Once again my Z88 with the internal RAM installed came up with BAT LOW. But the other one still had batteries that did not come up with the BAT LOW. Was there something wrong with the computer or was it because I might have used batteries with less life left And how much current did the Z88 draw under normal use? I had questions and no answers.
So I built a break out box so that I could put meters on the current and the voltage. So after visualizing the break out box I found a little box to mount the connectors on and went to Radio Shack. By the time I got the breakout box built I realized that was only half the answer. On the internal battery/power supply how would I measure the battery voltage and current draw to get the difference between the operating mode and the sleep mode? And that was a puzzle, at first I thought of a metal strip taped to the batteries to get the leads out of the computer but then that would only allow the voltage to be measured. So I came up with an idea and I took a smaller diameter dowel and made a battery emulator connector.
I carefully measured the length of four AA batteries and subtracted the amount the solder lug and flat head screw would take up and cut the dowel and then made one more cut in the middle. I then wound masking tape around the dowel near the end that was the contact end until it was slightly less in diameter than an AA battery. When I mounted the solder lug and the wire to bring the contacts outside the computer I then cut a grove in the masking tape to seat the wire in and put a turn of Scotch tape around the masking tape and the wire to hold in place. At the end of the two wires on the outside is a jack to plug in a power supply or battery box with a standard Z88 type plug. To keep the polarity correct I used red for positive and black for negative for ground.
I had a HeathKit training unit that had a variable ±15 volts DC so I had an easy way to get the proper voltages. I had my newly purchased digital multimeter to monitor the voltages. I used an analog multimeter set for milliampers to read the current drawn by the Z88 under test.
On the Z88 with the internal 528K RAM and no internal batteries and using the HeathKit to supply the power by way of the battery compartment (internal batteries) I set the HeathKit power supply to as near 6 Volts as I could. First I checked the current draw and with the computer turned on the current draw was 80 MA. But in the sleep mode it was .8 MA. So back to the computer on mode and I slowly decreased the input voltage until the BAT LOW came on at 4.40 volts.
I next checked the external battery jack and with the power supply set for 6 volts it drew 100 MA when turned on and .5 MA in the sleep mode. The BAT LOW came on at 5.10 volts.
So next I installed batteries internally and their unloaded voltage was 6.19 volts and I applied 7.5 volts DC through the external jack and the computer drew 50 MA from that and stopped supplying current at 5.87 volts. I found four batteries that gave a total of 5.01 volts and with the external jack supplying 6 Volts the current draw from the external jack was 90 MA, it stopped supplying current when the external battery voltage reached 4.85 volts.
I have one other Z88 and it is set up with 128K RAMPAK and drew 60 MA from the power supply and at 5.2 Volts it reverted back to internal batteries. I suspect that the batteries are an their last legs from that since the other computer with low batteries cut out at 4.85 volts. That is why NiCad batteries do not supply very long running time. A NiCad should supply 1.25 volts right after being charged and they self discharge quite rapidly. In fact in about 20 days they are so far discharged that we would call them discharged. That is why if you use NiCad batteries you should use 6 and get 7.5 volts to start with. I know, I know that they are sold as 7.2 volts when in battery packs but they use the same logic in stating battery voltage as the auto industry does with the lead acid battery in automobiles. That battery fully charged is 6 times 2.2 or 13.2 volts.
I junked out a 2/3 height disk drive in a case. The case and the drive was all one piece. When I got to the bottom of the case there was this battery pack of ten AA NiCad batteries. I don’t know how long the batteries had been laying around but they had a little charge left in them. The unit had a power supply requirement of 18 VDC. But I had a Commodore power supply that put out 15VDC without a load. I found the proper polarity and plugged it in and the batteries charged up nicely to almost 13 volts. But since I had heard that NiCad batteries will self discharge I check them every few days and sure enough after a couple of days the voltage started to drop. It would be far safer for the Z88 to retain data if alkaline dry cells are used and the battery voltage is checked often. One other alternative is to use the RENEWAL batteries by RAY-O-VAC which are rechargeable alkaline batteries and if you don’t discharge then too much like only using them for 20% of their life you can recharge them (according to the company literature) for up to 100 times. However, they are more expensive than regular alkaline batteries and they use a special charger that is not cheap. The charge lasts like the shelf life of the battery and that is a long time.
I use AA RENEWAL batteries in our clocks and they seem to last a long time. However, I recharge the batteries every few months so as not to discharge the batteries too much.
On the Z88 the books report that the most current demanding action is when you SAVE to an EPROM. So that is something that I will have to test out and I will report when I do it. While an the subject of EPROMs I have a Z88 EPROM eraser and it is designed to be plugged into 220 VAC. Of course since it was designed for use in the UK they would go with the standard voltage of that country. There several ways to go to 220 volts but the easiest and most expensive is to get a 110 to 220 VAC transformer. I saw several and Radio Shack has one too but they are heavy and bulky and run from $35.00 to $45.00 each. But there is another way without tearing into the eraser and that is to use two transformers. One to drop the voltage to 12.6 VAC and then connect that output to a transformer that has a 6.3 VAC secondary to the 6.3 volt windings and then the formerly 110 VAC will have 220 VAC. But in prowling the transformers and wall outlet power supplies I found a unit that supplies 18 VAC but the primary is 220 VAC. The other day I found a plug-in deal that supplies 18 VAC and will carry the load of an EPROM eraser then I have that problem solved. I connected them up temporarily and I got 216 VAC on the output. Close enough I think. More later on that.
If you have any suggestions or questions let me know. For my part I have thought of a battery box for the Z88 that includes 5 D cells to give a total voltage of 7.75 with fresh batteries. I have thought of putting a IN4000 diode in series with the power until the voltage drops below 7.2 without the diode if that is a problem with the Z88, BUT! and external battery really needs a means of testing for when the voltage gets below 6 VDC. That way you would never inadvertently deplete the internal batteries. 0/0.