This guide will explain the basics of Lithium Ion battery technology and the criteria that should be considered when purchasing a used battery for your laptop computer. The basic information regarding Nickel Metal Hydride (NiMH) and Lithium Ion (Li-Ion) batteries is the same for all laptop computers. Buying a used laptop battery can be a great bargain or a terrific waste of money. Take a few moments to educate yourself on Lithium Ion batteries and the information that should be in a battery listing before purchasing a used battery.

Lithium Ion batteries have become the preferred power source for most current laptop batteries. Older laptops used NiCad batteries, and more recently laptops have used NiMH batteries. This guide will primarily discuss Li-Ion batteries because they are the predominant  battery technology for current generation laptop computers.

Li-Ion batteries combine the best qualities of alkaline batteries with the best qualities of rechargeable batteries. Li-ion batteries provide constant voltage during discharge and do not suffer from the memory effect that plague Ni-Cd batteries. Li-ion batteries also provide the highest available energy density of any available battery technology (i.e. provides more power per ounce as compared to NiMH or NiCad) Li-Ion batteries also have a low self-discharge rate (about 5% per month) compared to Nickel Metal Hydride (about 30% per month) or Ni-Cd (about 20% per month). A little known characteristic of Li-Ion battery technology is that Li-Ion batteries lose approximately 15% usable capacity each year (depends upon storage temperature). Because of their characteristics, Lithium Ion batteries can operate for between 300-500 cycles and last for about 5 years from the manufacture date.

Many Li-Ion batteries have a set of built in LEDs to determine battery charge status. Just push the button and you can readily determine the charge state of the battery. The LEDs DO NOT provide any information regarding the health (or capacity) of the battery. It only indicates the battery's charge state. If the battery only holds a 360 mAH charge, all the lights will illuminate, but the battery only may only have 10% of it's original capacity.

An average laptop battery operating time is a tricky number to accurately quantify. Battery operating time is dependent upon power settings selected by each individual user and the processor speed. A battery that operates for 2 hours on one laptop with maximum energy saving options selected (e.g. dim screen, reduced processor speed, disk spin down, etc.) may only operate for 1 hour on the same laptop with no energy saving features selected.

A simple test that I use for determining battery capacity is to play a DVD. This is the most energy intensive test that you can perform on your laptop. I usually set up this test with all energy saving features disabled (disk spin down = off, full bright display, processor performance = highest, no sleep, etc.) This provides the worst case scenario. Normal operating conditions will usually be better than my test conditions. I would rather have a customer get a battery that was tested to operate for 90 minutes and actually operates for 2 hours (under normal conditions), than for the customer to get a battery that only operates for 1 hour.

Here's What You Should Consider Before Purchasing a Used Battery:

1. Li-Ion batteries have a 300 to 500 cycle lifetime. Mac OS X Tiger (10.4.3 and on) allows you to monitor the number of cycles on a battery using the System Profiler. I assume there is a similar capability in Windows or some shareware that is readily available. Try to avoid a used battery that has a very high number of cycles on it.
2. A new battery should be able to play a DVD for over 2 and a half hours. A good used battery should be able to play a DVD for at least an hour and a half. A used battery that plays a DVD for about an hour and a half will probably operate for over 2 hours under "normal" operating conditions (this is just an educated guess based upon my experience).
3. Li-Ion batteries tend to lose about 15% capacity each year, depending upon storage conditions. Try to avoid purchasing a battery that is more than 5 years old.
4. Be cautious if you're considering a battery where the listing doesn't provide any test data. It's a simple matter to test the battery and provide operating conditions and run time in the listing.
5. Be cautious when a battery listing makes claims like "holds full charge" or "all 4 lights illuminate" . As I stated above, this means virtually nothing regarding the health of the battery.

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