Do I need to add a disconnect switch to the cranking battery circuit to add the 3-bank of the charger? Looking to change to the Dual Pro PS3 Professional Series.

What would you expect the disconnect switch to do? The cranking battery should already be separate from the trolling/house batteries. The instructions that come with the chargers are pretty much self explanatory.

I was concerned about damaging the alternator on the outboard. I have heard/read about damage being done to them if not using a disconnect and I want to know if this was the norm or an anomaly I guess. I fully understand that the trolling motor batteries and the cranking battery are isolated from each other.

I didn't, just connected each battery to a bank. Each bank should charge independently and should not cause a problem. Look at the owner's manual for the charger and they'll tell you how to hook up their charger

each bank should have a fuse also.

Thanks for the input. Just wanted to make sure I was not opening a can of worms by not putting in a cut off switch to the cranking battery.

Yes, there's risk to boat equipment anytime a charger is attached to a battery that is not isolated from the boat equipment. If you want to hear what an electrical engineer has to say about it, this is your chance.

Will normal operational charging harm any boating or engine equipment (aside from trolling motors we are already instructed to disconnect)? Typically, no. Is doing this a good idea? I say absolutely not.

If the charger circuits were to fail in a nasty destructive way, there is a chance damage could occur. This doesn't happen very often with well-designed chargers as the goal is to design the charger in such a way that failures pose little risk to attached devices.

The greatest risk is from a destructive power surge on the AC power circuit. Example: A lightning strike causes a surge on AC power circuits that eventually connect to the charger. If it's bad enough, it can cause catastrophic damage/failure inside the charger in the blink of an eye. Voltage spikes/surges could reach thousands of volts, passing right on through the charger to anything connected. You can guess what might result.....

It doesn't happen a lot, but it does happen, and when it does, it can be a very bad thing.

There have been cases of extensive damage to engine electronics (wiring, ECM, regulators, etc.) along with everything in the boat with any connection to a battery being charged. There have been cases of boats experiencing complete loss of all connected electronics. (Being turned "off" is of no consideration for such surges.) Shallow water anchor and hydraulic jackplate power connections not interrupted will put those items at risk also. Surge protectors can help, but I have seen those fused in the event and be rendered ineffective.

I NEVER rig a boat without a main battery cut-off switch. One bad lightning strike from a passing thunderstorm could theoretically do $10,000 in damages, depending on what's rigged on a boat. In fact, the only disconnect switch I use for my own boats is one that disconnects both the + and - terminals of the battery connections.

A Dual Pro charger is heavy and contains heavy transformers allowing it to provide steady (non-pulse) charging currents. It is much safer for engine and boat electronics than a small light charger that is using pulse charging.

Never the less, the Dual Pro charger will at one stage in chargin output well over 15 volts.(varies with temp compensations) It will also contain some small AC leakage voltages atop the DC. ( Most engine alternator's voltage regs otherwise never output more than about 14 or so volts.)

So yes, chargers add some additional stresses to electronic components. It is best to use a battery disconnect regardless of charging though many users may get by without it. If you use a lightweight charger not having heavy transformers, a battery disconnect should always be considered.

I have been using a 3 bank charger for many years and have had no damage to the charging system. A master switch is cheap insurance to prevent damage from a short. I have one on my cranking battery and my trolling batteries. If you open the circuit by opening the negative lead on the cranking battery you should reduce any impact to the charging system. Never leave the charger plugged in all the time. I charge my batteries as soon as I get back from fishing and I will plug the charger in again the night before I leave to make sure they are topped off. Most of the time the charger on my boat shows the cranking battery is fully charged when its connected.