Title says it. Do you measure the voltage from time to time, or do you just replace them when you feel like doing so, or do you hear something when they get empty? I installed the EMGs in my Charvel 750XL by myself. The guitar does not have a dedicated battery compartment which is easy to access. So I cannot simply put them out and check. I have to unscrew a plastic plate and the screwholes are already pretty worn out after 15+ years... That said, is there an easy way to tell when to replace them?