.he Ni-Cd Notes Bookbinder NI-CD NOTES I've had a PX-8 for almost a year, now. In that time, largely because I've had some trouble with them, I've learned quite a bit about the Ni-Cd batteries that power this computer and its portable peripherals. Following is a summary of what I've been able to glean, from reading and from observation, about Ni-Cd batteries in general and about how they are handled within the PX-8 in particular. The PX-8's Batteries ..@1The PX-8's Batteries The PX-8 main unit contains two rechargeable batteries. The main battery supplies power when you are using the unit and is recharged with an AC adapter that provides about 6 volts DC. This battery is rated at 1100 mAH at a nominal 4.8 volts. The backup battery supplies power when the unit is switched off or turns itself off. This backup power keeps data and programs stored in the PX-8's main memory intact. The backup battery is rated at 90 mAH, also at a nominal 4.8 volts. To insure that the backup battery is always in a fully charged state when it is needed, it is recharged in one of two ways: either by the AC adapter or, if the PX-8 is on but the adapter is not connected, by the circuitry that powers the LCD screen. The Batteries of Some PX-8 Peripherals ..@1The Batteries of Some PX-8 Peripherals PF-10 Battery ..@2PF-10 Battery Like the PX-8 main battery, the PF-10 disk drive battery is rated at 4.8 volts, 1100 mAH. It, too, appears to be built from four rechargeable sub-C batteries. The PF-10 also has a backup battery, said to be good for about 10 minutes of operation after the main battery runs down. This battery looks identical to the battery on the main PX-8 board. It is soldered to the PF-10's main printed circuit board. RAM Disk and Multi-Unit Battery ..@2RAM Disk and Multi-Unit Battery The RAM disk battery is rated at 4.8 volts, 450 mAH. It is built from four 'AA' size cells. CX-20 Modem Battery ..@2CX-20 Modem Battery The external modem battery is identical to the RAM disk battery, except that it uses a slightly different connector. .pa General Characteristics of Ni-Cd Batteries ..@1General Characteristics of Ni-Cd Batteries Ni-Cd batteries come in a variety of basic types. These include: Resealable Vented and One-shot Vented When sufficiently overcharged or over-discharged, Ni-Cd batteries can build up enough internal pressure to cause a vent to open. This releases the pressure and prevents an explosion, but it also releases some of the battery's electrolyte. Some Ni-Cd batteries are built with one-shot vents; these vents, once open, do not reseal, and the battery eventually dries out. Many modern Ni-Cd batteries are built with vents that will reseal once the situation creating the excess pressure has been corrected. These batteries may be somewhat reduced in capacity once they have vented (as is a lead-acid battery that has been allowed to run low on acid), but they will, in many cases, still be usable for quite a while. Cells that have vented will have white crystals growing near the positive contact or will show other obvious signs of leakage. Normal Charge and Fast Charge Some Ni-Cd batteries can only be recharged at a rate less than or equal to the so-called "10 hour rate," which is one-tenth the rated ampere-hour capacity of the battery (C/10). For example, the 10 hour rate for the 1100 mAH PX-8 main battery is 110 mA. Due to inefficiencies in the charging process, the battery must be recharged at this rate for 14 to 16 hours to achieve full charge. Other batteries are designed to be able to withstand a charge rate equal to one-fourth their rated capacity (C/4). These batteries are capable of sustaining repeated "fast charges" without damage. The batteries used in the PX-8 appear to be of the "fast charge" variety, since they are intended to withstand a charge rate greater than C/10 without shortening their life. Normal Capacity and High Capacity Normal capacity Ni-Cd batteries will continuously operate a device for approximately the same period of time, on one charge, as ordinary dry cells. High-capacity batteries will operate the same device for a longer period of time. (For example, the Radio Shack high-capacity 'C' cells have almost twice the capacity of the standard Ni-Cd 'C' cell, and the high- capacity 'D' cells have nearly four times the capacity of standard Ni-Cd 'D' cells.) How Ni-Cd Batteries Can Be Damaged ..@1How Ni-Cd Batteries Can Be Damaged Ni-Cd batteries can be damaged by improper use (over-discharging or discharging too quickly) or by improper recharging (overcharging or charging too quickly). Overly discharging a Ni-Cd battery can cause the weaker cells to be reverse-charged by the stronger cells. This can permanently damage them by causing them to vent. (In general, no cell in a Ni-Cd battery should be permitted to fall below 1.0 volts.) Overcharging Ni-Cds at a rate up to one-tenth the rating of the battery may temporarily weaken them, but the damage can often be mostly reversed, as described below. On the other hand, overcharging at too great a rate, as wellas discharging at a rate greater than C/2, can cause Ni-Cd's to overheat. Overheating can lead to venting or can cause internal shorts to develop. (Internal shorts can sometimes be repaired by burning them out using the technique described below.) Ni-Cd batteries are also subject to a "memory" effect. This can happen after the batteries have been subjected to repeated, identical partial discharge/full recharge cycles. A battery with this problem will appear to be discharged when it "remembers" the point in its discharge cycle at which it is usually recharged. This effect, too, can be reversed and the battery restored to nearly full capacity. How the PX-8 Charges Batteries ..@1How the PX-8 Charges Batteries Main Battery ..