Help with battery issue diagnosis

[bad_panda]

I bought a new (decent) battery for my car a month or so ago as it'd been struggling to start and then died altogether. It was fine up until a few days ago when it's been struggling to start again.
On doing a bit of diagnosis yesterday I found that the battery voltage was very low at 12V when I first checked it with the engine off. I spent a while checking for parasitic current draw but the max current draw was only 20.5mA, so that seems fine.
With the engine idling the battery voltage increased to 13.3V, which is obviously still low. After a long-ish drive, the battery voltage increased to 12.3V, so it's being charged but just barely.
What's confusing me is that when I check the voltage at the alternator itself (i.e. using the positive on the alternator connector as the + and the metal case of the alternator itself as the ground), it reads 14.3V at idle, which is spot-on as far as I can tell.

Does this mean that the alternator is ok and the fault lies somewhere in the wiring between the alternator and the battery, or am I measuring the alternator voltage incorrectly?

[Polly Grigora]

Now connect voltmeter between battery positive and alternator positive and it will show the volt-drop along the alternator positive supply

Nicely done, you've found the problem, alternator positive supply needs sorting

[bad_panda]

I've just measured that volt drop at idle from a cold start and it started off at 0.25V then dropped to 0.05V after a few minutes. Does this sound too high or is it vehicle dependent?
I suppose ideally I need another pair of hands to wiggle the cable around a bit while I'm measuring it (the multimeter probes are the spike kind for electronics rather than electricals) as it could be a loose connection.

[Panamax]

What's the car?
Does it have start/stop?
Does it have an AGM battery?
If so, is the new battery absolutely IDENTICAL to the old one in EVERY specification?

[bad_panda]

It's a 1998 Mitsubishi Shogun 2.8 TD, so no start/stop and just a lead acid battery.

The new battery is essentially the latest/updated version of the Yuasa battery that was already in it, as per the Mitsubishi spec.

[Panamax]

OK, fair enough. A loose or corroded connection may well be the culprit. I'd start by checking the condition/tightness of the battery clamps.

Beyond that, getting an automotive electrician to have a look might be your easiest way forward.

[bad_panda]

Yeah I was thinking maybe it's worth getting an auto electrician involved if I can't see anything obvious. I've at least narrowed it down a bit.