AGM cells are more resistant to damage from overcharge than flooded lead acid cells. They are much tougher than FLA cells. You're charging system would have to be operating far, so far outside capacity for a very, very long time to damage an AGM battery. Long before the AGM battery is damaged, the liquid electrolyte in an FLA battery would have been lost due to electrolysis and vaporization.

Both flooded and AGM cells use sulfuric acid as electrolyte.

As stated above, not all X5s came with AGM batteries and flooded lead acid batteries are available for the X5. What's in your X5?

The charge profiles and end of charge voltages are different for each type of cell.

Frequent under or partial charging of both types of cells reduces their service. Periods of open circuit in an under or partial charged condition is severely detrimental to both types of cells. Cells subjected to such abuse can usually be recovered with some careful work.

I've worked with lead acid cells and batteries for almost 20 years. From the standard 12V SLI and deep cycle batteries to 2V cells that weigh two tons each, AGM, gel, FLA, VRLA, vented, sealed, etc.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim