When the coin battery on the mobo dies, you either won't be able to boot, get strange error messages/failures or both. Has happened to me twice in the last month (2 different machines). Seems mobo batteries used to last 5-10 years. Now, more like 18-36 months. When you replace the battery, go into "Setup" and check all settings. Removing the battery leaves the mobo in default configuration. FWIW...
On the missus' PC, I had to press some function key (as indicated by the error message) to continue and/or set the time manually and continue. So I was able to boot her PC when the mobo battery was gone. Of course, replacing the battery is the solution to this problem.
In my Dell Precision T3500, I could not enter "setup" (nor "diagnostic" mode) at all without a functioning battery. I assumed that when you rebooted after exiting "setup", your manual settings would be lost again. Maybe it's not the same with all mobos. ??
Quite possibly. This was a while ago and was with an old ASUS P4 mobo. Naturally, the process would have to be repeated if the PC's power was cycled, but after the initial setup of the time and date, the boot continued normally. The Precision motherboards are in a different class so that behaviour does not surprise me.
Most of the time I do not lose my bios settings if I am fast enough on changing out the battery. I have the new battery in my fingers next to the battery holder, try to get the old battery out and the new battery into the battery holder as fast as I can. I write the date of installation on the new batteries and change them out if they have been in use more than about 5 years or I notice any time slippage in the system time posted in the bios standard cmos features. However, sometimes I am too slow or as I am putting the new battery into the battery holder the new batteries + side shorts across the + - contact points of the battery holder and clears the cmos settings ( this happened to me yesterday). I often see motherboards that are about 10 years old and still have the original Panasonic batteries in them without losing the cmos settings.
There may be some weird symptoms when the CMOS battery is out. For me, one case: when Windows booted up, Windows said something was corrupted. Chose the option to "Repair".... after rebooted, Windows no longer recognize the additional 8400 GS graphics cards. A 8-monitor computer was down to 2 monitors working. So for some motherboard, the configuration details (Windows relies on it) are stored in memory powered by the onboard CMOS battery. Battery out, configuration gone, Windows weird. I posted the case here: http://www.elitetrader.com/vb/showthread.php?t=280141
My last episode.... 1. BSOD... citing "shutdown to hardware overheating"... but there was no overheating. Rebooted 2. 1 Beep, system froze. Rebooted 3. Wouldn't boot. POST message, "no drive mounted"... of course there was 4. "USB device failed"... nothing was wrong with any of them 5. Woudn't boot. Diagnostic lights indicated "other failure". So..... I replaced the battery. On next boot, I got message, "Mobo battery low". Of course that was after the battery had been replaced. Squirrely problems when mobo battery dies. FWIW....
I'm more than a little surprised by all this. I would have thought that modern mobos use flash (or some other non-volatile) memory, which doesn't require battery power to maintain settings, and the battery's function is simply to keep the clock running. Very strange.