;^) My old D600 was able to use two batteries at once, so it's possible your Inspiron 4150 would be able to, too.

Pull the main battery out and look into the cavity where it goes.  You'll be able to see a row of electrical contacts at the back of the cavity.  These correspond to the contacts on the battery itself, obviously.  If you pull the removeable media drive out, too, you'll see a similar pair of contacts for powering and communicating with the media drive.

Well, on my D600, the second bay has two sets of contacts, one to connect to the media drive, and a second set to connect with a battery.  Soooo, because each bay has the appropriate contacts for batteries, it's possible to plug two batteries in at once.  They'll be connected in parallel, which means that the voltage will be the same, but they'll last twice as long....

If your Inspiron 4150 likewise has battery contacts in the media bay, you should be able to use them both together.  As a test, try removing the regular battery, and running only with the second battery.  If you can boot up, then you can use batteryies in the media bay, and thus you can use them both together.

;^) The only trade-off is that you won't be able to use the media drive at the same time.