Hi , fit a power isolation switch . This will isolate battery when not in use. It has to be accessible as door locks will have no power .
Autoelectrician booked for early Jan - I’ll let you know how it goes!
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Hi,
I have a Eunos Mk1, and it’s battery runs down, so I disconnect it each time I leave it for a few days.
I’ve just found out from this thread, that the satnav plugged into the socket, my be draining it.
Also there’s a whirring sound when I connect, I’m not sure what this is.
C
There are many things on modern cars which can leech current; the only way you’re ever going to solve it is via an auto-electrician. It is 8yrs old now, so there could be many culprits.
I hope the sparky can solve the problem for you.
My MK3 does not get driven much in the winter and has been known to flatten the battery, so like above I keep it attached to a ctec charger to avoid the problem of the alarm draining the battery. The battery does appear to be small, but then I have a couple of Volvo’s with monster batteries on diesel lumps.
Good luck OP
I had a similar problem with my Copen and took it to an auto electrician who resolved the problem (bad cell on the battery that didn’t show on a standard test). He recommended I get one of those plug in voltmeters to see when the battery dipped. Nowadays you can get a Bluetooth version that connects to the battery and will synch the data to a suitable mobile phone.
Generally, if the voltage drops noticeably when it’s cold, it’s likely to be a battery fault (experience of Discovery 4 alarm going off at 3 am daily!). If it’s a steady decline it’s a vampire draw which is when you end up chasing boot lights, shorts, handbrake switches (Hyundai/Kia) etc.
Good luck and report back.
Firstly, if not using the car much, just disconnect the neg terminal. If the battery is not faulty itself then the car will start when you need it to.
Go to one of the big sheds like ATS. They will do a free battery test with pro tester.
Finally, use a proper auto electrician to fault find. They usually offer a mobile service.
About four feet, two in reverse and two in first. I’ve bought mine, just to experience RWD and a non-coupe sports car. For now I will struggle through the winter with the TT. I did change my insurance for five days to pick it up and have a looksee….
TBH it’s pretty crude
There was a recall on early NDs because the cruise control would drain the battery if left live when the car was switched off. The cruise control kept operating and drained the battery. Check with your main dealer and get the software update and that will cure the problem.
So, the auto electrician (good chap, fixed my mk2 when it had alarm issues) spent a whole morning investigating. Here’s his summary:
Vehicle charging well at 14.5v fully loaded.
Battery test good. Battery quite new.
Vehicle battery drain at rest is stable 110mA. This is high.
Investigated all possible causes including removal of all fuses, one by one.
The drain never fell below 100mA.
Search vehicle for after-market ancillary equipment and signs of consumption but nothing came to light.
Extensive further investigations are required to locate the issue.
Alternatively the vehicle can be placed on a charge conditioner if left for long periods.
I have opted for the alternative for the moment!
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Did he check for a leaky alternator?
I’ve experienced this on two different cars over the years, although not on MX5.
One was in a rectifier where the sealant was incomplete in the rectifier stack and carbon brush dust was shorting directly across the silicon chip of one of the nine big diodes. A good clean up and some silicone bath sealant fixed it.
The other had a faulty control module, but that was a new car so fixed with a warranty exchange.
It has none of the signs of a faulty alternator, and is fine when used regularly - the problem occurs when the car is left standing for more than a few days.
Often a “leaky” alternator will work just fine.
The test is simple, disconnect both hot wires to it while doing the dark current measurement.
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Do you know how long the auto electrician left the car locked and undisturbed for when taking the parasitic draw measurement?
No I don’t, but i do trust him to have done a thorough, professional job. 
Sadly, he hasn’t found the fault though.
It’s also very frustrating with these type of faults.
From experience don’t rely on the fact it’s a “new/newish” battery.
Mine was tested and they said it was fine, but in the end it WAS the battery that had a defective cell.
Now that happened on my current MX5 NC and was eventually replaced under the battery warranty.
On my Volvo, the main dealer “also” said the battery was fine, (only just 2 years old) but the stop/start would just never work properly.
They eventually plugged the car in and ran a FULL diagnostic check for around an hour and hey presto the battery was duff and replaced and has worked properly ever since.
I would be very surprised if it was an earth fault/bad connection too.
Failing all that he will have “missed something” if it is still continuing to discharge when left standing still as it’s still discharging too fast.
Was all the testing done when the vehicle was left switched off and key removed for a while?
As the vehicle needs to be tested when in sleep mode, (as clearly that’s the problem area).
As on these modern cars a few electrical components will still draw current until they actually go to sleep.
Have you added something electrical recently to the car, as that would also be worth checking.
A good CTEK charger is also a sound investment.
Just an opinion and good luck. 
In the absence of an answer then revisiting things like the battery would be a wise move.