2009~2013 Bad battery drain and rough starting

Okay guys here's a question. I don't post much but when I do it's usually something stupid because I've done something stupid and this is no different so I needed to put a new head gasket on my 2009 Mazda3 2.0 liter. I thought I blew the head gasket, took it to the mechanic he said I had no compression so it looks like you need a new motor so I just took him at his word so I went and got a motor and when I started to take my motor apart to get it out of the car I disconnected the everything but mainly the wiring harnesses not just from the the PCM or ECU but from the motor itself where they just sat on the ground and I didn't label anything...So now I'm at the point where I'm trying to start it and I've got a bad battery drain and it won't stay if it drains while I'm trying to start it with a charger on it, brand new alternator, brand new starter and not a brand new belt...All suggestions would be very appreciated.
 
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South Carolina
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12 MZ5 13 CX-5
Use a DMM set to current measuring. Unhook the positive battery cable. (Or negative, doesn't really matter). Place one lead on the battery terminal, the other on the battery cable. Look at the current draw. Now, pull a fuse, see if that number changes. No change? Replace the fuse and go to the next one. Rinse and repeat until you find the circuit that has excessive draw.
 
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2010 Mazda 5 Sport
Should be obvious but on a battery drain test, where you have the Digital Multi-Meter in-line (in series) with one of the battery terminals, do NOT attempt to start the car. You will fry the meter. This also works for SMALL drains. Like 3amps or whatever the meter is rated for.

ALTERNATIVELY:

So, more and more with modern cars, when you disconnect the battery and reconnect it with the DMM in-line with the battery, the car will "wake up" and you have to wait 1-2 hours for it to go to sleep and have it cease its drain.

To prevent this, using a DMM with the clasp/clamp probe to verify current draw is preferable as it just clamps around a battery cable...no disconnection necessary. You record this. Then you take the DMM and measure voltage drop across the fuses and there are charts out there that tells you what the voltage drop across that particular rated fuse equates to in regards to current drain. You measure all fuses until you get them to add up to the total draw.

Another benefit, is that you can't destroy your multi meter this way.

I don't believe our cars are new enough to require the alternative test. But some people find that easier.

I'm sensing that the battery drain is SO BAD that OP can't get the car started from a fresh battery. If this is the case, I would NOT put the DMM in series with the battery. Something is seriously wrong. If your battery is getting hot...then you are in extremely dangerous territory. This is where cars burn to the ground territory.
 
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