Voltage spikes

I was helping out my uncle on his boat (30' Sea Ray) with installing a stereo system and when the boat is running, the voltage would spike to 17-18 volts. Any thoughts of what might be causing this issue?

Bad voltage regulator?

How long are these "spikes" ?

If the meter just occasionally gives a reading that is too high then I would suspect that the meter is being hypersensitive to noise.

An automotive/boat electrical system is perhaps the harshest environment for electronics to work in. With all current surges and DC motor brushes and other noise generating devices it is a wonder that anything electronic works at all.

Before you worried about it too much I would get a second opinion: get a different volt meter and see if it indicates the same thing.

Already checked with 2 different volt meters ... spikes would last 5-10 seconds then go away. I'm leaning towards bad diodes (regulators). The ship-to-shore radio did not like the spikes at all.

Can you provide more details? I would expect a 30' Sea Ray to have two engines, do you get the same result off both engines or is it just one?

Does the alternator(s) connect to an isolator and then the batteries, or do they connect directly to the electrical system?

Where are you measuring these voltage spikes at?

Do you see these same voltage spikes if you measure directly across the batteries?

How old is the boat?

If it is a dual engine boat then there is a possibility that the engines / alternators are wired in a funny way. I have seen arrangements where one alternator was used to recharge the starting batteries and the other altnerator was used to charge the "cabin" battery. Depending on how you had the switches set you could end up running the cabin directly off the alternator with no battery online, which would be very very bad.

If you see the voltage spikes at the radio, but not at the battery then it indicates that the wire between the alternator and the battery is either too small or takes way too long of a path.

If the output of the alternator goes through a circuit breaker before it gets to the battery then it is possible that the contacts on the circuit breaker are getting bad and causing a significant voltage drop. This would make the alternator side (which is probably the "cabin" side as well) be a lot higher than the battery side.

If you see 18 volts directly across the battery then either the alternator is pumping out some serious current (which means the voltage regulator is bad) or there is something seriously wrong with the battery. The battery is supposed to act as a reservoir, taking excess current during the surges and supplying additional current during the lulls.

If the battery was bad then I would expect it would have a hard time starting the engine, assuming that it is used for starting. If the battery in question is just used for the cabin then check it out, make sure it has water covering the plates and that it hasn't completely died.

Anohter thought: is the boat plugged into shore power and the battery charger on while you are doing this? Try turning off the battery charger and repeating the test.