When it is really cold outside and you have to keep an eye on your car battery, I find this image useful in determining battery condition. By simply measuring battery voltage, you can know approximate charge level.
Here is the image :
[IMAGE: https://steemitimages.com/DQmV6RQQwg3fKmPDQMctNqFwdyP5nHbJRR1dnsboxUfcNgf/%EC%BA%A1%EC%B2%981.png]