I apologise if this has been covered before but I couldn't find it.
Firstly, as far as I can tell the 1.35litre mazda 2 up to about Sept 2010 is in £135 road tax band.
From about Sept 2010 it is in £30 tax band. (ours is Dec 2010 in £30 tax band).
Not sure exactly what Mazda did to change it but I think the bhp went down by 2 and they added a battery monitor to the negative battery terminal which helps reduce the alternator drag on the engine as far as I can make out.

The problem; Bought car in 2015 2nd hand. In 2017 the battery failed which I think is safe to say is original battery.
I replaced it with a budget brand battery and a battery size which I think was same AH rating but 1cm less high as the proper size one was quite a bit more expensive and I could just put some spacers on the battery retainer.
Sometime later in 2021 I was thinking the battery did not seem to spin the starter at a reassuring speed and then when very cold in winter the battery did not have enough power to start the car. I did some checks with a voltmeter checking voltage with engine running and concluded that my 4 year old budget battery was no good. So I replaced it in Dec 2021 (with another budget battery of incorrect size). Not wanting my wife to be left stranded

After that, it's never failed to start but I still think it sounds like the starter does not spin at a reassuring speed. Not wanting my wife to be left stranded, and with weather getting colder (and lights now on for most journeys which are always short) I thought I would do some checks. Voltmeter on battery terminals shows 12.2 volts (nothing turned on), turn ignition on, goes down to 11.8 volts. Start engine, goes up to 13.1 volts but over a period of about two minutes gradually comes down to 12.6 volts(including when revving it, not just when idling). Turn headlamps on, goes up to 13.1 volts (including when revving it, not just when idling). This does not sound all that reassuring.

So I checked/ cleaned connectors including taking earth straps off. made no difference.
I own a Ford Fiesta and on Ford Forum there is often mention of resetting the battery monitor when changing the battery and it says how to do it (and battery light on dash blink to confirm when done). This does not seem to work on the mazda. I can't find anything which tells me how to reset the battery monitor to work properly with a different battery. Does anyone know how?

What I have done now is unplugged the two thin wire plug from the battery monitor on the negative battery terminal connector. Volts at battery terminals is now 14 volts with lights on or off, idling and revving. So that seems good and I would imagine is exactly as a pre Sept 2010 Mazda2 would be. Does anyone agree or dispute that?

I am hoping that there is no downside other than the fuel consumption and emissions will be 0.001% higher than they were before.
so
a) can anyone tell me how to reset the battery monitor system to adapt to my present battery?
b) does what I have done sound OK or am I missing something which will end in disaster (leaving the battery monitor disconnected - although I think that just makes it the same as a pre Sept 2010 Mazda2)?

(Alternator is 90amp and says FoMoCo on it (Ford) which seems to match the pics of Mazda2 alternators for sale on ebay)


updated a few days later: with the battery monitor at the negative battery terminal disconnected the dash battery light is on which is a bit annoying but it's definitely charging as the voltage is 14 volts. but, of course if the alternator later fails or belt comes off I have no extra warning

 

Let me tell you about my Mazda 2 2011 (UK)
In 2020 My original battery died - so I've purchased the CHEAPEST in...... Euro Car Parts - so far....... I've had 4 battery replacements as they sell them........ worn/damaged even their tool they CONFIRM battery is....... WORN (after just 3-5 months after replacements)
My multimeter shows CORRECT charge rate