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Friday Mailbag: My BMW’s battery keeps dying

Welcome to the end of another week with the Friday Mailbag. Next up, it’s BMW batteries. Why are they so complex, and why don’t they always work?

If you’d like to participate, drop me a question at mike@machineswithsouls.com. And don’t worry, you’ll remain anonymous.

Question

Hi Mike! My question is in regards to the battery on a BMW. I’ve had to replace mine multiple times, but the car is only a few years old, and I do drive it regularly. Well, semi-regularly.

The dealer insists I need to “code” the battery too. It sounds like they just want more money out of me. Any insight would be appreciated.

– Dan

Answer

Dan, you don’t need to get out the pitchforks yet; the dealer is telling the truth. Sort of. Let’s start with what makes BMW batteries so special (i.e. difficult).

You drain me

Remember last week how we talked about some reliability issues that stem from simply not driving the car enough? Well, meet exhibit A.

“A” can stand for alternator here, because it works a little differently on a modern BMW. Around 2006, they introduced something called “Efficient Dynamics”, and maybe if you’re old enough, you remember the window stickers showing off how efficient you are.

One of the components for this is reducing the amount the alternator charges the battery. As you drive around town, the alternator is programed to charge the battery at a lower rate. Once you enter the highway and cruise at a more constant speed, the alternator is awoken from slumber and asked to send more love to the battery. Overall, this helps to reduce parasitic drag on the engine, and give you better fuel economy.

Your BMW is pretty smart too. If you’re using a lot of things at once, maybe the defogger, stereo, wipers, headlights, etc all at once, the car knows to increase the charge to the battery even if you’re driving at a slower pace.

bmw-f80-m3
Modern cars like this F80 require a lot of juice to keep everything running.

Age is just a number

Every battery, from your kid’s toy to the one inside your BMW, is meant to degrade over time. It’s just how the science works. If you have your car’s battery at 100% the day you buy it, by the time you’re a year into ownership, you’ll only be able to charge it up to say, 98% of it’s original capacity. No doubt you’ve noticed this with your cell phone.

Eventually, it won’t be able to be charged up enough to start the car. But in BMWs, there’s a computer that senses just how tired the battery is and adjusts the caffeine sent. This helps to ensure the battery lasts as long as possible by not getting overcharged.

Give me the code

Eventually, when it’s time to replace BMW batteries, you go someplace and just pop a new one in, right? Nah.

The dealership needs to hook up your car to a computer which tells it “Hey, new battery. Please don’t charge is as much as the old one k thx bye”.

If you don’t code the battery to the car, it won’t explode. But it won’t last nearly as long, and could cause a pretty Christmas tree display on your dashboard (assuming one isn’t there already).

Why do BMW batteries wear out so fast?

I’ve toured the BMW plant in Spartanburg and watched them build an X3 from scratch. They had said that if you unraveled all the wires from the car and tied them end-to-end, it would be something like a mile long (don’t quote me on that, but it was an astonishing number). An F30 335 may have over 100 actuator motors and control units, all of which need to be supplied with power. Even when the car is off, power is needed for systems like your alarm or Connected Drive, otherwise known as “parasitic drain.”

The point is, BMWs have a lot of electronics. That means they need more power, and if you only drive your BMW locally, you’re probably not giving the car enough of a chance to charge the battery.

Trickle me pink

If you don’t drive your BMW that often, you might consider a trickle charger. It’s something I utilized with my E92 when storing it for the winter, or in between periods of rest for the car.

You can find out more about storing your BMW here.

bmw-f80-m3-engine
You can hook the charger into the battery under the hood, even though the battery itself is in the trunk.

With that routine, I kept the original battery in the E92 for almost 10 years, though it was due to be changed out.

Consider also the type of BMW battery you have, which will effect the charger you get. Most popular is the one made by CTEK, and it knows when the battery is topped off so you don’t over charge it.  It’s important to use the right charger because…

BMW batteries are special

I’m not sure which car you have Dan, but BMW actually picked the F80 M3 to install a new type of battery, a lithium ion variant as oppose to AGM or lead-acid. There are some good reasons why. Alert – science ahead:

  • Li-ion batteries are maintenance free.
  • They weigh less than AGM or lead-acid batteries by as much as 50%.
  • They offer higher current consumption, so they can achieve a higher state of charge under load.
  • They offer increased cycle stability and higher current consumption, which results in faster charging.
  • They’re able to completely cycle around 14 times more than a conventional lead-acid battery, making them suitable for use with the automatic engine start/stop function.
  • The battery life is about double that of a traditional lead-acid battery.
  • They’re safe – lithium iron phosphate cells generally have a lower hazard potential than other lithium-based batteries.
M3-side
BMW introduced the Li-Ion battery with the F80 M3.

Compare that to say, the battery in an F10 with an N63, which pretty much always died, and you’ll understand why BMW made the switch. You can tell which battery you have by going into the truck and checking the label. If it says LI-ion on the top – you got it.

To help you out, check out BMW’s battery page and find your specific car.

I’m not sure what the difference in “Advanced” and “Performance” is – but the cost includes coding. Some cars like the F80 only list the Performance battery, so assume that would be the Li-ion version.

Can you code a BMW battery on your own?

In short, yes, if you’re technically inclined. Apps like Carly go on your phone and allow you to connect with your car to code in a new battery. If you have multiple older BMWs, it might be worth the investment.

So Dan, your BMW might have 99 problems, but a battery shouldn’t be one.

Cheers!

bmw-f80-m3

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