How to fix Lenovo/IBM Thinkpad T40 T41 T41P T42 T42P T43 T43P battery not charging problems.
High quality Lenovo/Battery for IBM Thinkpad Series are made from Li-ion cells. IBM Thinkpad T40 Series Batteries  are built-in overcharge and short-circuit protection.

Papatek high quality rechargeable battery for IBM Thinkpad T40 Series. Every piece of battery has been tested and validated on IBM Laptop to ensure it will work with your IBM Thinkpad T40 Series notebook computer.

However, you may meet the battery of your IBM Thinkpad T40 series wont hold charge. Sometime in the last 24 hours the battery on your IBM Thinkpad T40 stopped charging. So how to fix Thinkpad T40 series charging problem?
A few things to  troubleshoot before replacing a new batery:

1 - Verify that the battery actually charges - windows should report charging and the battery charge light should be on.  If you don't see this then your battery may not be the issue.  More than likely the DC/DC board is malfunctioning. 

2 - If the battery is charging but not holding the charge then you should see the battery charge as usual and then the laptop should die after a minute or so.  This would indidcate an issue witht the battery, also after charging the battery you can remove it from the notebook and use a voltmeter to read the voltage, it should indicate the voltage marked on the battery which for a T40 should be about 10.5 to 11 Volts DC.  If you see this kind of behaviour then you would replace the battery.

If you see above behaviour in 1 then most likely it is an issue with the charging circuitry and most likely the DC/DC board, in some cases these are seperate boards and can be replaced but in others the circuitry is part of the mainboard.

IBM Thinkpad T40 5200mAh 6 cells Battery Series Laptop Battery

Voltage: 11.1V

Capacity: 4400mAh

Type: Li-ion, 6 cells

UL certificated cells inside!

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Fit Laptop Models:
IBM ThinkPad T40P, IBM ThinkPad T41, ThinkPad T41P, IBM ThinkPad T42P, IBM ThinkPad T43P, IBM ThinkPad R50E, ThinkPad R50P, IBM ThinkPad R51, ThinkPad R52

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How To Use IBM Thinkpad T40 Series Laptop Battery?
Usually new IBM Thinkpad T40 Series batteries come in a discharged condition and it must be charged before use (refer to your computer manual for charging instructions). Upon initial use (or after a prolonged storage period) the IBM Thinkpad t40 battery may require two to three charge/discharge cycles before achieving maximum capacity.

It's normal with rechargeable batteries, when charging your IBM Thinkpad t40 series notebook battery for the first time that charging is complete after just 10 or 15 minutes. Just simply remove the IBM Thinkpad t40 series battery from the laptop and repeat the charging procedure.

Try To avoid over-charging, especially, do not let your laptop battery over discharge. It is normal for IBM Thinkpad t40 series notebook battery to become warm when charge and use you battery. The battery exhaust much faster under the high temperature, you should avoid using your laptop battery in shady, cool place. Hit the battery, and other acts will also accelerate depletion of battery. So, pay attention to your battery as much as possible.

If you don't use your IBM ThinkPad T40 battery laptop batteries for 1 month or longer, you should remove the battery from your notebook, make sure leave your battery power about 40%, stored in a shade, cool, dry place away from heat, metal objects and direct sunlight, but not in the refrigerator. It’s objected strongly store your IBM Thinkpad t40 series laptop battery for over three months without any using. A charged IBM laptop battery will eventually loses its charge if unused. It may therefore be necessary to recharge the battery after a storage period.