The battery will drops to the very beginning of the yellow part when at a stop. If I release the brake so the car walks a little the battery will go back to half. The only times it seems to do it, is when I speed as if I am racing. I have checked the battery it is at 88% life.
My car has 187000 miles.
My car has an automatic transmission.
Even though you have already checked the battery, the charging system will need to be thoroughly tested to see if it is charging at idle and at about 2000 RPMs. Part of a thorough charging system test will be to test the battery again. If the system is not charging enough or the battery is failing it will cause the problem you are having. I suggest having the charging system tested by a qualified professional, such as one from YourMechanic.
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