Understanding and maintaining laptop batteries

It’s vital hardware, and your laptop isn’t going to get very far without it, but how often do you give a thought to your battery? In this Daily Feature, I’m going to give you some powerful facts that will get you charged about batteries.

What’s under the hood?
Three types of batteries power the laptops you’ll find in service today, nickel cadmium (NiCad), nickel metal hydride (NiMH), and lithium ion (Li-ion), with Li-ion being the most common in newer laptops. Each battery type has a different chemistry for generating a charge and, therefore, different characteristics.

Capacity
The capacity of batteries is measured by power delivered per battery weight, rate of self-discharge, and useful cycles. Power is often measured in watt-hours per kilogram, or Wh/kg. Self-discharge refers to the fact that all batteries lose power when they’re stored. Cycle refers to how many times a battery can be taken from a full charge to a full discharge before the battery can no longer hold power.

The fact that batteries discharge over time is affected by temperature. At higher temperatures, the discharge rate is faster. The general rule to follow is to store them in a cool place and use them at room temperature.

Nickel cadmium
In tests performed in a lab by Cadex Electronics, Inc., nickel cadmium batteries had a life of 1500 cycles, which means they went from full charge to full discharge 1500 times before giving out. Because of the chemistry of this battery, however, NiCads can suffer from a so-called memory effect. The memory effect is a chemical reaction that takes place if the battery is not fully discharged before recharging. In this reaction, crystals form that inhibit a full charge. The battery seems to “remember” a lower level of charging. Incidentally, full discharge never means taking the battery down to 0 volts, which can irreversibly damage the battery. It means taking the battery down to a rated End of Discharge Voltage (EODV).

Cadex found in its tests that if NiCad batteries are not “exercised,” or periodically maintained by full discharge and charge cycles, their useful life could be reduced by up to three times. In that case, the batteries would last only 500 cycles. Therefore, it’s important if you’re using a computer (or any other battery-powered equipment) that you maintain these batteries. Of the three types, NiCad cells have a middling shelf life. If stored at room temperature, they’ll lose about 20 percent of their charge per month. In fact, they lose 10 percent of their capacity within 24 hours after you take them out of the charger. Within five months, you’ll have dead batteries unless you maintain them. On the other hand, if they’re properly stored and maintained, they have the longest useful life of the three. They’re great performers, too. If taken care of, they’ll be able to store nearly the same charge late in life as they did when they were young. Wouldn’t we all like to have that capacity?

Nickel metal hydride
The chemistry of this battery gives more power for its weight, which is why it seemed to be a good replacement for NiCad batteries a few years ago. On the other hand, it has a very short shelf life. NiMH cells discharge 30 percent per month. In only three months, unless you maintain them, you’ve got dead batteries. Another disadvantage is that they last for only 500 cycles, making them the lowest performer in the useful life area. And like NiCad batteries, if you don’t fully discharge and recharge them periodically, their useful life is reduced. You could potentially get only 175 cycles out of them. On the other hand, they do better than NiCads when you discharge them only a little bit before recharging. Unlike NiCad cells, you increase the number of cycles when you work with your laptop unplugged for just a little while, then plug it in and recharge the batteries. Remember, NiCads like to be fully emptied, NiMH batteries prefer to be only partially drained.

Lithium ion
Of the types of batteries mentioned here, lithium ion cells have some powerful advantages. They have an energy density of 150 watt-hours per kilogram. Like NiMHs, Li-ion batteries have no memory effect. That is, they don’t lose the capacity to fully charge if they aren’t fully drained. Left on the shelf, they lose the least power. Their discharge rate, at 10 percent per month, gives them a good storage life (but they must be used within two years of manufacture). This means that you can grab one that you charged up three months ago, and it will still have about 70 percent of its charge left. If you’re maintaining a number of laptops, that’s good news.

