Back when I was a kid, car batteries used to be huge heavy lumps of plastic filled with lead and acid. They used to weigh almost as much as a mobile phone (slight exaggeration there, sorry).

45 years later, car batteries still look the same and weigh the same.

So, in this modern age and emphasis on fuel economy, why do batteries still weigh 40lb? Why have advances in technology not been able to make them lighter and more efficient?