Battery Drain

I have a 2014 mustang GT convertible 5.0L it has an intrusion alarm that was installed before i purchased the car.I purchased the car June 2019 and from what i gathered had already had the battery replaced as it didn't have a motorcraft one it had a motomaster one installed,in march 2020 i replaced it with a new one as for some reason it would not hold a charge that long and now it's Oct 27 ,2021 and the battery is dead flat again i know that one of the main reasons is i don't drive it i have only put 1800 km on the car since i purchased it so i was wondering how long do these cars usually hold a charge for i last drove it Saturday sept25,2021 and i went to start it today and it's absolutely dead, if there is that much of a draw on the battery will my 2A battery tender keep it charged....i will charge the battery this weekend as we are getting slammed with rain at the moment and saturday is supposed to be sunny maybe i'll get it charged and put a permit on it....i was wondering if there is anywhere specific i should look at for the draw

You could do a parasitic draw test, but chances are you won't find much. Letting it sit is hard on a battery. Keep a battery tender on it. Start it occasionally.

i just got back from the auto parts store i needed to get a new battery charger as the one i had got to 71% and then through an error code and would not start charging again....the weather is supposed to change this weekend for the better but get a little colder so i will spend some time looking things over i never have an issue with it in the warmer weather just when it gets cold...but like i say i haven't driven it as NOBODY in my family will ride in it so the car just spends it's time sitting i also need to invest in some moisture grabbing pellets (DRI-Z-AIR) so the car will be going to the highest bidder when i find them...i did manage to get a name brand battery maintainer (battery tender) that i can mount under the hood and directly to the battery connections so i just have to find a spot out of the way for it