Quote:
Originally Posted by 493263
We all know the dreaded problem for city drivers needing new batteries prematurely. I live in DC so it's a problem for me. I tested some behaviors with the car on how to get it to charge if you're in low speed traffic. You can tell when it's charging because the fuel economy bar underneath your tach swings to the blue side. Here's what I found.
Problem: If you drive in normal mode, it rarely charges the battery under 45mph. That's a huge problem as you don't get to go 50+ in the city.
Solution: Use Sport Mode or hold a lower gear longer. Basically, you are engine braking at a higher rpm and forcing it to engage the alternator. You burn more gas but use less brakes.
What I found: It seems like it is based on rpm rather than speed. In normal mode, it doesn't like to go past 2.5k rpm. If you hold a gear higher than that when decelerating(engine braking), it will charge your battery. I took a 2 mpg hit but I've never had to use an external charger anymore. On the highway, whatever mode doesn't matter as the rpm will be high anyways. However, if you are in sport mode, then it just engages the alternator a little quicker(nothing significant).
This has worked for me and kept the battery charge up at optimum level. Even when I left my car in the cold for 3 weeks without driving it, it still cranked up. When the temperature dropped recently to 6F, it still cranked up without a hiccup while every other car in the parking lot had problems.
I did about 4 months of testing with this on the highway and city(I drive mostly city). It was the only way to get the car to charge the battery more often in city driving.
Recommendation: Drive in sport mode in the city. It's fine to use normal or comfort on the highway.
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I just plug in my BMW installed trickle charger once a month in the winter. Don't plug it in in the summer. After 14 months, no warning messages yet. Driving in a lower gear just burns more gas so I'm not sure there is much saving there.