T4400 Battery Tips
A T4400C contains three (!) NiCd (Nickel Cadmium) batteries:
Although the circuits within the T4400 that charge the two smaller batteries are very simple, in my experience these batteries never cause any problems. It is however wise to note that a machine that has not been used for quite a while looses its setup. Since the HDD type is always automatically restored the system will boot fine but you may have to reset some settings.
- The main battery; 16,8 Volts, 2400 mAh
This battery is user exchangeable and runs the system. When it contains a charge, it also charges the other two batteries.
- The backup battery; 6.0 Volts, 120 mAh
This battery supplies power to all relevant chips when the system is in AutoResume mode.
- The RTC (real time clock) battery; 3,6 Volts, 50 mAh
This battery supplies power to the chip that maintains the date, month and year as well as the CMOS system setup RAM.
The main battery is the cause of problems. This battery, due to its rather rare square cells, is very expensive and needs to be treated properly to enjoy a long life. Basically it comes down to really use the battery (i.e. not using the system on mains power for too long) and use the machine on battery power until it enters AutoResume (i.e. until the battery is fully empty). The T4400C is a battery powered system so it should be used as such.
The above sounds easy. But still some of my well treated batteries broke down well before their sell-by-date. I have read a lot of literature about NiCd batteries and have come to the conclusion that one can treat a NiCd cell as the literature prescribes (such as no overcharge, always discharge fully before charging, no high temperature etc.) but that one simply cannot properly treat a NiCd battery that well. The problem is that within a battery there are a number of cells (in a T4400 battery there are 14) of which there is always at least one ugly duckling. This worst cell will for instance always deplete first or will always run hotter than the others. And since you cannot cater for the needs of this one cell seperately, it will die first. The stronger cells more or less fight against the weak one by almost certainly reverse polarizing it during a discharge (the discharge current from the other cells actually reverses polarity of a weak cell which causes havoc with the chemistry within the cell), making it even weaker.
An example; I have one battery that probably has one weak cell. If I could isolate that weak cell I could deep discharge and fully charge it a couple of times and that might strengthen it. But since it cannot be isolated I can only discharge the whole battery which means that the weak cell will be reverse polarized again and will grow weaker still, and charging the battery stops much to soon since the charger detects that 13/14th of the battery was already more or less full. There seems to be no way to do this properly. Overcharching the whole battery so that perhaps the weak cell finally will get a full charge has bad effects on the now heavily overcharged strong cells. Sigh.
The only cure is buying a new battery and (again!) catering for Toshiba's profits.
NiCd literature mentions that between the Ni and Cd electrodes sometimes a crystalline needle will form. I had one battery that appeared to hold no charge (i.e. probably one cell remained empty). I performed a very drastic solution by sending a couple of more than 10 Amps AC current pulses through the battery. It has worked fine since then. Not a cure for the fainthearted and not for all batteries. (Also a T4400's battery contains a fuse, so if the current is too high the fuse will blow rendering the battery worthless.)
A number of observations remain:
- The power supply electronics within the T4400 monitor the battery's voltage to check whether it is almost empty in which case it will enter AutoResume. Some NiCd cells exhibit 'voltage depreciation' which means that the cell's discharge curve shows a small dip when the cell is only slightly discharged. This voltage dip of the battery will trigger the T4400 to enter AutoResume. (See picture.) Since the battery isn't empty at all a subsequent charge will only take a couple of minutes. The literature prescribes a deep discharge to solve this problem, which needs to be done by an external load (eg. a 22 ohm 10 W resistor) and needs to be stopped at 14 Volts to make sure no cells are reverse polarized.
- Since the main battery continually (over)charges the backup battery, the main battery is always automatically discharged. The Toshiba manual suggests that a full main battery will be depleted in 10 days. However, this also means that when you leave a main battery in the T4400 for a long time, you run the risk of deep discharging it. The symptoms then might be that it may take an hour before it will start charging again (the 'Battery' LED remains red instead of becoming orange) or the battery will now contain a cell that has been reverse polarized one time too many rendering the whole battery unusable. Better not take any chances if you not plan to use your machine for a while; take out the main battery.
- The main battery will discharge out of its own in about 2 months.
- The two internal batteries are not user-serviceable. A bad backup battery means that AutoResume will not work at all (you then risk loosing any unsaved data) or will work unreliably (in which case you need to hook up the mains adapter really soon after the system has entered AutoResume).
- Small side note: Unfortunately do not rely on AutoResume too much. When the system is in real mode (i.e. Windows enhanced mode, DOS EMM386 enabled) and a disk I/O is in progress, AutoResume might fail. Especially when doing floppy I/O this is a risk since these I/O's take a long time. When you need to access floppies and the 'Battery' LED is flashing or the battery alarm sounds, do save your work.
- Literature suggests that NiCd batteries should not be run at high temperatures. Especially with higher environmental temperatures a used battery might run fairly hot. Let it cool down before you charge it again. Unfortunately the internal charger does charge batteries that are too hot.
- In case you wondered how Toshiba managed to design an A4 TFT notebook around 1990 using a normal i486 (there were no low-power versions at that time) using a 40 Wh battery as large as the T4400 's battery is, consider the fact that they more or less cheated. Almost all of the electronics for charging the battery are located in the bulky mains adapter. In modern notebooks those circuits are within the notebook itself. Funny side-effect; the Desk Station IV thus also contains all battery charger electronics. Another reason for its high price I guess.
- If only battery producers would include a terminal for every cell so that each cell could be externally deep discharged separately, battery life would soar.
- The RTC battery is only charged when the system is on. When the T4400 is switched off, regardless whether it is connected to the mains or not, the RTC battery will discharge in approx. 1 month.
- If your T4400 came without AC Adaptor and you want to find out how to connect an external power supply, click here.
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