What must be considered when upgrading technology from start-stop batteries, from EFB to AGM batteries?

Our upgrading recommendation basically covers 2 areas, increased power requirements (capacity and/or cold start) or technology upgrade - which is what this article is about.

Note installation location

The installation location and the resulting maximum ambient temperature must be taken into account:


In the case of the AGM battery, it is essential to avoid a „build-up“ of thermal load due to (extremely) high ambient temperatures (thermal runaway). 

For this reason, the ambient temperature must not exceed +55 degrees Celsius. +85 degrees Celsius, not an uncommon value in the engine compartment, especially in summer, is considered uncritical for a maximum of 3 hours per day. More than that cause the premature total failure of the AGM battery, because the separator dries out and the grids can become extremely corroded.




AGM battery installed in the engine compartment.





Where is the start-stop battery installed?


  • EFB batteries are usually installed directly in the engine compartment, exceptions prove the rule, as always.
  • AGM batteries are often installed in the boot or in the passenger compartment. Recently, however, some car manufacturers have also increasingly installed them in the engine compartment, rather away from the engine block and often equipped with a thermal protection cover.


Why are starter and on-board power supply batteries sometimes installed in the boot or, less frequently, in the passenger compartment?
Primarily for reasons of optimal battery and ambient temperature, the aspects of safety and optimal weight distribution in the car are rather secondary.


EFB battery installed in the engine compartment.


Directly next to the engine block (combustion engine), the more heat-resistant EFB battery with fitted thermal protection cover is to be preferred if this was already installed ex-works as the original battery.


The situation is completely different if the battery is installed in the boot (e.g. in many BMW models) or in the passenger compartment, where an upgrade to an AGM battery is highly recommended. Especially with a large number of additional electrical consumers, maximum vibration/cycle resistance and optimum on-board power system stability. This is because the thermal load on the battery due to the effect of heat is negligible here, in direct comparison with the engine compartment.

The same applies to an on-board power supply battery in an electric car, because electric drives emit only little heat loss to the environment due to their high efficiency.


Banner tip: When replacing batteries, we generally recommend "equal for equal" or, under certain circumstances, "equal for better" in the sense of upgrading, as explained in this article.

Please never downgrade the battery technology! Slight differences in performance in terms of capacity and/or cold start are tolerable - in the same battery box.

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