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PC battery charger

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


A supermagnet for using with a hook, without hook and directly connected (soldered), adhering on a CR 2032.





My simple CR 2032 battery charger with USB cable as power cable.
In this design i use four 1N4148 in series with a 100 kOhm resistor, because from the 3.3 V battery voltage there is 1.7 V difference to the 5 V USB and the forward current of one diode is 20 µA at 425 mV. So an empty battery with a voltage of 1.3 V gets a current of 20 µA and a full battery with a voltage of 3.3 V gets a current of about 4 µA, which is about two times the self discharge current. I tested it for more than three years and it works good for long-time charging. After three years charging one half empty battery with 2.90 V at the begin has 3.12 V at the end and a full battery with 3.30 V at the begin has 3.49 V at the end.





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.
These Li batteries have a capacity of 8.4 Ah, a voltage of 3.6 V and a weight of 50 g, while a high capacity NiMh accu of same size has only half the capacity, only one third the voltage and 20 % more weight. A high capacity alcali mangan battery of same size has 1.5 V / 7.8 Ah at 45 g.

From the USB bus 5 V / 500 mA are available, from the Firewire bus 12 V / 1 A are available and that's enough for battery charging.