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Choosing the Correct Batteries for Your Palmtop
There are a number of types of batteries you can use in your palmtop. What are the advantages and disadvantages of each type?
Then there are all the circuits that come to life each time we press the (ON) key of our little Palmtop computers. But batteries seem to take this paradox of simplicity and complexity to an extreme.
This is especially true of rechargeable batteries, which have chemistry which is so complex that there's no definitive information available to tell you exactly what takes place inside as you charge and discharge them.
Fortunately, we don't need to understand the chemistry in order use batteries effectively. But, just as knowing a little about how your car works can help you get the most performance out of it, knowing a little about your batteries can help you get more performance out of your palmtop especially when using high-power PC Cards.
There are currently four types of batteries available for use in the HP Palmtops. Of course the most common are alkaline, which are very inexpensive to purchase, but are single use (primary cells.) That is, when they are drained, you just throw them away. The exception is the renewal rechargeable alkalines. However, these do not perform well in the palmtops due to their internal resistance (more on this subject later).
The Eveready Energizer Lithium L91
The second type of battery is the Eveready Energizer Lithium L91. These are the highest capacity AA cells available, but like alkalines, are single use. The Lithiums make great spare batteries since they have about a 10-year shelf life, but they make poorer main batteries since they cost about four times the price of alkalines, and last only about three times longer.
NiCd and NiMH rechargeables
The third and fourth types of batteries are rechargeables: NiCd and NiMH. NiCd are more readily available and are recommended by HP, but have the lowest capacity of any of the types discussed here. NiMH have higher capacity, and although many people use them, you should note that they are not approved for use by HP.
To help you understand battery life, we need to introduce some of batteries' characteristic parameters.
Voltage is the parameter that is the most used, and the most misunderstood. Voltage is a measure of electrical pressure not capacity. Think of it this way: If two people are asked to push equally-loaded wagons up a big hill, one may be able to push harder, but may get tired sooner. It's the same with batteries: A higher starting voltage is not necessarily an indication of higher capacity. And just like an athlete that suddenly "burns out," some types of batteries may start out with higher voltages, but as they near the end of their lives they lose power very fast.
Alkaline batteries start out with a voltage of about 1.5 volts per cell, or a total of 3.0 volts. Alkalines are unique in that their voltage drops at a slow and relatively regular rate as you use them. Knowing this rate and the present voltage of the battery makes it possible to determine remaining power. The Palmtop has built into it the hardware to measure the voltage of the batteries, as well as the software (in the SETUP program) to estimate the remaining power of the batteries.
This built-in software uses an average rate of discharge for alkaline batteries. It is relatively accurate for these batteries, but not accurate for lithium, NiCd or NiMH batteries.
Rechargeable batteries start out at around 1.2 to 1.4 volts each, or 2.4 to 2.8 for a pair. However, unlike alkalines, they maintain an almost constant voltage for most of their useful life, then the voltage drops quickly at the end. This is why the battery life scale in SETUP disappears when you switch the battery type to NiCd.
Also, the starting voltage for rechargeable batteries seems to vary greatly from pair-to-pair, even for batteries from the same manufacturer. This is a very important point, since many users of rechargeable batteries tend to be overly concerned with the maximum voltage they get when they charge their batteries. But the final voltage value doesn't provide a reliable indication of how long they will last before they start their rapid fall.
Battery life is measured in milliampere-hours (mAh). Amperes (or simply amps) are a measure of the rate of electrical current flow, so multiplying the amount of electricity that flows by the amount of time that it's flowing gives you a measure of the total volume of electricity used. (If electricity was a liquid, then amps would be liters-per-hour, so an amp-hour would be equivalent to a liter-per-hour flow rate for one hour, or one liter of volume.) Since one amp is a very large amount of current relative to the consumption of the palmtop, the measurements are in thousandths of an amp (mA).
The palmtop's built-in SETUP program displays a battery bar that gives an estimate of the remaining power in a pair of alkaline batteries. The estimate of the power remaining in non-alkaline batteries is not accurate.
Typical alkaline batteries have a total capacity rating of 2500mAh. The ratings assume that you are using the batteries until they drop below about 0.9 volts each. However, the palmtop will not operate on batteries totaling about 2.2V for the pair. This means that you will not get the full 2500mAh from the batteries (although you could take them out of the palmtop and use the remaining life in something else that can run the batteries to lower voltages).
Most NiCd AA batteries have a capacity rating of only 500 to 600mAh. High capacity NiCds, such as the Radio Shack Hi Capacity NiCds or the GP brand that we sell, are rated at 850mAh. NiMH AAs are available in capacities of 1200mAh from my company, Shier Systems & Software.
While the ratings listed for rechargeable batteries seem substantially lower than for alkalines, remember that the rechargeables maintain their voltage for more of their rated life than alkalines do.