These batteries also have some important disadvantages, however. They have a volatile chemistry. Each lithium ion pack in your laptop must have its own electronic protection circuit that protects against overcharging, over-discharging, and too-high current. Each cell of the battery pack incorporates a built-in safety vent. All three battery types have a safety vent, but unlike NiCad and NiMH batteries, which can withstand some overcharging, the vent in lithium-ion batteries is designed to disconnect the battery permanently if it ever has to be used. It does not reseal. The potential danger of this type is why Li-ion batteries come as a sealed unit including safety electronics. As a helpful part of the circuitry, each Li-ion battery provides a charge indicator. Pressing a button will cause a row of lights to be lit, indicating the charge, as shown in Figure A.

I was surprised to learn that of all the batteries, lithium-ions have the worst useful life. Though they have a slow discharge rate on the shelf, all Li-ion batteries irreversibly lose capacity over time. Even after one year, they lose the ability to be charged to full capacity. Cadex tests indicated that their practical service life is only two years. According to Cadex’ document, Batteries for Mobile Computing, “Li-ion batteries should not be stored for long periods of time but consumed like a perishable food.” On the other hand, while they don’t deliver as many cycles as NiCad batteries, they do provide up to 1000 cycles if they’re used within that two-year lifespan. They are also maintenance free. Not only do they suffer no memory effects, they don’t need to be periodically discharged and topped off. Like NiMH batteries, shallow discharges actually increase the number of cycles. Unfortunately, these cells are expensive. Keep in mind that your IT department might be better off with a smaller, rotating inventory.

How should I maintain my inventory?
Store batteries in a cool place, preferably refrigerated and placed in a sealed plastic bag to avoid condensation. Bring them up to room temperature before using. To store NiCads, first fully charge and then discharge. When charged NiCads are allowed to self-discharge, they create crystals (the memory effect), which reduces their capacity. NiCads as well as NiMH batteries need to be primed with a slow charge when they come out of storage. On the other hand, Li-ion batteries need to be stored charged. If not, they can lose the ability to charge to full capacity in as little as three months. Further, cells may leak and corrode. When coming out of storage, both NiMH and Li-ion batteries need several charge/discharge cycles for the battery to come up to its optimal capacity.

These days, batteries often come with extra circuits that can tell a charger how best to recharge them. These units are sometimes referred to as “smart” batteries. Consider buying a number of smart chargers that can understand this coding. The charger will automatically apply the right type of voltage, will know whether to use a trickle charge or not, and will basically be able to sense any of the three major battery chemistries discussed here. In addition, superior models will also handle “dumb” batteries. There are even Windows-based software packages that allow you to schedule battery maintenance and monitor battery analyzers/chargers. In any case, get the most advanced chargers you can and use the right charger for the battery chemistry. Many problems with battery life and capacity are due to incorrect charging and recharging. Using the wrong charger for your battery type can be dangerous.

More resources
Much of the information in this Daily Feature came from resources published online by Cadex Electronics, Sony, and Anton Bauer. If you want to learn more about batteries, the Cadex Electronics Web site has excellent content that non-engineers can read and understand. Try The Battery Book, The Battery Faq, and their Articles links. Sony has a good primer on Li-ion batteries, the Lithium-ion Battery Home Page. Anton Bauer, maker of batteries and charging systems for professional video operations, has an online resource called The Video Battery Handbook. While this book covers batteries from the perspective of one type of use, you’ll find the technical information to be excellent, especially on chemistries, charging, and capacity.

If your company has many mobile users, then this simple and often overlooked piece of vital hardware can cause your IT department to waste a lot money if it’s not correctly maintained and inventoried. In this Daily Feature, I’ve discussed some of the basic features of the three major types of batteries currently used in notebook computers, and I’ve also explained how you might best maintain and inventory these batteries.

Mike Jackman is an editor in chief of TechProGuild, an editor of PC Troubleshooter and Windows Support Professional, and also works as a freelance Web designer and consultant. In his spare time (when he can find some), Mike’s an avid devourer and writer of science fiction, parent to two perpetually adolescent cats, and a hiking enthusiast.

The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.