official conky doc says this btw: (for exec)
"Executes a shell command and displays the output in conky. warning: this takes a lot more resources than other variables. I'd recommend coding wanted behaviour in C and posting a patch. "
execi 30 in your conf is better, it only does that every 30 seconds, but if you use the built in battery variables at least it will update on every pass with little to no overhead. Plus you don't have to divide it by 10 any more.