How do I connect an external battery to an IBM486SLC21-EC1 motherboard. There is a jupmer pin set next to the on board battery, J6 that identifies the #1 pin. It looks as though 2&3 should be jumpered to use the internal battery. Pins 1&4 should be for the battery. What is the polarity and the voltage requirement?

Try to identify the onboard battery. If you can find information describing that battery, then you can determine an external battery to replace it.

Older batteries used pin 1 and 4. Common replacements used 4.5 or 6 volts. 3 or 4 AA Cells. Lithium Battery Packs used 7 volts frequently. A good starting point is to equal the existing battery voltage.

The onboard battery has no visible markings. It is glued in place and has small leads soldered directly to the motherboard. The battery looks like three coin cells stacked together with a shrink wrap sleeve and measures 1.2 - 1.7 volts. I wired a 2032 cell (3v) to Pins 1(-) & 4(+), but the clock stops when powered down.

If the 3 cells are about 3 quarter thick each, then you have a bad NiCd battery. It's voltage is 3.6 colts. The external battery may have to be greater since there may be a dropping resistor and/or reverse current protection diode. Polarity is important.

I had used 4 AA in a battery holder and plugged it in using an old cd-rom 4 pin end to the pins you mentioned on the MOBO, everything worked fine for years afterwards.... Keep the old stuff running

IBM486SLC21-EC1 If I recall this mobo, this has some association with a IBM 486 Blue Lighting! If correct, that mobo was one of my favorite mobo's back in 1994 as it was a 75mhz 486 25mhz clock 3x multiplier. Anyway, if what your describing, the need for a Nickle Cadiium three stacks cells each 1.2v to gain 3.6v series configuration originally made by SAFT or Varta, the replacement is using a Ni-MH rechargeable 3.6v 70mAH assembly. Each cell should be 1.2v 60-70maH cascaded one on top of another to form a soda can assembly. I believe it has three poles but two of the poles or tabs are on the same potential. You can get more information about this either from me on Ebay, my ID is CMOSBATTERIES or you can go to this website http://www.partstore.com/Part/Batte... 3.6 Volt NiMH Battery HI-Capacity Part Number: B-425/PC3 in a nutshell, the B-425 part number is associated with Energy Plus or Hi-Capacity, DSMiller, BatteryBiz, Duracell Direct, Fedco Electronics, and a varitey of others sell them directly or via the Internet.