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The Battery Rundown: A Brief Battery Primer

The Battery Rundown: A Brief Battery Primer

Check out the different kinds of batteries available for the HP Palmtops and the advantages and disadvantages associated with their use.

By Stanley Dobrowski

Batteries: Graphic

 The HP Palmtop's portability and functionality have made it an indispensable part of our lives. The ability to store large amounts of contact information, appointments, spreadsheets, and other data is simply amazing. And the (however slight) possibility of loosing all that precious data if your batteries die before you can change them is simply frightening! A basic understanding of battery technology along with knowledge of battery types available for the HP Palmtops can save you hours of work and help eliminate that queasy feeling that accompanies a low battery warning.

The HP Palmtops are powered by two AA batteries, and there are two types of AA batteries available on the regular consumer market: Primary and Rechargeable batteries.

Primary batteries

Primary batteries are designed to be used once, then discarded. They are not rechargeable because the chemical process that produces the electricity is not reversible. Once the chemical process has been used up, the battery is useless. There are two different chemistries for primary batteries: Alkaline and Lithium.


Alkaline batteries are by far the most predominant primary battery in use. There are many different brands and sizes available. Every manufacturer claims to have the longest lasting battery, but in reality, I doubt there is much difference between the major brands in terms of capacity.


The latest entry into the primary battery market is the Hi Energy Lithium Energizer batteries from Eveready. The lithium batteries have about 2-3 times the capacity of the same size alkaline cells. Since the lithiums are fairly new and only available from one manufacturer, they are still rather expensive at about 4-5 times the price of alkalines. The Lithiums also have a longer shelf life (up to 10 years) and are lighter in weight than alkalines.

Finally, because you use one pair of Lithiums for every 2-3 pairs of Alkalines, you have less of a negative impact on the environment.

Rechargeable batteries

Rechargeable batteries are designed with a chemical process that is reversible. The process that takes place when power is drawn from the battery, eventually runs out just like a primary battery, but when the battery is placed in a charger and electricity is fed into the cell, the chemical process is reversed and the battery can be brought back up to full capacity again. For most rechargeables, this discharge/charge cycling can be done over 500 times before the battery just wears out and will not hold a good charge anymore.

The benefit to the environment is even greater with rechargeables because you use one pair instead of a couple hundred pairs of alkalines.

There are two types of rechargeable batteries available: Nickel Cadmium and Nickel-Metal Hydride.


The most common type of rechargeable is the Nickel Cadmium (NiCd) battery. NiCds have become quite mainstream lately and are available from several manufacturers and in many different brands. A full charge on the best NiCd battery will still only have about 1/3 to 1/2 the capacity of an alkaline battery and the NiCds cost more than alkalines, but since they can be used over and over, they are more economical in the long run. NiCd batteries have a nominal voltage of 1.2-1.25 volts versus the 1.5 volts of an alkaline cell, but this usually does not present a problem for most devices. Unfortunately, many NiCd batteries suffer from the "memory effect" which keeps the batteries from completely discharging so that they appear dead even though there is reserve capacity. This is caused by repeated partial discharge/full recharge cycles. The battery "remembers" this partial discharge point and appears to be empty when that point is reached again.

The highest capacity NiCd battery that I know of is the Radio Shack Hi Capacity NiCd. It is rated at 850 milliampere-hours (mah). This means that at a constant discharge rate of say 85 milliamps, they will last for 10 hours. Also, I have heard that these Radio Shack Hi Capacity NiCds are designed to minimize the memory effect. Another very good brand of NiCds is Millennium. They have a complete line of different sizes and an array of chargers for them. The batteries come with a lifetime warranty (but you need proof of purchase and who keeps that stuff). The capacities are not marked on the packaging or the battery label, but I believe the AA size is rated for about 700-750 mah. These Millennium batteries can also be quick charged in 1 hour in their special charger. Many of the bargain brands of NiCds are rated at about 500-600 mah, so I would not recommend them.