Another very important point is that rechargeables tend to perform much better under heavier workloads than alkalines. Therefore, when using modem cards or certain flash memory cards in your palmtop which tend to require short surges of high power you may find that alkaline batteries will not be up to the task unless they are very fresh, whereas the lower-capacity rechargeables can support these cards even as they get close to needing a charge. The net result is that you may find that you get close to the performance of alkalines from your rechargeables.
The issue of how the work load affects the useful life of batteries is even more pronounced when we compare alkalines to the Eveready Lithium batteries. The specification for the Lithiums rates them at only 2600mAh. This is almost identical to the alkaline rating, yet we know that they tend to last about three to four times longer than alkalines in our palmtops! So what's going on here?
It turns out that there are two factors working in the Lithiums' favor. The first is that they have a flat discharge curve much like that of the rechargeable batteries. This again allows them to work in the palmtop for more of their rated life than the alkalines can. The second factor is that they work far better under heavy work loads than alkalines do.
Looking at Eveready's specification sheets, and making some assumptions about the typical draw of a palmtop using a 14.4kbps modem, we find that the Lithiums should last for about 12 hours of steady use, whereas the alkalines only last about four hours. However, if we compare their lives when used to maintain the memory of a palmtop turned off in a desk drawer, we find that they both should last about the same three months!
Different brands, different characteristics
It is interesting to note that different brands of alkaline batteries can have different chemistries, and therefore very different discharge characteristics. For example, the Duracell brand that HP ships with the palmtops operates very poorly with heavy loads. These batteries have a capacity rating of 2450mAh, which is very close to that of the Eveready brand. In a palmtop without any accessories, they may provide similar performance as compared to other brands. However, palmtop users find that they get the "low battery warning" almost immediately when they use modems or flash cards with Duracells. The Rayovac Renewal batteries also perform poorly under high loads, and we don't recommend them for use with PC Cards in the palmtop.
This shows that the more you use your palmtop, especially with PC Cards, it's important to chose your batteries carefully to get the best performance.
At Shier Systems & Software we strongly recommend to our customers that they get a pair of high-capacity rechargeable batteries as their primary power source for the palmtop. Many times, I'm told that rechargeable batteries seem to be too much of a bother. Most palmtop users forget, or simply never knew, that they can charge the batteries right in the palmtop. I personally open the battery cover no more than once or twice a year! (If I'm on a long business trip and don't bring the AC adapter.)
A bad choice for spares
Note that rechargeable batteries don't make good spares. Rechargeable batteries tend to discharge themselves much faster than non-rechargeables. Of course, this is less of a problem, since you can simply recharge them. However, NiCds can lose about 10% of their capacity in less than a month and be basically useless in less than three months. NiMHs have an even greater rate of self discharge losing almost half their capacity in the first month! This is why we don't recommend rechargeable batteries as spares, since by the time you need them, they may be as dead as the ones in the palmtop you are replacing. As spare batteries, we recommend the Eveready Lithiums. They have the best shelf life.
Can batteries remember?
An article which discusses rechargeable batteries wouldn't be complete without a few words about "memory." This is the term used for the reduction in capacity seen when you don't completely discharge the batteries before recharging them.
What is typically called the "memory effect" is really a function of a host of sins, most of which have nothing to do with memory. Almost all premature failure of rechargeable batteries can be linked to overcharging, heat, or reverse voltage due to over-discharging. Lately, a lot of people have been saying that the "memory effect" does not exist at all. This is not completely true, but clearly makes the point about how little information is available about battery chemistry.
One cause of rechargeable batteries exhibiting "memory" is the formation of large crystal cadmium. The smaller the crystal structure, the greater the energy transfer. However, with time, or extremely slow charge rates (long trickle charging) the crystal structure of the plates inside grows larger (like growing sugar crystals into "rock candy") and the cell appears weaker. Fortunately, this can be corrected by simply discharging the batteries (without shorting them out) which will break up many of the large crystals and allow new, smaller ones to form during the next charge.
On the other hand, overcharging batteries can cause permanent damage. Heat is the enemy of batteries. When you have fully charged a cell, but continue to pump energy into it, the energy must go somewhere. Where it goes is into the generation of heat. When you heat the cell by overcharging, you can change the chemical properties and lower the output voltage, thus reducing the cell's useful life. Even if you don't generate much heat, such as when you overcharge for long periods with a trickle charger, you still reduce the useful range of the batteries as noted above.
As a final endorsement of rechargeable batteries, I want to remind you about the price we all pay when we throw away all those alkalines. Not only are we throwing away money, but we're also contributing to the waste in landfills. While most alkaline battery manufacturers have eliminated toxic chemicals such as mercury from their formulas, using rechargeables greatly reduces the burden on the environment from our use of these electronic wonders we call palmtops.
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