The PC battery, usually a lithium battery of type CR 2032, has to be replaced after around
two years, although in theory it should be only necessary after 10 years,
because BIOS and RTC do need around 2 µA and the CR 2032 has a
capacity around 220 mAh with a self discharge (at 23°C) of less than 1 % per year.
There are many reasons for this discrepancy: Often no low-power design and BIOS/RTC
and on poor boards BIOS/RTC is powered from battery even when the PC has standby voltage or
when the PC is on.
The solution of this problem is simple: Do a conservation charging of the battery and compensate the discharging of the battery!
This prevents the battery from discharging and self-discharging when the PC is on.
Because i want a very soft and careful charging of not fully charged batteries, i'm using this circuit: From 5 V one 100 kOhm resistor and four simple cheap diodes (1N4148) in series to the +-pole of the battery (in the picture of the first prototype are only 2 diodes):
Below the adapter for charging, you can see the cheap components of the middle cable
section: One resistor and two diodes, covered and stabilized with two heat
If you don't switch off the PC by switching of the AC input, you should use the 5 V Standby, e. g. from a Wake On LAN connector or similar connectors.
When there is no battery holder on the mainboard where you can connect the alligator crimp with a conducting connection to the +-pole, you can solder a short wire of diameter 0.5 mm and length 5 mm carefully (fast) on the +-pole or conductive glue a short wire or connect battery and crimp with a small neodym magnet clamp (around 0.5 EUR) or solder a wire on the mainboard.
I have measured an open circuit voltage of 4.2 V and a short-circuit current of 34 µA. At a resistor of 823 kOhm 3.35 V / 4 µA, which means at a new battery the charging current is 4 µA and this compensates the discharging current on a mainboard of about 5 µA. So the net effect is reducing the discharging and this is compatible with the fact that the Li-battery vendors do say that every charging is not allowed.
You can monitor the voltage with a program, e. g. sensors under Linux:
> sensors | grep -i bat
Vbat: +3.28 V (min = +2.70 V, max = +3.63 V)
The real value can be 10 to 20 mV lower or higher because the onboard sensors are not calibrated.
A supermagnet for using with a magnetic hook, without hook and adhering on a CR 2032, as one example for connecting the battery. The minus pole on a mainboard is usually connected to ground, so you only have to connect the plus pole.
My simple CR 2032 battery charger with a USB cable as power cable.
My simple Li baby battery (C cell) charger with a USB cable, with two batteries from an cypres and with two 1N4148 + 1kOhm per battery.