by now, most likely as good as dead.
Replacing it should solve the problem.

RealBlackStuff wrote: ↑

Fri Sep 12, 2025 12:41 am

https://data.energizer.com/pdfs/cr2032.pdf

The original CMOS is about 10 years old by now, most likely as good as dead.
Replacing it should solve the problem.

Hi RealBlackStuff,

Thank you for your answer, but as I wrote in the original post, I already replaced the cmos battery(of course I need a new one) and the laptop started up, the question were if anyone had experienced that defective Cmos battery could prevent the L540 from starting up, because I never seen that before in a ThinkPad?

My experience with this is, that I only seen that happen on an Acer Travelmate 15 years older than the L540.


Second question were the capacity of the Original cmos battery(because of conflicting information online), I know about the Energizer as that is a big brand here, and mostly what I buy for rechargeable batteries.

Jan

FRU 02k6541 02K6572

Hey Jan, what exactly happens with the faulty cmos battery? Give me a play by play.

If I were to guess, the L540 was just running a self diagnostic.
This happened on my X1 Carbon when its cmos battery died.
The self diagnostic data is cleared when there is no cmos and main battery.

It's going to be 220 mAh for the original battery, but I know for sure the 1050mAh is fake. There is no way to cram that much battery capacity in a CR2032.

Not all CMOS batteries have the same +/- plug.
Some are smaller or larger than the average Thinkpad CMOS-plug and some even have + and - swapped.
The voltage should be no less than 2.6V, best around 3.0V or more.

Newer laptops (after ~2005) should start without a CMOS, or with a dead CMOS.
In both cases you should be able to boot after pressing Esc or F1.
If not, remove everything you can (HDD/SSD/CD/DVD, RAM, wifi card, Bluetooth card, batteries) and try to boot with only the charger.
If it beeps 1-3-3-1 that's a good sign, if not, something is rotten in the state of Denmark laptop.
After the 1-3-3-1 start adding 1 item at a time and try to boot.
Rinse/repeat.