I am looking at buying a van. The owner advised me that since he started parking his van on the street with the car alarm armed, the battery drains after 3 days. He thought it may have been the new stereo that was installed in July.

He went back to the installation shop and they asked him if he arms his alarm and lets the van sit. They changed some fuses for him and asked him to leave it sit again.

He has not let the van sit long enough to test if the problem has gone away.

Could a car alarm drain a battery in 3-7 days?

 

sure if the battery is going bad. You don't say how old the battery is.
A bad battery can discharge on its own in a few days.

 

has brand new costco battery. Stereo shop replaced some fuses and retested -approximately 13 Volts at battery. No discharge in 5 days.

 

If the battery is new then there's obviously some sort of parasitic charge going on somewhere, and if this only started after a stereo was installed then I'd say that something in the stereo is drawing power at all times. It doesn't take much to drain a car battery. An interior light or even a small glove box light can drain a battery overnight.

 

I asked the owner to let the vehicle sit idle for a week to determine if the problem was corrected. He wants to sell and said no. I politely declined to purchase the van as I don't want to be part of an experiment determining if replacing a few fuses worked. I have seen enough vehicles having batteries replaced for battery drain problems as it usually never solved the problem. Stereo and alarm installers don't always get the wiring done correctly and the problems can pop up years down the road.