Save ECU settings using i-stop sub-battery? 2013 Mazda5 Voyager

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2013 M5 Voyager
Hi Guys, newbie posting here, hopeful of getting some good advice!

I have a 2013 M5 Voyager with i-Stop. The main battery was recently fully discharged (0 Volts) due to the rear cabin light being left on. I jump started it and ran it for about 15 minutes. After charging the main battery is around 12.6V but overnight it drops to 12.25V.

This seems a bit low to me, so I'm considering removing the main battery and charging it with something like the Ring RSC612. The car normally does short runs and hasn't had a good long run since the discharge.

Although I wasn't unduly inconvenienced by the loss of ECU memory and other settings after the initial discharge, I would like to try maintaining 12.6V across the system while the main battery is removed.

And so to the question:
is there any reason why I shouldn't jump the sub-battery across to the main battery leads to retain the ECU settings (plus central locking) ?

Sundry notes: Both main and sub-batteries were replaced 2.5 years ago by a main dealer, at a cost of £235. I would have expected at least 5 years use from the OEM Panasonic batteries. The sub-battery is in good shape, and keeps 12.6 Volts.

Thanks for any help you can offer!
 
12.25v is fine. Not sure why you're worried about that.

You can get an OBD2 port settings saver for like $10. You can hook that up to the OBD 2 port and a battery, then take the original out. ECU settings will be saved.

Or any 12v source that will put out 100ma to the main cables will do the same. Just don't try to start the car or use anything that will overpower your source.
 
Thanks Negusm. For a regular lead-acid battery, 12.25V represents somewhere around a 50% charge, so I was concerned that the deep discharge may have damaged the battery and limited the amount of charge it may accept. That's why I wanted to give it a long charge using the Ring, which starts using a pulsed voltage to try and reverse any sulfation.

On the memory saver, I just thought it was weird that one should have to use another battery when you might be able to just jump the +ve terminal of the sub-battery to the +ve lead on the main, before removing the latter. I would do it when the main was as fully charged as possible, to get the voltage on the two batteries nearly the same. I just thought it was strange that I've never seen this discussed on any of the threads here. Maybe I haven't searched enough!
 
I would use the battery for a few months, then reevaluate. Half life means you can get 2 more years out of it. My tester on my 4Runner battery says I'm at 50%. It's been saying that for 6 months now.

Using the OBD2 port, you can hook up to a small battery and just have it out of your way while working in the engine bay. It's just handy. Especially when you're doing long jobs. Trying to keep battery terminals connected to something and not shorting on anything is challenging.
 
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