I would also add number of months before losing the first bar, since batteries degrade over time not just mileage...
in its current form poll does not capture people that still have all 12 bars, my guess this is the largest group so maybe you can add option for that.
actually I would rework the poll (if possible) to look like this
Current number of the battery capacity bars:
-12
-11
-10
-9
-8
-battery replacement
Mileage when last bar was lost / battery replaced / current mileage for those with 12 bars:
0-10,000
10,001-20,000
20,001-30,000
30,001-40,000
40,001-50,000
50,001-60,000
60,001+
Months FROM the car manufacture date TO when the last bar was lost / battery replaced / current date for those with 12 bars:
0-6 months
6-12
12-18
18-24
24-30
30-36
36-42
42-48
48-54
for example my answers would be:
12 bars
0-10000 miles
6-12 months