The latest development in rechargeables are the Nickel Metal Hydride (NiMH) batteries. They have several attractive features over the NiCds. Because the chemistry is completely different, there is no memory effect with NiMH batteries. Partial discharge cycles do not cause problems. The NiMHs are rated at 1000-1200 mah depending on brand. They can be charged in just about and trickle or standard rate charger designed for NiCds. I would not recommend fast charging NiMH batteries. NiMHs are more expensive than the NiCds, but the total lack of memory effect and higher capacity may be worth the extra cost.

The Backup Battery

In a special holder under the infrared port cover is the button sized 3 volt Lithium backup battery for the HP Palmtop computers. This battery supplies power to the memory inside the 95LX or 100LX when the main batteries fail or are being replaced. Normally, this battery should last for about one year before it needs to be replaced. However, if there are problems with the main batteries and the computer has to go into "backup mode", the tiny button battery can become drained before that period of time. It is a good practice to set an alarm or event in the Appt application for every 10-12 months as a reminder to change this battery. The backup battery product code number is CR-2032 which is available in many department, camera, drug and electronics stores. The instructions for changing this battery are described in the User Guides (page A-4 the 95LX Guide, page A-6 in the 100LX guide). The battery monitoring programs mentioned in the next section will give a readout of the backup battery voltage. It is always good to check this periodically, perhaps monthly, so that a premature failure or forgetting to change the battery does not result in data loss.

HP 95LX Battery Requirements

The HP 95LX is designed to be used with alkaline batteries and that is the only kind HP recommends. The battery voltage sensing circuitry and software algorithm monitor the gradual voltage drop as the batteries discharge and sound the alarm at a particular voltage. This was designed for the alkalines, not for battery types with flat discharge voltage curves and sudden voltage drop offs like lithiums, NiCds or NiMHs.

The 95LX will report these other batteries as full or almost full for most of the discharge curve. These other batteries die quickly at the end, and the 95LX's low battery detection may not catch them in time. This can result in a loss of data.

Another important consideration is the reverse voltage protection in the 95LX. This was designed for alkaline batteries. The electronics which protect the 95LX will burn out if you insert rechargeable batteries in backwards. If you use rechargeables, extreme care must be taken when inserting them. I double and triple check the batteries in my hand against the legend in the battery case before putting them in.

Many 95LX users, myself included, have successfully used the four kinds of batteries described in the 95LX. The alkalines are the safest. The lithiums last the longest, but you have to be careful about rapid voltage drop off and loosing your data. The NiCd and NiMH rechargeables are the most cost effective, but you have to recharge them out of your 95LX. The NiCds and NiMHs can also have a rapid voltage drop off.

If you use any battery other than an alkaline, you should invest in some battery monitoring software.


To compensate for the simplistic battery monitoring method in the 95LX, ACE Technologies has a program called BATTman which is a TSR program that watches the battery voltage over time and monitors usage hours of the Palmtop in a sophisticated algorithm to predict when the batteries are spent. BATTman has settings for all four types of batteries with a different algorithm for each. I consider this program required if anything other than alkalines are to be used in the 95LX.

HP 100LX Battery Requirements

The HP 100LX was designed to accommodate both alkaline batteries and NiCd rechargeable batteries. There are electronics and software to allow NiCds to be recharged inside the 100LX, even while it is being used. The battery monitoring algorithm in both Palmtops will safely shut down the unit when the main batteries can no longer provide enough power. It is very important to have the battery type setting in Setup (press (CTRL)-(FILER) Options Battery) correct or a loss of data could result.

The built-in circuit that protects against backward insertion of batteries appears to be able to handle the higher-impedance rechargeables, but HP does not recommend using any batteries other than alkaline and NiCd in the 100LX. I and other users have successfully used pairs of all four types of batteries in the 100LX without any problems. (The advantages and disadvantages discussed at the end of the 95LX section apply here also.)


