Car starts and charges fine if its driven day to day, but as i have a work vehicle it only gets driven once/twice a week.

Gonna have to bite the bullet and pay some $$$ to get it sorted i reckon

Do a resistance check first mate. If its less than 100 Mohm, check your stereo...Most aftermarket stereo setups tend to have power fed to the stereo via a direct feed from the battery and bypasses the accessories relay which causes the discharge. Any sections of wire that are spliced together, make sure they are heat shrinked or appropriately isolated with electrical tape.
I promise you 80% of the time this tends to be the issue.

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STi Ver 6 | 2.1L Stroker | 418whp | 11.5 @ 199kph |

Not fast enough dammit!

Im pretty sure part of the stereo setup is direct connected to the main battery that goes flat. It connects from there to the RedArc, then to the boot battery.

I'll check out the Ohms tomorrow, see what i get

Hmmm a resistance check with voltage present, not the best way.

Disconnect the positive lead on the battery, then remove the stereo fuse. Go between the load side of the fuse, and the negative side of the battery, or the engine block. Also disconnect the battery in the boot.

Failing to do this is like shorting the battery through you meter.

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Race Torque Dyno 399 NM, 248Kw @ 20PSI

Straight roads are for fast cars, twisty roads are for fast drivers. I have or am neither.

http://tttmsp.smugmug.com/

Most multimeters these days are current protected in resistance mode. Well, my ones at work are? But yes, I should have mentioned to disconnect the positive terminal first and go from the positive cable to ground

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STi Ver 6 | 2.1L Stroker | 418whp | 11.5 @ 199kph |

Not fast enough dammit!

My flukes aren't. Your putting in a shunt, and assuming he has the same equipment as you.

Best bet is spend $50 or so at an auto sparkie, or if you have one, tong the battery leads, or disconnect one side and put in a RC ammeter to see how much your using with the car off, and the alarm on, plus whatever is draining the battery. If its more than a free 100 mA it's something else rather than the alarm. If more start pulling fuses and see what that circuit draws.

But if unsure, see an auto sparkie.

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Race Torque Dyno 399 NM, 248Kw @ 20PSI

Straight roads are for fast cars, twisty roads are for fast drivers. I have or am neither.

http://tttmsp.smugmug.com/

With the Brant alarm it used to do the same. When removed I could leave the car for a fortnight or more with no issues.

Seems to be not alarm related as i first thought but stereo related.
Disconnected the big red amp lead from the battery that goes flat.
Also disconnected the leads off the boot battery.
So basically the 2 amps, sub and 2 6x9 speakers are no longer hooked up.
Car started perfectly this morning after 5 days of it being unused.

Getting somewhere now i guess

Are the amplifiers on when the car is off?

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Race Torque Dyno 399 NM, 248Kw @ 20PSI

Straight roads are for fast cars, twisty roads are for fast drivers. I have or am neither.

http://tttmsp.smugmug.com/

Thats the strange thing, no they arent