Ford Motor and General Motors are looking beyond cars. If history rhymes, investors could come to regret their wandering eyes. Will this time be different?

In May, Ford's shares surged as much as 45%, adding nearly $23 billion of market value, in the weeks after it formally announced Ford Energy, a grid-scale battery-storage business that will serve the power needs of artificial-intelligence hyperscalers and utilities. Its shares have since moderated, but are still up more than 20% since the Ford Energy launch. The company is also re-entering the defense sector, having recently won a contract to build tactical trucks for the U.S. Army.

GM's shares have climbed 16% since last week's earnings call, when it raised its full-year guidance and provided some details around its own defense and grid-scale battery businesses. On the earnings call, GM Chief Executive Mary Barra said these are opportunities to improve margins and become less cyclical.

Investors might want to keep their expectations in check. These businesses are promising but not proven, and automakers have a checkered history with diversification.