Unfortunately, ACE does not yet have a version of BATTman for the 100LX, yet. But there is a freeware program by Mark Scardina in the CompuServe HPHAND forum, library 11, called BAT100.ZIP (ON DISK ICON). This program will display the main and backup battery voltages and the good/bad status of the memory card battery. The newest version, available on this issue of The HP Palmtop Paper ON DISK, will even log this information to a disk file each time the program is run. With this program, the voltage of the main batteries can be monitored by the user and changed or charged at the appropriate time. I usually do not let the main battery voltage fall much below 2.45 volts before I recharge my batteries. I always like to have some reserve capacity available in case I get caught without a charger or an extra set of batteries.

[Editor's note: Another program, BATSET.COM (ON DISK ICON), will allow you to adjust the charging time for batteries being recharged in the 100LX. BAT95 .EXE (ON DISK ICON) will display battery voltages on the 95LX.]

Other Important Advice

1. Do not take the "Low Battery" warning lightly.

Depending on the type of battery installed, there may be minutes or seconds of computing time left in the main batteries. Either stop working and shut off the Palmtop or plug it into an AC power pack to continue working. Allowing the Palmtop to go into "backup mode" because the main batteries die can result in data loss and a drain on the backup battery.

2. Insert batteries with correct polarity.

There are battery orientation symbols inside the battery compartment. Both batteries must be inserted with the positive tip or bump pointing towards the serial port connector. For some reason, HP placed a spring type contact for the negative end of one battery and the positive end of the other battery. This is a bit confusing since all battery holders I have seen prior to the HP Palmtops use a spring contact only for the negative end of the batteries. So be careful when inserting your batteries.

Remember, the Palmtop must be either turned off or plugged into and AC power pack when the main batteries are pulled out.

3. Conserve power to extend battery life.

The most obvious and effective thing to do is to keep your Palmtop connected to an AC adapter whenever possible. Keep one at the office and one at home. You'll be surprised how much longer your batteries last.

The Palmtops power down after 3 minutes of inactivity to conserve batteries. But if you got in the habit of pressing the OFF button when finished, you'd save about three minutes of full battery use each time.

A number of programs give the user control over power to the serial port (see 95BUDDY(ON DISK ICON), TIMOUT. ZIP(ON DISK ICON), HP95CT.ZIP(ON DISK ICON), HP95MD. ZIP(ON DISK ICON), ASERCL.ZIP(ON DISK ICON)). An active serial port uses battery power. Shut it down when not in use.

When the Palmtop is waiting for a key to be entered by the user, it shuts down the CPU for a short period to conserve power. Calculations, searches, copying files, or other CPU intensive operations, keep the CPU active, using the batteries. Keep the Palmtop connected to the AC adapter when doing these types of operations to reduce the load on the batteries and extend their life. Also PCMCIA I/O cards can drain batteries if left in the card slot. Remove them when you are not using them!

4. Alkalines easiest solution.

Probably the easiest route to take with either HP palmtop is to use alkaline batteries. These work very well in either palmtop and provide an adequate life span, especially in the 100LX. Just keep spare pairs handy and put new ones in the computer as soon as the low battery warning appears. Since the low battery warning comes before the batteries are truly dead, you can use them in other less critical applications such as portable audio devices. Of course, using these expendable, single use batteries creates quite a bit of waste which may not sit well with environmentally conscience users.

5. Best rechargeable solution.

The best solution for rechargeables would probably be the Radio Shack Hi Capacity NiCd batteries. They can be charged right inside the 100LX but would have to be swapped out of the 95LX so that they can be charged in an external charger. There is a somewhat higher initial cost for the batteries and the separate charger if needed, but in the long run that will pay off because there can be hundreds of charge/discharge cycles for a single pair of batteries.

An excellent article by Mark Scardina (page 13, Jul/Aug 92) compares life and cost of the four AA battery types mentioned and reviews the BATTman program mentioned here.

iPhone Life magazine

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