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I charge my 2017 X4 M40i on a battery tender every weekend - I just don't think that ECO alternator feature is ever charging these batteries enough. BMW is always trying to eek-out every last mile per gallon for those EPA ratings, and with the alternator clutch disconnected most of the time, our batteries are really taking a good amount of daily punishment. When I put mine on my advanced CTEK charger, it takes a good 8-12 hours to top it off after the end of the week's driving, so it's no joke.
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