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Battery Power - Pt2 Don Bradbury continues his look at sources of battery power Last time we saw how improving battery technology was steadily bringing benefits to the computing scene. This time we take a deeper look at battery technologies and their treatment. If, for now, we accept that users of portable computing gear, given the option, will opt to use either NiCad, NiMH, or Li-Ion rechargeable batteries for normal use, reserving the non-rechargeable Alkaline batteries for emergency use, then we should first look at the advantages and disadvantages of such cells. Memories are made of this Aside from higher initial cost, NiMH has the advantage over NiCad technology in that it has both a higher charge capacity for a given cell size, together with less of a ‘memory’ of how it has previously been used. This memory effect manifests itself when a set of NiCad batteries are recharged before they have been completely discharged. The battery thinks it can hold less charge than its nominal rated capacity. It therefore charges only to that limit, and consequently lasts less time in use before ‘going flat’. The treatment for the problem is to ensure that NiCads, and to some extent NiMH batteries which also have vestigial memories, are fully discharged before they are put in their charger again. Here the fun starts because what your digital camera, for example, may believe is a fully discharged set of batteries may be nothing of the sort. The voltage may simply have dropped to an unusable level, though the charge may be only 80% spent. The notebook user Notebook users face may the same difficulty. If their NiCad or NiMH battery has been left inactive for several months it will have self-discharged to a voltage/charge level that cannot support the heavy-drain boot process. Worse, when connected to its charger and topped-up, the memory effect may permit only a fraction of full charge to be added before the cell voltage rises to cut off the charging current. But when you disconnect the charger, the system runs for only a short time before shutting down the computer because of inadequate battery voltage. The answer is to run through the discharge/charge process several times, letting the notebook run its battery right down in each case, even to the extent of it shutting down the system. This might, if you are lucky, restore most of the lost capacity, though, personally, I’ve found it rather unreliable. Conditioning If you are unlucky - after all, the computer will probably shut down too early to be very effective as a discharge device - you could need recourse to a commercial charge/discharge unit that might be successful in fully reconditioning the battery. If you are still unlucky, a new battery is the only answer. Another trick is to disconnect the charger when full capacity is indicated, leave it a few minutes, and then reconnect. That forces more charge into the batteries. New cells benefit from this especially, and in addition you should routinely run through one or two charge/discharge preconditioning cycles before using new cells if you want to set them up them properly prior to use. Lithium for life Lithium Ion batteries, essentially, do not suffer from the memory effect. Nevertheless, proper conditioning is beneficial with new cells, and careful handling is also desirable if a long life is to be extracted from them. Their charge capacity is higher than the other common types, and their self-discharge rate is relatively low. However, their initial price is higher. It’s worth bearing in mind that you may also have difficulties locating a source of the typical AA size Li-ion cell. That’s because their cell voltage is significantly higher than that of NiCad (1.2 volts) or NiMH (1.2 volts) cells. Hence, they are not simple alternatives to NiCad or NiMH. Note that Lithium batteries (not Li-ion) offer a capacity which is 30% higher than Alkaline batteries, and a potential of only 1.7 volts, but these are not rechargeable. So, generally, devices using Li-ion cells come with a dedicated battery configuration and charger. The latter is especially important as Ni-Cad/NiMH chargers should not be used with Li-ion cells; their characteristics are essentially different. Next time we’ll look at battery types vis-a-vis their chargers, because each type demands its own dedicated charge routine if maximum life is to be gained and preserved.
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