AGM versus AGM No Longer Necessary
After AGM and EFB, there is now another battery type that can be installed in vehicles with start-stop systems: the AFB battery. It can replace both existing types and offers two decisive advantages in the replacement market.
Curiosity was immediately aroused when Moll approached the Krafthand editorial team to present a completely new battery type for the start-stop applications replacement market. What else could there be besides AGM and EFB? Most importantly: What about compatibility with battery management? And what are the two main advantages of the "AFB start-stop", which is more cost-effective than an AGM battery while simplifying inventory management? The AFB battery is not only suitable as an affordable alternative to AGM but can also serve as a replacement for a defective Enhanced Flooded Battery, known in workshop jargon simply as EFB battery. The resulting simpler inventory management is not to be underestimated for workshops. After all, when it comes to the number one cause of breakdowns - "defective battery" - business often goes to whoever has a replacement battery immediately available and can install it spontaneously. It's no coincidence that ADAC's yellow angels always carry batteries on board and often snap up such jobs from workshops.
Disguised AGM
But back to the new AFB (= Advanced Flooded Battery), whose designation indicates that, like the EFB, it is a wet battery. This is in contrast to AGM batteries, where the electrolyte is bound in glass mat separators. Not least because of these different technological approaches, the question arises: Can a vehicle's battery management system designed for AGM as original equipment even work with the AFB?
Dr. Klaus Eichhorn, CEO of battery manufacturer and original equipment supplier to VW, Audi, Škoda, INEOS Grenadier, and Mercedes-Benz, Moll, doesn't beat around the bush during Krafthand's visit: "Of course I understand this question. But with our AFB, the rule of 'AGM only against AGM' loses its validity." Simply put, through modifications to its internal structure, the AFB is "a disguised AGM that is recognized as such by the battery management system." He says this not least with reference to the fact that Moll has relevant expertise not only in battery technology but also in energy management. The supplier, headquartered in Bad Staffelstein, Franconia, contributed to developing the integrated electrical system for Volkswagen Group vehicles over a decade ago and introduced the algorithm for battery condition detection.
Moreover, with the advent of start-stop systems, Moll's developers designed the EFB battery for Wolfsburg to save on the significantly more expensive AGM battery for series applications. Another advantage of the EFB, which essentially forms the basis for the AFB: It can also be installed in hotter areas of the engine compartment, for which AGMs are less suitable due to their higher temperature sensitivity. This is due to the completely different internal structure of AGM, where, as mentioned, the acid is not liquid but bound in the separators.
The Resting Voltage Trick
To understand how a battery management system (BMS) designed for AGM can work with an AFB, one must understand its operating principle. Two essential factors are decisive for "battery type detection": the battery's current consumption and its state of charge (SoC), which the BMS doesn't allow to drop below 60 percent or charge above 90 percent. The BMS cannot measure the SoC directly but determines it based on corresponding parameters. A very important characteristic for this is the resting voltage.
In an AGM, this is somewhat higher compared to an EFB. If an EFB were installed in a system designed for an AGM battery and there is no conversion to EFB (if even possible), it results in incorrect charging and shortened battery life. Simply put, too much current is supplied to the battery because the battery management system – assuming an AGM – interprets the lower resting voltage of the EFB as indicating too low a charge state (detailed explanation in the box on page 14).
"This cannot happen with the AFB," explains Eichhorn. "Through constructive measures, we have raised the resting voltage curve. This means the system recognizes the AFB as an AGM battery, and there are no misinterpretations or incorrect charging scenarios."
Constructive Measures
He also outlines which measures in the battery's interior contribute to this:
- Use of particularly corrosion-resistant alloys
- Lower self-discharge and reduced electrolyte consumption through calcium grid technology
- Specially developed active masses
- Robust gravity casting technology with reinforced grid design
Ultimately, according to Eichhorn, these modifications have even led to the Moll AFB performing slightly better than an AGM in capacity reserve and starting power tests, and also offering advantages in service life and in the so-called micro-hybrid test (the most important test for start-stop capability). He also shows Krafthand a diagram reflecting tests of current consumption (important for recuperation, among other things) at three different charge states. In all three scenarios, the AFB outperforms the AGM.
According to the Moll CEO, workshops can therefore install the AFB without hesitation in cars with either an AGM battery or an EFB. This gives them the opportunity to save their customers money. An AFB costs about 20 percent less than a comparable AGM. Ultimately, it's increasingly about repair appropriate to current value, says Eichhorn, and underpins this at the end of the conversation with an interesting statement: "The AGM was originally developed as a supply battery, for example for consumers in the living area of motorhomes. With the advent of start-stop systems, it was then discovered as a starter battery due to its high cycle stability. As a very expensive one, though." What the expert means is: AGMs undoubtedly have very high cyclic load capacity for large charging and discharging volumes, but this "overperformance" isn't needed in modern vehicles, yet must be paid for. Ultimately, this was the incentive for Moll to develop AFB as an alternative to AGM.
KNOWLEDGE: Relationship between BMS and State of Charge when Switching between AGM and EFB
As shown in the case example here, if an EFB were replaced with an AGM, the board management system (BMS) would assume an incorrect charge state of the supposedly installed AGM based on the different resting voltages of the two battery types (at 70% SoC: AGM = 12.58V, EFB = 12.53V) and set an incorrect charging current. The problem: The actually installed EFB in this example has a charge state of 73% (red line), while a BMS for AGMs assumes 70% based on this battery's voltage level (blue line). Consequently, the EFB cannot absorb as much current as assumed by the BMS, since current absorption capacity decreases with increasing charge state. On the other hand, the BMS interprets the apparently too low current absorption as aging/wear of the battery and constantly tries to adjust charging behavior and charging current. Due to the control loop, this results in mismanagement, leading to the deactivation of certain (comfort) functions and the start-stop automatic system. In the end, the incorrect detection of the resting voltage negatively affects the battery's service life.
In the reverse case, i.e., if an AGM were replaced with an EFB, the situation is opposite. Then the AGM battery has a lower charge state than the system expects, consequently the current consumption is higher than expected due to the lower charge state. However, this would be less problematic for the battery itself, as apparently better current consumption is not critically evaluated.
Note
For distribution, Moll is planning a workshop partner concept where interested automotive businesses would receive a basic stock of batteries, likely with generous payment terms. Since not all details were finalized at press time, more information will be provided in upcoming issues.