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On an Atari ST computer (1985 -> 1987), there is an optional battery backup circuit board for the clock chip (6301). The voltage output by the board to the battery cable with zero load is 4.17 to 4.20 volts. Based on measurement of charge time, it appears the maximum current from the board to a battery pack is less than 30 ma. The 6301 clock chip is a CMOS chip, so the voltage needs to be 3.5 v to 5.0 v, and the board's 4.17 to 4.20 volts is sufficient. When running on battery power, it is reported the board pulls about 1.3 ma current.

The battery pack is 3 x AA nicd or nimh pack. If using an external charger, the packs are normally charged only to 4.1 volts, in case there is a potential over voltage issue with the circuit board.

I'm wondering if the 3 x AA pack could be replaced with a 1S 850 mah LiPo battery, which would eliminate the self-discharge issue of a nimh pack.

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If it is really true that the battery charging circuit never puts out more than 4.20V then the Lipo should be OK. However you must also ensure that it doesn't go below 2.8 V or it could be damaged by over-discharge, and attempting to recharge it might blow it up! If the Lipo doesn't have a built-in PCM (Protection Circuit Module) then you should add one.

Another option might be to simply replace the 3 x AA NiMH pack with LSD (Low Self Discharge) cells, eg. Eneloops. This will provide much longer backup time with greater safety.

When running on battery power, it is reported the board pulls about 1.3 ma current.

At 1.3 mA an 850 mAh Lipo would last about 3 weeks. AA LSD cells should last about 8 weeks. If you don't use the computer regularly then you might want to put a switch in the battery circuit to turn it off for long-term storage.

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  • \$\begingroup\$ Thanks for the info. I was looking for pros and cons of using the LiPo instead of NiMH, since this came up in an Atari forum. One concern is that a 3 x AA NiMH can temporarily be charged to 4.5 volts, but should drop down to 4.2 volts after a few days (maybe just one). As for myself, I'm more concerned about running the 6301 clock (also keyboard) chip 24/7, so other than testing, I leave the battery disconnected. This just means I have to enter date and time once per power up. \$\endgroup\$
    – rcgldr
    Commented Mar 28, 2020 at 15:50
  • \$\begingroup\$ Running the 6301 24/7 won't hurt it. Having the 3 cell NiMH charged to 4.5V shouldn't be an issue either. I wish I could be more specific but I know very little about your board. Do you know where I can find a schematic, pictures, or any other information about it? If I ever get an ST myself I might want to reproduce it. \$\endgroup\$
    – Bruce Abbott
    Commented Mar 28, 2020 at 17:19
  • \$\begingroup\$ Image of the board time-saver. Red and black wires to go battery pack. The 40 pin 6301 is removed from the 40 pin keyboard socket and the circuit board plugs into the 40 pin keyboard socket, and the 6301 is plugged into a 40 pin socket on the circuit board. At AtariAge forums, someone posted their own circuit, but it is different than the board shown. The key issue is using diodes to prevent battery power going beyond circuit board to rest of keyboard. I got the Atari ST with the circuit board and 1MB ram upgrade as part of dev kit in 1985. \$\endgroup\$
    – rcgldr
    Commented Mar 28, 2020 at 18:56
  • \$\begingroup\$ Link to the circuit diagram for a battery backup circuit. It's somewhat different than the "time-saver" circuit board. \$\endgroup\$
    – rcgldr
    Commented Mar 28, 2020 at 19:06
  • \$\begingroup\$ That's the image I found! (should have looked the url and then I would have known it was yours). Any chance of seeing the bottom of the board? \$\endgroup\$
    – Bruce Abbott
    Commented Mar 28, 2020 at 19:07

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