A long shot but is there a common issue with the 300 that drains the battery? 

I'm sure it’s intermittent but didn’t think much about it as I put it down to a dodgy battery but fitted a new battery a month ago and had the kill switch turned off so all was fine. Yesterday I left the kill switch in at 8:30am and by 2am my alarm starts going off!! Couldn’t turn it off or open the doors so pulled the kill switch. I don’t want to be leaving it like this if it can drain a brand new battery in less than a day. 

I've never had a problem on any of my cars unless I leave the hardwired dashcams on for more than 3/4 days, then the battery is flat! My current car can be left for weeks and will still start first time. I'm not much of an expert on electronics, but maybe go around with an ammeter to see what is drawing power?

This guide shows you how to trace a parasitic drain problem:

https://www.wikihow.com/Find-a-Parasitic-Battery-Drain

The boot light switch can be problematic (boot light and interior light staying on).

Alarm not arming properly (fault in one of the sensor/triggers).

Edited July 20, 20214 yr by AndrewG

Thanks guys will try that. ?

when i bought my zed it had all kinds of additional wiring which was not original.

Having the same issue i started to unwire anything that didn't look oem.

If its an aftermarket alarm try disconnect the fuse for it and test the batter 24 hours later.

Other things like the stereo or amps can simply be unplugged.

 

Unfortunately may be a slow game trying to find the fault.

 

I find that if I leave the car for a couple of weeks, with the alarm armed, then the battery will drain.

If I don't arm the alarm, the battery stays charged..... The Zed is safely locked in my garage though, and since the alarm is cat 1, the engine remains immobilised.