How to prevent battery from charging while on UPS

I'm running on UPS power and would like to prevent the laptop's battery from charging, to increase the amount of power available to other devices. Is there a way to do this?

update

The machine is a Dell Latitude D400. If people want more details, just ask.

Also, I'm gathering that I need to explain my desired setup a little better. I've gotten a bunch of suggestions about taking the battery out. I'm not sure if people are suggesting to take the battery out while the machine is running — this, as I understand, is not a good idea with most laptops — or to just remove the battery altogether.

The latter option is not optimal, because ideally I'd like to use the 30-60 minutes of power in the laptop battery and then switch over to UPS power. The details of the switch-over may constitute a separate question, but if I can't find a way to keep the laptop battery from charging, then removing the battery from the machine altogether may be the best way to do this. I'm not sure yet if this machine will run without a battery, but I'll check that out.

Other than the laptop, the UPS is just supporting a cable modem and router and a USB hub.

Again in the idealized version of this setup, all the power management changes would be automated, i.e. not require replugging anything or pressing Fn-keys. I'd like the machine to start using laptop battery power when apcupsd indicates that the UPS A/C is out, and then start using UPS power, but not charging the battery, when the battery is almost depleted.

At least some Dell laptops have a key that you can hit to disable battery charging. I think it was Fn-F2 or so. The key has a little blue battery icon on it.

Given that manual intervention on your part would be required, I'd suggest a low tech workable solution. Plug the laptop into a switchable power strip and plug the power strip into the UPS. Switching off the power strip will stop your laptop from charging and the laptop battery takes over powering the laptop without missing a beat. The UPS will now have more available power for your network components.