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<\/a><\/p>\nWhat\u2019s under the hood?<\/span> Capacity<\/span> 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.<\/p>\n Nickel cadmium<\/span> Cadex found in its tests that if NiCad batteries are not \u201cexercised,\u201d 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\u2019s important if you\u2019re 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\u2019ll 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\u2019ll have dead batteries unless you maintain them. On the other hand, if they\u2019re properly stored and maintained, they have the longest useful life of the three. They\u2019re great performers, too. If taken care of, they\u2019ll be able to store nearly the same charge late in life as they did when they were young. Wouldn\u2019t we all like to have that capacity?<\/p>\n Nickel metal hydride<\/span> Lithium ion<\/span> 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\u00a0Figure A<\/b>.<\/p>\n
\nThree types of batteries power the laptops you\u2019ll 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.<\/p>\n
\nThe 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\u2019re 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.<\/p>\n
\nIn 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\u00a0memory effect<\/i>. 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 \u201cremember\u201d 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).<\/p>\n
\nThe 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\u2019ve 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\u2019t 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 Hp 593554-001 batteries<\/a><\/strong>. Remember, NiCads like to be fully emptied, NiMH batteries prefer to be only partially drained.<\/p>\n
\nOf 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\u2019t lose the capacity to fully charge if they aren\u2019t 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\u2019re maintaining a number of laptops, that\u2019s good news.<\/p>\n