Today we’ll take a look at how to charge a 12V battery in your BMW. You’ll also learn how the BMW battery charging process is different from conventional systems.
Let’s get started.
Where to Connect The Trickle Charger?
If your BMW is equipped with a lead-acid or AGM battery in combination with an intelligent battery sensor (IBS), you must connect the trickle charger at the jump-start terminal points in the engine bay.
This way the IBS will recognize the charging. If you charge the battery directly at battery terminals or removed it from the car, this could lead to misinterpretation of the battery condition or unwanted check control messages and fault memory entries.
For BMWs with lithium-ion 12V batteries, you can use both jump-start terminals and battery leads. BMWs with lithium-ion batteries use built-in battery electronics instead of intelligent battery sensors (IBS).
Forget The Cigarette Lighter Socket
The 12V charging socket is supplied with voltage by the power distribution box via relay. This relay drops out after the ignition is turned off. This means that the trickle charger will be disconnected from the battery.
Which Battery Charger to Use?
Theoretically, you can use any 12V automotive battery charger, but I strongly recommend using a trickle charger. If you are willing to invest some $150-200 for a battery charger, look no further than CTEK MUS4.3 Test&Charge.
Pay attention when charging a lithium-ion battery! The charging voltage generated by standard chargers for lead-acid or AGM batteries is too high for the lithium-ion battery.
If you want a charger for both normal and lithium-ion batteries, I recommend going with a BMW Accessory Battery Charger (made by CTEK for BMW). The price is about the same as CTEK.
How Long Does it Take to Charge a Battery?
This solely depends on the battery state of charge (SoC) and size (Ah). For most BMW batteries (80-110Ah) charging an empty battery will take about 20 to 30 hours with a 4-6 ampere charger. To boost the battery enough to start the engine will take about 4-6 hours.
Charging The Battery While Driving
If you’re considering charging the battery enough to start the engine, and then let the alternator do its job, do it only as the last option. Some people think they can just jump-start a car and drive around to recharge the battery, but it doesn’t work this way.
In most cases, the alternator will only add a surface charge to the battery. That means you’ll be able to restart the engine only for a limited time, like 10 to 20 minutes after you’ve turned off the engine. And after a few hours, the battery will be dead again.
The batteries’ main purpose is to start the engine and then work as an electrical load balancer.
So, if you need to recharge the battery, do it with a charger.
Intelligent Alternator Control (IGR)
To make the engines more efficient and prolong the battery life, most modern BMWs (2005+) use intelligent alternator control (IGR). This means the alternator charges the battery based on a complex algorithm. In simplified terms, that means that the alternator is charging the battery only when the engine doesn’t consume fuel.
Here’s how it works.
The IGR is a part of the Advanced Power Management (APM) in BMW cars with internal combustion engines. The APM system has three main objectives:
- Save fuel (up to 3%)
- Prolong battery life
- Secure the engine start
For this strategy to work, the alternator no longer fully charges the battery because a fully charged battery cannot accept any more energy.
Here are three phases in which IGR operates depending on the battery state of charge:
- IGR Low
The IGR-Low phase starts when the battery state of charge drops below SOC_S1 (state of charge, stage 1). The IGR increases the alternator voltage during the overrun (coasting) phases. The battery charge level increases with an increased number of coastings and their duration. The state of charge can reach 100% during the IGR-Low phase.
- IGR Medium
This is the sweet spot – when the battery state of charge is between about 70 – 80% (between SOC stage 1 and stage 2). During the IGR-Medium phase, the alternator only maintains the battery at this level.
- IGR High
In this phase, the battery is discharged and partially supports the vehicle’s electrical system.
So, after you fully charge the battery, your BMW will start at the IGR_High phase and slowly drain the battery to IGR_Medium. And that’s the best way to do it.