Very confused about what batteries I need

[cky_stew]

Hi - I've got a gen 2 2009 mazda3 LF engine with iStop.

I've had issues since getting the car a few years ago with batteries draining themselves dead if the car has been sat there for 5-7 days without usage.

Been a good 2 weeks and they are both totally dead. AA was confused about it (both times they've looked at it).

The manual says I need 12v - 36ah/5hr for the main battery. And 12v 21-AH/5hr for the sub.

I'm really not sure which is which - there is a large one and a smaller one.

Pic here:

 https://photos.app.goo.gl/4bVfZY5RyaxKfSFu7

I think whatever batteries are in there may not be suitable for the car. But really I would just like to know exactly what I need to replace them with as my local mechanics and the AA are just baffled about it!

I don't mind splashing out to make sure I get exactly what I need - however I'm not a fan of the iStop anyway I must say.

Any help would be massively appreciated.

[qbanista]

Hi, I've got the same model and went through the batteries research myself.

So for the main battery (the big one closer to the front of the car) you need an EFB battery (due to i-stop) with at least the OEM specs (>36Ah), which is quite hard to find in this size. I concluded that non-EFB battery (as the Halfords one in your car) is just not suitable, even if you're planning to manually switch off the i-stop every single time. It will just go flat much more quickly and might cause some ECU errors. I went for Yuasa YBX7053 (https://www.yuasa.co.uk/ybx7053). It seems retailers stock 2 different versions, 370CCA and 450CCA, I assumed the higher spec is better. Best offer: £135 on eBay.

As for the sub one (the smaller one closer to windscreen), it doesn't have to be an EFB and the choice is much wider (>21Ah). I went for Yuasa YBX3009 (https://www.yuasa.co.uk/batteries/automotive/ybx3000-smf-batteries/ybx3009.html). Best offer: £45 on eBay.