Many cars have nasty voltage spikes & stuff. I would put a transzorb across that pin as well.You can expect 14.5V when the battery is charging. Vout = Vin R2/(R1+R2)You know you want Vout <=5V, Vin up to 14.5V, pick an R2 of say 6.8K and Solve for R1Vout(R1 + R2) = Vin R2VoutR1 + Vout R2 = VinR2VoutR1 = (VinR2 - VoutR2) or (Vin-Vout)*R2R1 = (Vin - Vout)*R2/Voutso (14.5 - 5)*6800/5 = 12920using a standard 15K:14.5*6800/(6800+15000) = 4.55Vand if Vin was lower12*6800/(6800+15000) = 3.74V so you get a good high over the expected Vin range.Current draw, V=IR, so V/R = I14.5/(6800+15000) = 0.66mA
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