Looking at the weather outside and winter ahead when my car is not used as much… Any reason not to charge through the 12v “Cigarette Lighter” socket in the passenger footwell? Seems to work, other than it prevents the electrics from shutting down… Just simpler when needed than having the bonnet up:-) Peter
Hi P,
I think you will find that this socket is not ‘live’ unless the ignition is at least in the ‘accessory’ position. This means that you will not be able to charge the car via that socket when the car is switched off as it is NOT connected to the battery.
Hi Peter. Malc is right about the aux. socket, see your handbook section 5-30. I would suggested that you get a “smart” charger (plenty of posts on the forum) with “permanent” fixture leads to fit to the battery then you can just plug it in, switch it on and leave the bonnet closed. The beauty of these is you can leave the charger connected and switched on safely while the car is un-used and it ensures the battery doesn’t deteriorate or go flat.
Roger
You were correct:-) “socket is not ‘live’ unless the ignition is at least in the ‘accessory’ position” it worked for a while but then I guess it timed out and “shutdown” stopping charging the process - plan A no good!
I’ll have to go to plan B with the smart charger (that I already have for my motorcycle) I was hoping to find somewhere to connect the lead meaning I would not have to open the bonnet, but I can’t see anywhere obvious and it is not a huge problem…
Thanks for the hints above:-) Peter
The OBDII socket is always live, just be careful in how much current you feed back up its power pins.
But then you have to open the car, and squash a lead in the door seal, and the connector is a pain.
During the first six months of lockdown when we didn’t use either car, I tried a Solar panel connected this way in my Mazda3 (which shares much of its electrical system with the ND).
Unfortunately the Solar did not produce enough Ampere-hours each day to offset the dark current in the Mazda3. So I’ve added the usual connector to the battery terminals and Smart charge it with lead going in via the bonnet.
you should be able to charge it through the obd connection. this saves opening the bonnet, but you must use a low charge rate. but does not need the ignition on.
Get a smart charger with a hardwire kit, on my MK3 just two wires onto battery then fed wire with connecting plug discretely hiding in the front grill.
Excellent idea.
Thats more or less what I had on my Ducati 999, a capped plug discreetly tucked away behind the fairing, ready to be used instantly.
In the grill would be perfect.
Way to go.
Connector through grille, pulled it out for photo but when connected push it through grille to keep dry. When not connected pull it out and push connector head in next grille segment. Charger is in a Tupperware box under car with slots cut out for wires, it is IP rated as waterproof but feel it’s safer this way.
Ideal
My Optimate 4 Chargers are safely mounted on the internal garage wall and with the maker’s extension lead the total length is about 6.5 metres and the end plugs into the lead permanently fitted to the battery via the grille that way the charger itself is not exposed to the elements. The power lead from the charger to the mains socket is also about 1.6 metres long which give plenty of scope for charger mounting site. 
That is a great arrangement, will have to look behind my bumper, I don’t have the nice honey combe grill you do so would need to find somewhere to attach it neatly so it did not flop about in the wind when driving - thanks for pickie too! Peter
if you do fit a connector at the front; ensure it has a suitable fuse.
The hard wire lead that came with my smart charger does have an in-line fuse.
So does the permanent lead for my Optimates.
The official Madza line is don’t leave a semi permanent charger connected ! If you have a modern Madza with the start/stop when you come up to a traffic light etc. the Battery is different than the normal ones. and the old chargers ,even the ones you leave connected all winter will damage the battery. The latest ones will cope with these new batteries, but as I have said Mazda don’t recommend. Having said that my car 30th Anniversary orange edition is in a block with no mains power. I have a solar panel on the fascia above the door and connected it via a Photonic PU0512EU Solar charge controller and connect to car via the OBD 11 connector by the steering wheel. Photonics says it should be OK and it certainly seems OK ,but I disconnect it occasionally.
The OBD 11 plug has Pin 4 Ground and Pin 16 positive live all the time. But Mazda will not want to know if you mess it up !! I found that Honda took the same attitude to anything added that was not on the car from new !
My 30AE has been connected to a new (bought for it) Optimate 4 since I got it 11 July 2019 with no ill effects. My ND SE-L Nav took over the older Optimate 4 on 27 Aug 2015 that I had for my Honda again with no ill effects. I wonder if you’re confusing “trickle” chargers with “smart” chargers as the former can cause problems because they don’t “read” the battery’s condition and just continue to put the same charge in until they’re turned off.
Don’t forget that the stop/start on the MX5 does NOT use a second battery. The system mostly relies on the fact that one cylinder is stopped fully charged just as the spark is due. When you put the clutch down that plug fires the fuel mixture and starts the car. The starter appears to be available to help if required no doubt drawing current from the capacitor if available.
The batteries on both our MX5s look like relatively low capacity ordinary lead acid ones but others may know differently.
My only concern with having the charging lead in the front grill is that it is a permanent live open terminal that is in the direct line of fire for water. Not ideal me suggests. Having it nice and dry under the bonnet sounds much safer to me. Apparently water and electricity are not compatible.
There is a tight fitting cap that goes over the socket end of the lead on the Optimate to keep the weather out and hopefully others too! 
Yes, that is a typical arrangement.
Most Smart chargers offer functionally similar leads in their kits of bits, even if not quite plug-compatible across brands.
Here is an old pic of my Noco lead, with built-in in-line fuse and waterproof end-cap protecting the connector contacts. The connector fits all the Noco chargers and extension leads.
