When the 12v Accessory Battery Dies

When it comes to Electric Vehicles, one of the bigger concerns of the last decade has been battery durability, and how long the battery can be expected to last.  Earlier EVs, like the Nissan Leaf, BMW i3, Fiat 500e, and Chevy Spark, made use of smaller (sub-24 KWh) batteries which required frequent recharging.  Battery life is shorted slightly with each charge cycle, so these smaller batteries have been worked harder throughout their life, leading to more rapid loss of capacity.  Newer EVs use larger batteries offering over 250 miles of range which no longer require daily recharging.  This, coupled with improved battery chemistry, is leading to batteries that will likely outlive the rest of the car.  This has quieted the concern over battery longevity.

A typical 12-volt battery for a gasoline-powered car.

Of course, this concern focuses on the longevity of the main traction battery.  The second battery in EVs is similar to the battery used in cars with a gasoline engine – it powers accessories and drive-train electronics.  When this battery starts to fail, the voltage level drops which causes the electronics to operate with suboptimal voltage, leading to unpredictable problems.  If it drops too low, the car won’t be able to start because the electronics that control energy flow won’t work.  Replacing the accessory battery has become part of regular car maintenance that EV drivers tend to forget about.  When you give up on oil changes, tune ups, transmission fluid replacement, radiator flush, replacing worn belts and hoses, etc., it’s easy to forget about the need to replace a worn battery.

The skinny 12-volt accessory battery in the BMW i3.

The other day my other half was driving our 2017 BMW i3 and called me from the car with the concern that the navigation system kept blacking out and restarting on the way home.  After a quick Google search, I determined that the 8-year-old battery had reached the end of its useful life and looked into how to replace it.  This task was barely a DIY job, but I found a YouTube contributor with a helpful video.  I was able to remove the battery with a small effort (including shutting off the main traction battery) and looked for replacements.  I found one that almost fit but had to return it.  I looked at another and realized it wouldn’t do either.  I relented and went to the dealer (who was open on Saturday for parts, thankfully) and bought the replacement.  Reinstalling it would have been straight-forward if I hadn’t dropped one of the metric retaining bolts somewhere (I found it the next day).  Once I finished the install and reenabled the main battery, it all worked like a charm.

If your EV is 7 years old or more, have the accessory (12-volt) battery checked soon and avoid the inconvenience of it failing on the road.