Exactly. Like I say, my car has its original 2015 battery (60,000 miles) but a free Halfords test proved that it still holds a sufficient charge. Why don't you pop somewhere so you can confirm what condition yours is in, just for peace of mind?
When I first looked, Eurocarparts was quoting around £450 for a new battery! Halfords' equivalent was around £150, but I'm sure you could find decent alternatives elsewhere. However, the support battery I changed was only around £35, so it didn't take as long to think about!
From what I understand, the main battery starts the engine, whether normally or through Stop/Start, and presumably powers most major electrical systems. The support battery powers the car's interior systems (infotainment, climate control, etc.) when the car is stopped during Stop/Start, but the main battery still starts the car again. It's also meant to take load away from the main battery so everything doesn't dim while starting the engine.
It explains why mine had no real problem starting, but presented strange issues with the sound cutting out and the 'battery charging' message.
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