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 discrepance: 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!
This prevents the battery from discharching 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 two simple cheap
diodes (1N4148) in series to the +-pole of the battery:
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
shrink tubes.
If you don't switch off the PC by switching of the AC input, you should use the
5 V Standby, e. g. from the 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 a short-circuit current of around 42 µA which means charging
from 4.2 V with a series resistor of 100 kOhm. A fully charged
battery with 3.33 V gets charged with a current of around 9 µA, so this
circuit is for PCs which are mostly powered off. For mostly powered on PCs you
should add a third and fourth diode in series for charging from 3.4 V.
The battery vendors of the Li-batterys do say that every charging is not allowed, but
this is nonsense for conservation charging with only few mircowatts which only
conserves the battery capacity and voltage.
