Question: I would like to have a higher output alternator installed on my boat to help recharge my batteries while I’m motoring. A friend of mine is telling me that there will probably be a lot of other things on my boat that may need changing if I do this. What are the other things I will need to consider?
Answer: Certainly this is a good question, and one where a lot of mistakes do get made. In fact I've discussed this before in Upgrading an Alternator to High Output. Let me expand a bit on the answers I gave to that reader.
To begin, it depends on exactly how big you want to go with the alternator in terms of increased amperage output. If your engine is currently equipped with say a 65-amp alternator and you want to go up over 100 amps, you will need to change the pulleys on the front of your engine to double belt-type pulleys. If yours is a newer engine with a flat serpentine belt set-up, you may need a different belt and be able to make a match with the new alternator pulley. Also, alternator bracketing will undoubtedly need changing. So, a bit of research is going to be needed up front to make sure you can in fact match things up on your engine. Beyond the actual mounting and belt drive for the new alternator, there are a fair number of electrical things you will need to deal with.
First, the output cabling from your old alternator may not be of a sufficient gauge size to handle the increased amperage. So that may mean a bit of re-wiring. Additionally, if your boat is equipped with battery isolators, understand that they are rated for amperage as well, and typically they are sized to match the maximum rated output from the alternator in the system. So, you may need to swap out any battery isolators in the system. Finally -- and this one can be a real problem -- you need to determine if your engine’s tachometer is receiving its pulse signal from the alternator you now have, and if your new one will accommodate this. Otherwise you may be looking at an entirely new way of determining engine rpm.
Finally, I’m going to assume that along with this new high-output alternator you are going to also add a matching external regulator. Make sure you or the installer carefully calibrate the new regulator to match the needs of the batteries you are charging.
Speaking of that, you might want to read another related article, Battery Tech for High-Output Alternators.