Help/advise please

I have a problem with my VHF radio
When im going along after about 10 mins my radio starts bleeping and flashing Excessive voltage on the screen so i put the deck light on or bilge pump and it stops?

So its getting too much power from the engine charging the battery?

Any ideas to prevent this anyone??
Thanks in advance

Could be your alternator/voltage regulater is over charging
When you rev the engine do the lights go brighter