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I have a high-end motherboard (Asus WS Z390 Pro) out of warranty. My bios looses its settings everytime cut-off the power. I found out something is draining the battery way too fast.

I decided to desoldered the CMOS BIOS IC, it's just an SPI flash (W25X20CL). After that, it's still draining the coin battery, that means the IC isn't the culprit.

  • motherboard alone
  • nothing is plugged in, no power cable, no SATA, nothing
  • sitting on my desk
  • CMOS chip desoldered
  • no other power source (everything unplugged)

I plugged the battery cell and I could see the voltage dropping very quickly, more than 10mV/s with the voltmeter.

I'm now investigating, I know the problem doesn't come from the CMOS chip since it isn't present on the board anymore.

enter image description here

  • What is the red SOT-23 component? Looks like a transistor but this package can contain anything... Can it be 2 diodes with a common point to the top? (diodes selecting 3.3V or battery's 3.3V)

  • What is the blue component? 01B?D1B? A diode should have a white line on one side... From measurement, it doesn't seem to be a resistor.

    • its top side is connected to the + of the battery and the bottom side is connected to the SOT.

Also, do you have an idea of the schematic of such circuit? I thought the battery cell wasn't connected to anything else other than the CMOS/BIOS IC via a diode but since I de-soldered it, nothing should drain current anymore, but something's left.

  • + and - of the battery aren't shorted, ohm-meter says 2.6Kohms, just good to know it's not shorted, nothing else.
  • All the caps around the battery and the ClearCMOS push-button are fine, not shorted.

I'll try to inject 3.3V with a current-limited PSU because it has drained out 3 of my batteries in 5min of testing...

Edit:

The blue component is a 1Kohm resistor, the red component L43, from https://www.s-manuals.com/smd/l4, it seems to be a Schottky dual-diode BAT54C.

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  • \$\begingroup\$ Shorted capacitor somewhere? \$\endgroup\$
    – winny
    Commented Mar 4, 2022 at 12:56
  • \$\begingroup\$ Just a guess, does the CMOS IC control the power distribution in some way? If so operating without the CMOS IC in place may be causing the excess power consumption. \$\endgroup\$
    – RoyC
    Commented Mar 4, 2022 at 12:58
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    \$\begingroup\$ The battery cell on modern motherboards is ONLY connected to the Real Time Clock, which is usually part of the main chipset. The BIOS stores its settings in nonvolatile memory that does not need to be powered to keep its settings and should not be connected to the battery. \$\endgroup\$
    – StarCat
    Commented Mar 4, 2022 at 13:23
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    \$\begingroup\$ 01B is most likely 810, for 81Ω or 810Ω resistor. Issue is most likely a cracked ceramic cap or partially failed discrete device, like the SOT-23 (if it is even connected to the battery.) \$\endgroup\$
    – rdtsc
    Commented Mar 4, 2022 at 13:58
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    \$\begingroup\$ The BIOS chip you removed is Flash memory. It's not connected to the battery, and stores the BIOS code which boots up & configures your motherboard. It does not store any of the settings, nor does it contain the Real-Time-Clock. In the distant past the settings & RTC were contained in a separate IC powered by the battery, but these days those functions are built into one of the large 'chipset' ICs. \$\endgroup\$
    – brhans
    Commented Mar 4, 2022 at 15:59

1 Answer 1

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The red component, SOT-23, marked L43 is BAT54CT, dual-diodes. This component was dead and kinda shorted on all the pins (<5ohms). I injected 3v instead of the battery and the voltage was then redirected to the main 3.3v inducing a 3mA current.

I replaced it and it works now. The current went back to single digit uA.

Thanks to all the commenters who helped me!

BAT54C

https://www.allicdata.com/news/generally-electronic-semiconductor/cmos-circuit-composition-and-working-principle.html

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