Successful 12v battery change

oldstumpybiker · 2025-03-08T11:15:35-0600

Not much remarkable in my replacement of 2015 Rex 12v battery, except the old one, as I thought, was the original, so 10 years old, no issues, was still working fine, only changed it because I thought I should. Still keeps a decent charge so I'll keep it just in case for a while.
Noticed the negative terminal adapter was loose, this hadn't caused any issues, maybe the cable clamp bolt was tight enough to compensate.
I noticed though the labels on old & new are a bit different, attach 1st pic the old, 2nd pic the new, although new one bought from BMW dealership. Some values are different. Any thoughts, should I be concerned?

Mondomensch · 2025-03-08T11:58:04-0600

Are they different sizes? Hard to tell from photo if old one is wider. Certainly new one seems rated at smaller capacity.

alohart · 2025-03-08T12:25:32-0600

The original battery is 20 Ah whereas the new one is only 16 Ah. With a lower capacity, its charge level would decrease more rapidly when the car is parked for a while (e.g., while away on vacation). The DC-DC converter would probably be on more of the time to maintain its charge level. It would cycle more deeply and more often, so might not last as long. However, your original battery lasted 10 years, so this one might last only 8 years. None of this seems critical because when an i3 is running, the 12 V battery capacity is effectively that of the HV battery pack because the DC-DC converter is powering an i3 most of the time that it's running.

I haven't seen this battery in the U.S., but if a BMW is selling it as a replacement, it's probably good enough.

eNate · 2025-03-08T12:29:30-0600

What's puzzling is the new battery has the identical part number to the old – can't get anything closer to a like-for-like replacement!

I am curious about the 2015 date on the new battery.