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In an earlier thread you asked for information on the BP-3 battery pack which is used with the TI 2550-III. I responded with some information and promised more later. I was in Florida last week and worked with my two 2550-III's. I started with the one which has a nine volt battery connector patched in. With a nine volt battery attached the operation is intermittent. Some of the keys wouldn't work some of the time. There was a curious orange line across the display of blue characters, similar to the retrace lines which I remember seeing wth the old Tektroniks oscilloscopes if the blanking of the retrace wasn't working properly. I tested the machine with other voltages connected with clip leads to the nine volt conector. With 1.5 or 3.0 volts the orange retrace disappears and no keyboard problems are evident.

My other 2550-III had a badly corroded BP-3 installed. Some damage had resulted to one of the connections to the machine. I took apart the battery pack and verified that it consisted of a single rechargeable AA cell. I installed a new connection in the machine and tried various battery voltages. The machine operated correctly with voltages ranging from 1.5 to 9.0 volts. There was no orange retrace in the display at any voltage. At lower voltages some of the display sements which were not needed for the number which was being displayed were also dimly lit. That characteristic got better with higher battery voltages.

During the testing I had nine volts connected to each machine for several minutes. Although the one machine exhibited intermittent operation with higher battery voltages connected there was no permanent damage

The baseline 2550 did not use a battery pack. Instructions in the cell well explained that the user should install either three rechargeable AA cells or four non-rechargeable cells. The installation of the fourth non-rechargeable cell operated a little Rube Goldberg switch. The instructions in the cell well also explained that an AC9120 AC Adapter was to be used when four non-rechargeable cells were installed and an AC9130 Adapter/Charger was to be used when three rechargeable cells were installed. It seems strange to me that fewer rechargeable cells were used.

There was also a 2550-II. It used a BP-2 battery pack which consisted of two rechargeable AA cells with no additional electronics such as with the BP-7 and BP-8 battery packs used on later TI machines. The 2550-II used the AC9130 Adapter Charger. One of my two 2550-II's came with a battery pack which showed no corrosion. After shocking the battery pack I was able to use it successfully in both of my 2550-II's. I have not tried higher voltages patched into these machines.

Clearly, the method for using rechargeable cells was evolving at TI during the development of the 2550 product line.