My 6530 does the same thing randomly but it’s no big deal. I start my tractor cold and give it a few minutes to warm up. I then start to slowly bring up the rpms to what sounds to be around 1500 rpms at this point I stab the throttle a couple times to rev the engine and the tach will start working and battery will start charging. I have no idea why it does this or why this works but what I do notice is when the tractor sets unused it happens more frequently