If you're going to do this, do what the posters above suggest and leave the battery connected. Battery chargers do not often incorporate much filtering since the battery filters the AC ripple/noise. If you disconnect the battery, that ripple/noise will be going to all the electronics in the car and depending on the charger, that could cause damage to the electronics. Some of the newer switchmode chargers are much better for filtering (because they aren't just a transformer and diodes) but there's still no reason to remove the battery.
If for some other reason you really want to disconnect the battery, then I'd suggest you get yourself a good high current 13.8V regulated power supply like what Ham radio operators and car audio shops (for demo vehicles) often use. These are easy to find on eBay (search for "Astron") and probably at Ham fests.
-Rod