@2Main Battery The main battery is recharged by a 6-8 volt DC power source connected to the "ADAPTOR" plug on the back of the PX-8. A diode in the PX-8 charge circuit is intended to protect the battery from discharging if the power supply connected to the computer falls below the battery voltage, and to protect the computer from damage due to reverse polarity power supplies or short circuiting the power supply connector. The circuitry also contains a Zener diode which will shunt moderate overvoltage (though I wouldn't go out of my way to test any of this protection, since it, too, is capable of being damaged). The computer is designed to recharge the batteries at a moderately fast rate (150-200mA) for eight or eleven hours, depending on whether the computer is turned off or on, respectively, when the charge cycle begins. After that, charge drops to a trickle charge of about 40 mA. This is intended to allow you to recharge the computer fairly quickly and keep it in a charged state, while at the same time avoiding overcharging the batteries if the charger is left connected. Unfortunately, if you are not careful about how you recharge the batteries, the circuitry that is intended to protect against overcharge can work against you. The manual supplied with the PX-8 does not adequately warn against this. Here's the problem: The PX-8 recharge is monitored by circuitry intended to protect against overcharge. It does so by detecting when external power is supplied to the unit and at that time initiating its 8 or 11-hour charge cycle. It does so regardless of the current state of charge of the battery. This means that if the battery is partially charged when the power supply is attached, it will be overcharged for some portion of the next eight or eleven hours, and it will be overcharged at the full charge rate. Repeated overcharges of this sort will eventually damage the batteries. Although modern Ni-Cd batteries can "take" continuous charging at the C10 rate (1/10th their current rating) without suffering permanent damage, the PX-8 charge rate considerably exceeds this. Therefore, you should not recharge your PX-8 unless the battery voltage has dropped below about 5 volts. (The PX-8 will switch from trickle charge to full charge by itself, if the voltage drops below 5 volts while trickle charge is in effect.) If you do need to recharge before this has occurred, you will be doing your computer a favor if you do not leave the charger connected for the full 8-hour period, or if you take the time to run the battery down by using the serial port or tape drive. Since the charge rate is boosted to full each time the power supply is connected to the rear of the computer, you should also not disconnect and reconnect the adapter once a charge cycle has begun. Each reconnection begins another full charge cycle. Backup Battery ..@2Backup Battery The backup battery is designed to always be at peak charge while the PX-8 is on, so that it can take over when the main battery needs to be charged. If the AC adapter is connected, the backup battery is recharged in the same fashion as the main battery (full charge/trickle charge). If the adapter is not connected and the computer is turned on, the backup battery is recharged through the internal power supply to the LCD screen. RAM Disk Battery ..@2RAM Disk Battery The RAM disk or Multi-unit battery is recharged in a manner similar to the main battery, and is subject to the same full charge/trickle charge cycle. It can only be charged when the RAM disk is connected to the PX-8, since it does not have a separate external power supply connection. PF-10 or External Modem Battery ..@2PF-10 or External Modem Battery The PF-10 and CX-20 external modem do not seem to have overcharge protection circuitry. Their batteries can be damaged by being continuously connected to a charger (I can say from personal experience that the PX-10 batteries will overheat if they are connected too long). If the batteries have not leaked due to overcharge, most of the apparent damage can be reversed by cycling the batteries through one or more charge/discharge cycles. If they have leaked, they should be replaced. This can be done by ordering a battery pack from an EPSON supplier, or much more cheaply by purchasing the appropriate replacement cells (from an electronics supplier or from Radio Shack) and building a new battery pack from the original one, as described elsewhere in this article. How to Squeeze a Little More Power Out of PX-8 Batteries ..@1How to Squeeze a Little More Power Out of PX-8 Batteries You can use the technique recommended for restarting the PX-8 after you have changed the battery to get a few minutes more life out of a discharged PX-8 battery -- enough to finish most low-power operations. If the unit has turned itself off because the battery got too low, turn the power switch to "off" and let it sit for a few minutes. While holding down the reset button, turn the unit on and release the reset button. The PX-8 will probably come on, and it will function for another five or ten minutes. PX-8 Ni-Cd DO's and DON'T's ..@1PX-8 Ni-Cd DO's and DON'Ts o Don't short out the battery. Do be careful when you remove or replace the battery from the PX-8. o Don't overcharge. This will happen if you attach the charger for the full 8 hours when the battery is not fully discharged. Wait for the "CHARGE BATTERY" messge, or use a battery monitor program like px.com, battery.com, or pxutil and don't recharge until the battery is near the cutoff point (4.7 or 4.8 volts). o Do let the battery rundown completely before you recharge. Don't partially discharge and then fully recharge. This can produce the so- called "memory effect." The memory effect, which can occur after repeated, nearly identical partial discharge/full recharge cycles, causes the battery to appear discharged at the point the premature recharge cycles previously began, therefore giving shorter service than it should. How Damaged Ni-Cd Batteries Can Be Restored ..