Maintaining and replacing a BMW battery isn’t as simple as it used to be. Modern cars consume electrical current all the time, whether the car is being driven or not. Additionally, the variety of battery types can be bewildering.

Battery types

Lead-acid batteries now come in three primary types:

1. Wet Cell (flooded)

Flooded or wet cell batteries are the most common type of lead-acid battery in use today. An outer plastic shell contains inner cells, each with a a grid of lead plates along with an electrolyte based on sulphuric acid.

2. Gel cell

Gel cell batteries have silica dust added to the electrolyte, forming a thick putty-like gel. This gel material allows electrons to flow between plates but will not leak from the battery if the case is broken. Many gel batteries also use one-way valves in place of open vents, which help the normal internal gasses to recombine back into water in the battery, reducing gassing. These are sometimes referred to as “silicon batteries.”

3. Absorbed glass mat (AGM)

AGM batteries feature fiberglass mesh mats between the battery plates which serves to contain the electrolyte. AGM batteries contain only enough liquid to keep the mat wet with electrolyte — If the battery is broken no free liquid is available to leak out.

Gel vs AGM

More often than not, AGM batteries are mistakenly identified as gel cell batteries. Both have similar traits: non-spillable, deep cycle, may be mounted in any position, low self discharge, and safe for use in limited ventilation areas. But AGM batteries outsell gel cell by at least a 100 to 1.

Also, gel batteries tend to have very narrow charging profiles and can be damaged easily by charging them inappropriately. So, a setting that is suitable for a gel battery may not be suitable for any non-gel battery. Be cautious of any battery charger that has “gel” or confusing “gel/AGM” settings. These batteries are similar, but have very different charging requirements.

Basically, an AGM can do anything a gel cell can, only better. However, since they are also sealed, charging has to be controlled carefully or they too can be ruined in short order.

Battery maintenance

Contemporary BMWs since the 1990s draw battery power all the time, even with the engine shut down. Multiple electronic control units are always drawing current. So, when the car is stored for periods of a week or more with no battery maintainer attached, the battery will discharge and fail to start the car. It is highly recommended that you use some sort of battery maintainer if you don’t plan on driving your BMW for more than few days.

Recommended battery maintainers:

• BMW USA
• Deltran Battery Tender
• CTEK

Battery venting

Due to their mounting locations — usually in the trunk or under the back seat — modern BMWs must provide adequate battery ventilation. Vented batteries have a vent tube, which leads outside the car and works to release potentially explosive gases from the battery enclosure. The failure to install a properly vented battery in these cars can result in a battery explosion. AGM type batteries may not require venting.

Battery registration

According to Mike Miller, technical editor of Roundel Magazine:

“Battery registration” is simply telling the engine management computer (ECU) and other components that the car has a new battery.

The negative battery cable has an electronic control unit on the battery end of it called an intelligent battery sensor (IBS). The IBS measures the state of battery charge and sends this information to the ECU. The ECU uses this information to control how much charging voltage is needed from the alternator to support vehicle electricity needs.

A battery deteriorates normally over time, depending upon service demands and driving profile. The IBS monitors battery condition and sends this data to the ECU. The ECU tells the charging system to increase voltage and amperage output incrementally as the battery matures. Therefore, at battery replacement time, if you don’t tell the ECU it has a new battery it will overcharge the new battery causing it to wear out faster.

The registration process takes place in the ECU, which is the main player in this system. Also, when the IBS sees that battery voltage is too low for normal vehicle operation it can start shutting down vehicle functions to conserve power while keeping the vehicle operational. This may be accompanied by a check control warning of excessive battery discharge.

Battery registration completes the following operations:

• Informs the vehicle that the battery has been replaced.
• Battery capacity is set to 80%.
• Current odometer readings are stored.
• Stored battery statistics (current, voltage, battery charge level) are deleted.
• Stored temperature statistics are deleted.

The following BMW models require battery registration:

• 2002 and newer 7-Series E65/E66 chassis
• 2003 and newer 6-Series E54/E63 chassis
• 2004 and newer 5-Series E60/E61 chassis
• 2006 and newer 3-Series E90/E91/E92/E93 chassis
• 2005 and newer X5 E53 chassis with N62 engine
• 2007 and newer X5 E70 chassis
• 2008 and newer X6 E71 chassis

Battery failure

Some technicians have reported battery failure in less than a year due lack of registration. The charging system often shortens the life of an unregistered battery by charging it too aggressively, especially when it is cold, and sometimes damaging other control units. Premature alternator failure due to lack of battery registration has also been reported.

Battery replacement

Always have a new battery registered after installation at a BMW-qualified shop. It’s not very expensive and will ensure a properly operating electrical system. If you prefer to replace a battery yourself, the following tutorial on the 1Addicts BMW forum may be helpful:

• DIY battery replacement

Sources

• BatteryStuff.com
• BMW E90 Voltage Supply and Bus Systems
• BMW Diagnostics
• Lifetime Maintenance Schedule by Mike Miller, technical editor of Roundel Magazine
• Munich Motorsport
• Optima Batteries