I have been battling low voltage on both batteries for some time now. I replaced one starting battery with a deep cycle but seemed it did not help. When you turn the keys to on the volts read around 12 but when engines are at idle they read 11.5. Shouldn’t they be read around 13? Do I have two bad alternators?

Does the voltage go up when you are off idle? Alt's don't charge at idle rpms.

The volts don't change much at all. Most of the time it is below 12 volts no matter what speed I am traveling.

Something is definitely wrong with the charging system.

First fully charge batteries with a battery charger. If the same symptoms exist, check wiring (including battery isolator if equiped, the switch, and field coil cricuit if battery switch has that feature) back to alternator. If good, you very likely need a new alternator. To double check, with engine running alternator output terminal should have at least 12-13 volts, and increase to perhaps 13.7 or so when engine rpm is increased.

If you are running multiple batteries in that bank, you need to replace them all. If you alternator is bad, they will just die and not charge back at all. You could have a loose ground.

I am having the same problem....well similar atleast. Did you figure it out? I'm hoping mine is just a loose ground. Sat night went to leave the dock party and fired right motor up then went to start the left and it killed the right one. All it took was a jump from a buddy to get them both going so off to anchor out i went. Sunday morning the boat started fine with no jump......hmmmmm..???? Interested to see what your results were

Bench testing alternator tonight. Checked ground last night, tightened and wire brushed. I will be using the boat this weekend. I have my fingers crossed.

first thing i would do is put a volt meter across the battery's when the engine is running. Make sure to give the engines a "burp" past 1500rpm on startup incase you have self exciting alternators. Then if you dont have 13+volts when engines are running would i start with testing the alternators. Also when you run the engines if you have more then one battery bank system make sure the battery switches are turned to their respective battery's....dont run the switches on "Both" or "All" That does goofy things to alternators.....think of two pumps trying to pump fluid from different sources into the same hose. One will always be slightly stronger and can back-feed into the other pump and overwhelm it! If you have good voltage at the battery's then the alternators are working fine...then start looking at your dash connections...you may have corrosion or a bad ground somewhere and causing resistance in the voltage gauge circuit.