EFB vs AGM batteries for a start/stop car: which is better and why? Can / should I replace EFB with AGM?

Question

I need to replace an EFB battery in a start/stop car because it doesn't hold the charge - the battery is at 12.3V after 3 days and at 12.1V after 7.

My questions are:

1. Is it always possible to replace EFB with AGM? Does it depend on the car? The manual of my car doesn't say anything on this.
2. Is it recommended? Do AGM batteries perform better?
3. Does replacing EFB with AGM require a specific reprogramming / registration?

I have found two mobile services that can come to me, replace the battery and dispose of the old one. I am not sure if I should go for the AGM, if AGM is better, or for the EFB, because Bosch is a better-known brand and EFB is sure to be compatible?

• One is a large car parts reseller in my country and can supply a 640 CCA AGM battery of its own brand (no idea who actually makes it)
• The other can supply a 560 CCA Bosch EFB battery
• Both batteries are 60Ah and both come with a 5-year warranty

Answer 1

AGM batteries are more advanced than EFB batteries, but are more expensive. If your car uses an EFB without issues you won't get much benefit from an AGM battery.

Cars that use EFB or AGM batteries have sensors and a battery management system (BMS), in general you can use an AGM battery in place of an EFB, but not the other way around. However, your car manufacturer's BMS may not be designed to use an AGM battery, so in the absence of information from your manufacturer or dealer I would replace like for like and go with an EFB.

Answer 2

I don't see why you would want to use AGM battery in place of EFB battery. The AGM is a better, more sophisticated and more expensive technology. For example, I would never use an EFB battery in deep discharge applications. But, in a stop/start car, the charging algorithms have been tailored for a particular type of battery. AGM requires slightly different charging voltages and if your car does not support those voltages, you could destroy the AGM battery (which is supposed to be more durable) quicker than you would destroy and EFB battery.

Use the manufacturer's specified battery type.

Battery university says:

As with all gelled and sealed units, AGM batteries are sensitive to overcharging. A charge to 2.40V/cell (and higher) is fine; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell); a direct replacement with a sealed unit could overcharge the battery on a long drive.

If, on the other hand, the car's charging voltage is tailored for AGM, then you will not want to use an EFB battery.

Answer 3

For the sake of other folk who find this, as you've said you have a Volkswagen, their electrical system service manual states:

An “EFB” battery may only be replaced with another “EFB” battery.

Those service manuals are for Volkswagen dealerships, and err on the side of being extremely cautious and thorough, so I wouldn't be surprised if installing an AGM battery would work just fine. But the official advice from VW is to always get work done at a dealership, and their advice to the dealership is to only replace like with like.