I never understood why Mazda moved the battery to the engine compartment on the NC vs. the NA/NB where it was nicely tucked away in the trunk.
They also managed to jam it just between the radiator and engine which blocks off all airflow from the upper part of the radiator.

I did a minor relocate while upgrading the IC pump to the space originally occupied by the washer reservoir here:  https://www.mazdaspeedy.com/supercharger-ic-pump-upgrade-and-relocation/
But it had some drawbacks:
- It required a smaller battery which can barely cope with the load
- Weight is till in front of the car, but higher.
- Looks non OEM. Reservoir moved to the firewall does not help either.

Later I also upgraded the IC heat exchanger to a massive lump which adds even more weight to the front of the car so to fix both the weight distribution and a higher capacity battery I moved it to the trunk.

I started off in the rear of the car to check for space and calbe layout. The left fender area was the obvious choice:
- Right fender is already occupied by the jack/tools.
- The positive wire will need to go to the fuse box located in the left side of the engine compartment.
- I can use the existing wire loom grommet on the left side of the firewall.

The wire I used was probably overkill. I was about to use the stock size wire but there are actually two (+) wires going to the the battery so I upped to the next size which was huge.
I kept the wire black for stealth.
The battery I use is this one: https://www.mazdaspeedy.com/new-battery-again/

Cable was routed by the shock tower and isolated with extra rubber hose  trough the sharper sections with risk of rubbing.
I also considered routing it under the car but it feels safer to have the (+) on the inside, and the length of the wire would be longer.

The ground wire is grounded to one of the shock mount bolts. Removed paint to get a good ground to the chassis.

The door sill had room for the wire without modifications.

Spent 20 min on my back under the dashboard in the most awkward positions to finally get the cable trough the lower part of the main wiring loom grommet. Had to drill a hole and use some grease.

The cable was spliced with the original battery cable. By removing some clips and a bit of tape on the OEM loom it reached all the way to the shock tower.
I have never spliced such a big a cable before. Was planning to use a piece of copper pipe as a splice but ended up using two terminals bolted together with some low profile hardware.
The crimp tool required some serious BFH action to make solid crimps.

Finished routing in the engine bay. Used plenty of isolation on the splice.

View of the engine compartment with no battery. Going for the OEM look...

For the battery tie down bracket I chopped up a very cheap aluminium ruler/straight edge from Biltema.no .  Since it's a measuring tool it has decent quality and stiffness.
For the "hooks" I used 6mm threaded rod.

For the (+) battery terminal I'm using this one that I found on eBay . Turns out it's from autolinkparts.com.au so shipping from STRAYA + COVID19 delays took 40 days. "It's the virus mate"

It's nice though with fuses and a couple of separate terminals for upcoming WI pump, but a bit bulky. Nothing is ever good enough so I cut off the clamp and boltet it directly to the MC style terminal on my battery.

I kept the original ground strap from the frame to the engine.
Everything works as normal so I guess this was a success.

I also replaced the MAF IAT sensor with a resistor to give a constant ~ 12C input which results in rock steady 13.8v vs the earlier decline to 13.0v when hot.
Looks like the temp info from MAF is the primary input used for voltage regulations by the stock ECU.  The battery seems to be more peppy on startup now, so this small gel battery might be sensitive to lower charging voltage.  ( Warning! I do not use the MAF or the stock ECU in my setup for anything besides alternator control and some boring environmental stuff, so do not mess with the IAT sensor on a OEM setup as it's used for fuel/spark calculations)