starting/electrical issues

[Jezza_hr]

Okay, I don't know where to begin with this one, as this issue is a rough one to diagnose and there are several different symptoms that could give clues as to where the issue could be. But here goes.

Since rebuilding my car in the summer and relocating my battery to the boot I have been having an intermittent starting issue where, from time to time, the starter motor and open circuit relay may click on and off on their first and second try and then finally start turning over on the third go. Eventually this either starts the car or kills my battery (where I will then swap in a healthy charged one and get it to start first time) so I eventually end up cycling between two batteries as one discharges over time from all these rough starts. The clue that the battery is about to die is when I'm met with a rapid clicking sound from the open relay as the car is trying to turn over.

However, once the car has been running for a while and has warmed up, the starting issue seems to be next to non-existent, but if the car has been sitting idle for a day or two, the same issue comes back.

When checking over the voltage of everything, there's a standard 12.5V through the the circuit when nothing is running and 14v at both the alternator and battery when it is, so that tells me it should be charging the battery, so I don't think the charge circuit is to blame.

Obviously, one thing that needs to be done when relocating the battery is making sure to get a good ground. Having extensive lengths of 4 gauge wire, I have done my best to ground everything in the best way possible with one ground from the battery to the boot floor and another running to the chassis leg to the front of the car where the block is also grounded, so I assume this wouldn't be an issue?

One other thing that seems to be a problem. Since I'm now running a tachometer signal switch from my Omex to the OEM dials, there seems to be some oddities when using other electrical items in the car. When using the windows or the indicators, the tachometer drops but the engine speed remains the same. I can only assume this a ground issue or something potentially related to my intermittent starting problem? But this also opens up questions for my charging circuit...

Either way, this issue is becoming infuriating, as I live each day wondering if the car will start as I go to leave the house, or if I will end up stranded somewhere. I'm having to keep one battery charged at all times on the off-chance the one in the car dies the next time it turns the starter motor.

Any pointers on where to begin guys?