So just got the above car and opened the bonnet to find the OEM battery was very old school - not even a sealed battery, it has top up plugs for adding acid. Thought they disappeared years ago. And the car has stop-start so in needs to be a decent battery…
These make for a nice engine bay.
I’ve only looked at ND2 at the dealership when getting parts for my NBFL but I believe the correct OEM battery for a ND2 2.0 is a 32Ah Panasonic Gel length 235mm / depth 125 mm / height 160 mm what you are describing does not sound OEM at all.
It ‘looks’ like it is (Japanese all over it etc) I’m doubting it was a replacement in the first year of the car’s life, sourced in the UK
If it was not driven a lot and or had a battery drain could well have been replaced.
My ND2 battery 2018 had caps for topping up. Replaced at 5 years.
https://forum.mx5oc.co.uk/t/nd-battery-failure-after-1-year/142683
See the link why the battery might have been changed after 12 months.
Apologies it appears that NC and ND do have lead acid batteries the Panasonic number for ND2 2,0 should be Panasonic 46BL24(S) which is 45amp CCA 325. Interestingly Mazda recommend replacements with a CCA 500 rating, sorry not been able to nail down a number for a Panasonic with CCA 500.
The batteries on the ND (with I-Stop) are enhanced flooded batteries (EFB), yes although quite “Old Skool”, have more plates than a conventional battery.
Yeah - I was just surprised to see a battery with those acid top-up ports, EFB or not 
On a separate point, do any of the electrics get ‘confused’ for a while if you disconnect the battery? And when it comes to changing it, does the battery management system need coding to tell it there’s a new battery? (Coming from Audi - a 8 year older car where all this is true)
The only thing you lose by disconnecting the battery is some of the trip data. This 2016 ND Shop Manual | ND Shop Manual (hexorcism.com) is a useful source of information even though it references ND1.
If your stop/start does not work it usually fixes it if you disconnect for ten minutes.
This Mazda I-stop Explained - Idling Stop - Discover Your Mazda explains how the system works, but in essence it does not need a big battery as the ‘combustion’ element removes a lot of the strain on the battery.
