My Friend's computer keeps losing date and time. It is a Dell system that he purchased over a year ago and did not set up until recently. He was "Busy and building an extention to his house". We have, up until now, replaced the motherboard battery three times, but the problem still appears after one or two days. If he leaves the computer connected to the mains, and the mains left on, it retains the time and date. If he unplugs it from the mains it loses the time and date. Please can you help, Dell support cannot.
Thank you
Ted

As you have already discovered, this problem isn't due to a bad m/b battery.
If it was the battery, you would lose your BIOS settings every time you powered down the computer.
Have you checked in the BIOS to see if the date and time are correct there?
How far off are the date and time, or do they reset to Jan. 1st and 12:00?
The computer's time is regulated by a quartz crystal circuit [not exceptionally accurate]. If the crystal is damaged it might cause this sort of problem.
A power supply problem might also be to blame [turning the power on introduces transient spikes in the input, poor filtering would allow them to compromise the time circuit].
The key is the change in power input. I would just leave the computer plugged in, unless power is really expensive and in short supply. Plug it [and other equipment]into a surge protector assuming you haven't already got one.
Your other options would be trying a different power supply to see if that solves the problem or a new motherboard if it doesn't.

Thank you very much for your responce.
The date and time revert back to when the pc was manufactured, 2005.
I shall put your suggestions to my friend and let him decide what course of action we should take. Thank you once again Ted