XTi on AC adapter giving "Low Battery" warning

[str0be]

Well, the title pretty much says it all, but I'll try to elaborate a little. I have a Rebel XTi, firmware 1.1.1, attached to my XP laptop via USB and being powered by the Canon ACK-DC20 AC power adapter (CA-PS700 and DR-700). I am trying to get EOS Utility v2.5 to act as an intervalometer to do some timelapse recording.

When I turn the camera on with the AC adapter plugged in, the camera starts but shows the battery low (one bar) icon. Under AC power, I am able to take pictures manually (as in, not through the laptop) but when I try to use the EOS utility, I get the following message: "Battery charge is low. Replace battery or use DC coupler." Since I'm using the DC coupler, I'm at a loss as to what I can try to do to fix the problem. Using a battery instead of the AC adapter allows me to use the laptop to trigger the camera, but the battery runs out after only a few hours which is too short for my timelapse requirements.

So, here's what I'd like to know -
Should the camera automatically sense that it is running from an adapter, or is there some setting I'm missing? Anybody else have this problem? Anyone have any suggestions for me to try? Do I just have a defective AC adapter?

I hope you guys can throw some light on this since my internet search has so far turned up nothing.

[SmartWombat]

I'm wondering if there is something power related going on.
When you get 1 bar on the camera under AC power, is it connected to the laptop, or not ?
I presume your laptop is plugged in as well ?
Try with the camera on the DC adapter, and the laptop on batteries.

[str0be]

Even when not connected to the laptop, the camera shows 1 bar under AC power. It's the same when the laptop is on battery.

[SmartWombat]

Thanks, wondered if it was some strange earth return path through the laptop USB connector.
So it looks like the AC adapter isn't giving enough output (can't tell if it's voltage or current limited) for the camera to register as fully powered.

At this point I have to bow out, because I don't have an XTi of my own to compare it to.
I suspect it's a defective AC adapter.

[Norfindel]

If the adapter is on warranty, it would be a good idea to go where you buyed it, and tell them what is happening, or go Canon directly.