Hi all,
Is there any reason why I can’t use a AGM battery in my 20vt ?

Possibly. AGM have a different charge/discharge profile and from memory have a higher resting voltage.

Would it impact the function of the electronics? Possibly not, but they were never designed for AGM. Does it make the airbag or brakes behave differently in a crash?

Will it damage either the battery or the alternator mixing the charge types? Again, don't know.

Why are you wanting to switch?

There is no problem using AGM batteries on the Coupe. The difference between AGM and regular plated lead acid battery is that the AGM's are better at deep cycling and faster at charging, hence why they are specified for cars with Start-Stop features. The higher resting voltage does not negatively impact the electronics. In fact, the alternator will work less when the resting voltage is higher.

I'd agree it's likely OK, but there are differences in the charge discharge profile. For example modern cars tend to hold an AGM battery less than 100% charge to prolong life as they can be sensitive to overcharge. The deep cycle nature means you don't see a functional difference. The Coupe's basic alternator can't support that. It'll work, but what do you get by fitting an AGM? Particularly as they tend to be a bit dearer to buy

Thank you, the reason is that I bought one for another car but didn’t use it.

The difference in charge discharge profiles doesn't matter. It just means the battery is capable of discharging and charging faster without negatively impacting the lifespan of the battery. Does the coupe's basic voltage regulating alternator need an AGM? Perhaps not, but the lifespan of the battery will definitely be longer compared to a basic plated lead acid battery. I'm an owner of an automotive Lithium Iron phosphate battery brand here in Malaysia, and our batteries have similar characteristics to an AGM battery...albeit with much more cranking power, an even longer lifespan, and in a much lighter package.

Since you already have the AGM battery, just use it mrwiggle