My original 9 cell Lenovo battery pack dropped from 3 hrs to 2 hrs capacity after a couple of years. So when it stopped working, "battery not detected", I thought it was forced into retirement due to low capacity. In fact I opened the pack and all cells were unexpectedly within millivolts and were good to at least 2 Ah. I get an aftermarket 9 cell pack, and it arrives with zero volts. eBay seller quickly sends me another. But the W540 cannot detect it. I order two 6 cell packs from two other vendors. None are detected by the W540.

So it strikes me maybe there is something wrong with the connector, and not that the handshakes had failed by these aftermarket manufacturers. Lo and behold, the two positive pins on the laptop were bent, and so was the plastic tongue. It turns out that you can gently force in the battery and hear the double click of the lock, and due to misalignment, normal insertion would bend the pins. No connection will be established if the male pins on the laptop are bent. After straightening them, which breaks the plastic tongue guide, I was able to insert the battery and the battery is now recognized.

So if you get a "battery not detected" error, before suspecting the cells or the pack, you may want to check the male connector pins on the laptop and make sure they are all straight. Also check the female connector on the pack as that can be bent, or the female connector pushed into the pack. I believe the alignment of the battery connector is a bit off with respect to the battery, and that is causing this bending of the pins. It just goes to show that there is an issue respecting the connector.