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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
  • Use batteries that meet, or exceed, the manufacturers original specification. – Solar Mike Apr 24 at 13:32
  • Can you be more specific? The manufacturer's manual doesn't say anything whatsoever on this point. So, as a non-expert, I kind of understand that AGM are 'better' than EFB, but I have no clue if I can replace EFB with AGM or if there would be some kind of compatibility issue. That was the question. – Pythonista anonymous Apr 24 at 13:37
  • Please keep the discourse civil and remember comments are for the clarification of the question, not to banter on about trivial things. I have cleaned up the comments which do not pertain to the question. Please use the chat link provided to communicate further. – Pᴀᴜʟsᴛᴇʀ2 Apr 24 at 15:50
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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.

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  • Thanks. When you say "more advanced", could you elaborate on that? Does that mean AGMs can be used in more starts? Or that they retain the charge for longer when idle? Is it true that AGMs can be recharged by the energy of the braking system while EFBs can't? I presume this requires specific hardware on the car, so, in the absence of specific information from the manufacturer, I am not sure if my car would do that. – Pythonista anonymous Apr 24 at 16:57
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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.

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  • Thanks. I suppose that, in the absence of any indication on the owner's manual (which is shameful!), it is safer to stick to EFB. Bosch says on its website that EFBs can be replaced with AGM ( br.bosch-automotive.com/en/internet/parts/… ) but VW gave no details about what my car would support – Pythonista anonymous Apr 24 at 20:35

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