@1How Damaged Ni-Cd Batteries Can Be Restored You can restore a weak battery if it has not become physically damaged (internal short or leakage of electrolyte). Usually, all you have to do is repeatedly put the battery through full discharge/full recharge cycles, preferably bringing the terminal voltage down to 4.0 volts (but not lower!) for a 4.8 volt battery. To restore the PX-8 batteries, try fully discharging and then recharging the computer a few times. If this doesn't improve things sufficiently, try bringing the battery down below the 4.7 volts at which the PX-8 normally cuts out. You can do this by first letting the computer run the batteries down, then disconnecting the battery and inserting the ends of a 100 or 220 ohm resistor in the connector that normally plugs into the PX-8. Monitor the battery voltage with a voltmeter clipped to the resistor leads. As a last-ditch effort, you can try to restore batteries that have one or more shorted cells by jolting them with a short burst of excess charge. This charge burns through small short circuits within the cell itself. You can supply this charge by building and applying the circuit below (from Hands-On Electronics, June '87). __ 25 + __/\__ blast button vdc o---////-------+----o o-----+------------o to voltmeter plus 150 ohm | | 1 watt | o blast | resistor | / | blast/charge | / | switch +----/ o-----+ | charge | | | | + | + 10,000 mf ----- ----- capacitor ----- --- cell to be blasted (or more) | | | | | | 25 - | | vdc o--------------+---------------+-----------o to voltmeter minus The idea is to leave the switch on "blast" and let the capacitor charge, then hit the blast button a few times, waiting a few seconds between each hit. Then switch to "charge" and watch the voltmeter; if you have succeeded in repairing the damage to the battery, the battery voltage will rise. If not, you may as well repeat the process a couple of times, then put the battery through a normal charge/discharge cycle and see what happens. I have also accomplished the same thing by momentarily shorting a dead cell of a battery across a six volt power supply, though I can't guarantee the safety of this procedure. In any event, batteries brought back to life with this method are not likely to last as long as batteries that have not been damaged, and you are probably better off replacing them. Replacements for PX-8 Batteries ..@1Replacements for PX-8 Batteries The main battery is easy to replace when it wears out (that is, will no longer hold a charge for a reasonable period of time). It is built from four 1.2 volt sub-C cells. A replacement pack is available from Epson, but one can also be constructed much more cheaply from tabbed sub-C cells available from Radio Shack and other electronics suppliers. On the other hand, the backup battery is soldered to the main circuit board of the PX-8 and would be difficult for a PX-8 owner to replace, even if a substitute could be found. A replacement battery pack for the PF-10 is available from Epson. The PF- 10 is also designed to run off ordinary 'C' batteries; with a simple modification, described below, you can enable it to use and to recharge commonly available standard or heavy duty Ni-Cd 'C' cells. (The heavy duty batteries provide almost twice as much power as the factory batteries.) The battery for a wedge (RAM disk or Multi-unit) can be replaced easily by removing the retaining screws of the RAM disk unit, unplugging the battery pack, and replacing defective or worn out cells with tabbed 'AA' cells available from Radio Shack or other suppliers. The CX-20 modem battery pack is similarly constructed and can be repaired in the same way. Modifying the PF-10 to take High-Capacity Batteries ..@1Modifying the PF-10 to take High-Capacity Batteries The following is adapted from a description by John Cooper. It describes how to modify a PF-10 disk drive to use Radio Shack 1.8AH 'C' Ni-Cd batteries (Radio Shack part #23-141). I have performed the same modification, following his directions. The project requires four batteries. The modification enables the PF-10 to recharge these or other rechargable batteries as well as the original battery pack. Note, however, that you should not operate the PF-10 with the charger attached when you are using ordinary, disposable batteries after you make this modification to the PF-10. 1. Set the PF-10 on a flat surface with the back of the unit facing you. 2. Remove the single screw between the Serial plug and the Adapter plug. 3. Turn the unit on its back and remove the battery cover and battery pack. 4. Remove both of the most rearward PF-10 case screws. Slide the plastic battery carrier and rear cover combined out to the rear. 5. Carefully disconnect the plug connecting the battery carrier to the main printed circuit board. 6. Turn the battery carrier over so you can see the small printed circuit board. Pry the board up a little to free the wires, then de-solder the white wire from this circuit board. Store the wire under the tape that dresses the plug wires on the carrier. 7. Connect the now-empty pad where the white wire was to the red wire pad, and then to the brown wire pad. 8. Reassemble. Make sure you plug the battery carrier wires into the main PCB the same way they came off, and don't forget the ON/OFF switch cover. A little organizing of the wiring harness may be necessary to keep it clear of the Serial cable sockets. .pa The PF-10 should run about twice as long between charges (though it will also take about twice as long to recharge). The new batteries should also be much less succeptible to damage from overcharge, since the charge rate of the charger that comes with the PF-10 charges the new batteries at less than the C/10 rate for these cells. David Bookbinder West Medford, MA October 10, 1987