PC Mainboard - CMOS Bios Battery Circuit

[Harold Bradshaw]

I could imagine the CMOS Bios Battery circuits are quite similar on all recent mainboards.

What does it look like and how does it work?

Background:

My mainboard started draining the bios battery (an ASRock H55M). I inserted a fresh battery with 3.3v. It only measured 2.9V when I removed it, after the PC had been turned off for about 8 hours. While booting, the BIOS is showing the error message "CMOS Setting Wrong" in combination with "CMOS Date/Time Not Set". My suspicion is, that the battery voltage - while being drained by the mainboard - likely dropped below the 2.9V and caused the error.

So started wondering what could have failed and what this circuit would look like.

 

[DPG]

Welcome to the forum.

First things first, are you using good quality batteries from a reputable source? There are a shocking amount of very poor ones on the market.

 

[Lucien Nunes]

Yes indeed. As well as low capacity and low terminal voltage, stale / faulty batteries can sometimes have very high internal resistance which is confusing if you measure them out of circuit. The off-load voltage sometimes looks OK to a multimeter with 10 megohms input resistance, but a few microamps of RTC load drags the voltage down once in circuit.

Alternatively, you might have a leaky capacitor draining it. The next test is to connect a meter in series with the battery and motherboard and measure the actual load. Many ordinary multimeters don't measure right down to microamps but a measurement can be made with say a 100k resistor as a shunt and the lowest voltage range. The drop in the shunt must not be so high as to artificially reduce the load e.g. by stopping the clock oscillator, perhaps aim for 0.25V. If the load is excessive, search for a small electrolytic with battery volts on its positive side, A less likely scenario would the the RTC . CMOS RAM itself, or even a leaky supply diode letting battery current back out onto a 3,3V rail.

 

[w0z]

I could imagine the CMOS Bios Battery circuits are quite similar on all recent mainboards.

What does it look like and how does it work?

Background:

My mainboard started draining the bios battery (an ASRock H55M). I inserted a fresh battery with 3.3v. It only measured 2.9V when I removed it, after the PC had been turned off for about 8 hours. While booting, the BIOS is showing the error message "CMOS Setting Wrong" in combination with "CMOS Date/Time Not Set". My suspicion is, that the battery voltage - while being drained by the mainboard - likely dropped below the 2.9V and caused the error.

So started wondering what could have failed and what this circuit would look like.

Higher probability that the new battery is at fault, voltage measurements as said are very deceptive on that type of battery, buy a good one and do a very quick short circuit dab current test on a suitable current range (100mA or 200mA) (- an analogue meter is better for this), then do the same test on a suspect battery, you'll find the current swing is very low in comparison to a good one. If the battery was good and you then suspect the motherboard there's a typical cmos power circuit here, should give you some idea what to look for. Have you looked at the board to see if there's any liquid damage