Altenator wire

S

Southern Pirate

Active Member
Nov 17, 2019
107
Boat Info
2005 sea ray 270 amberjack
Engines
5.7 Mercruiser w/Bravo III Drive
I want to change which battery my alternator charges first. I want the house battery charged then the starting battery. Can I just take the alternator wire off of the starter and route it to the house battery? I already have the Blue Sea ACR separating my batteries as well as moving the circuits that I want each battery to handle.
 
I am doing some modifications to a friends boat (2 engines, 2 batteries, wants to charge both batteries if either engine is running, and wants to be able to run both engines simultaneously). In figuring out how best to do this i have read quite a bit about ACRs, diode isolators, battery switches, alternators, etc; and came to the conclusion that you need to be careful when moving the alternator output from the battery powering that engine. You don't say if you have battery switches involved or not, if there are battery switches involved you run the risk of having the alternator output wire not connected to a battery while the engine is running and this can fry the alternator if the output has no place to go. Down the road some mechanic working on the boat may not realize the purpose of this additional wire connected to the battery and may leave the wire unconnected when working on the boat. This will generate the same condition, no where for alternator output to go. Also some alternators need to see battery voltage before they output anything (called an "exciter" wire), which can also be affected by battery switches.

For similar sized start and house batteries using the same chemistry, I think the ACR is doing what you want by charging both batteries and the position of the wire should not make a lot of difference in their charging. That being said, what you describe can be done, just make sure you safeguard against having the alternator output NOT connected to a battery. Also once you move that alternator output off the positive starter terminal you need to run a new wire sufficient for the alternator output back to your battery. This wire should be fused because you are no longer covered by the exception provided for the start circuit in a boat (fuse not required in start wiring provided a couple conditions are met). Also the wires connecting the ACR to the batteries need to be sized for the full alternator output and fused at something higher than the alternator output. A couple good references on ACRs:

https://marinehowto.com/automatic-charging-relays/
Pacific Yacht Systems - they have several videos.
 
jgoedd99 said:
I am doing some modifications to a friends boat (2 engines, 2 batteries, wants to charge both batteries if either engine is running, and wants to be able to run both engines simultaneously). In figuring out how best to do this i have read quite a bit about ACRs, diode isolators, battery switches, alternators, etc; and came to the conclusion that you need to be careful when moving the alternator output from the battery powering that engine. You don't say if you have battery switches involved or not, if there are battery switches involved you run the risk of having the alternator output wire not connected to a battery while the engine is running and this can fry the alternator if the output has no place to go. Down the road some mechanic working on the boat may not realize the purpose of this additional wire connected to the battery and may leave the wire unconnected when working on the boat. This will generate the same condition, no where for alternator output to go. Also some alternators need to see battery voltage before they output anything (called an "exciter" wire), which can also be affected by battery switches.

For similar sized start and house batteries using the same chemistry, I think the ACR is doing what you want by charging both batteries and the position of the wire should not make a lot of difference in their charging. That being said, what you describe can be done, just make sure you safeguard against having the alternator output NOT connected to a battery. Also once you move that alternator output off the positive starter terminal you need to run a new wire sufficient for the alternator output back to your battery. This wire should be fused because you are no longer covered by the exception provided for the start circuit in a boat (fuse not required in start wiring provided a couple conditions are met). Also the wires connecting the ACR to the batteries need to be sized for the full alternator output and fused at something higher than the alternator output. A couple good references on ACRs:

https://marinehowto.com/automatic-charging-relays/
Pacific Yacht Systems - they have several videos.
Click to expand...
Thank you for your response. What I was thinking is just moving the 2 wires that are connected to the fuse from the alternator at the starter. I was going to mount these on a buss bar, with the fuse. Then take that buss bar back to the house battery via a switch. The switch will allow me to go to either house battery or starting battery or off. Of course, I would not use off with motor running or switch batteries with engine running. From what I think I know, this puts the starting battery for starter only. The engine will run off of the alternator via the 2nd wire on the fuse that is now connected to a new buss bar. As far as the wires, I am using the 4.0 welding wires that are the same as Sea Ray attached to the batteries.

The reason for all this rewiring is Blue Sea recommends charging the house battery or batteries 1st and then allow the ARC to switch to the starter battery.
 
I think the key is that if you have similar (i.e. the same) house and start battery battery size and chemistry, you are not gaining much by doing this, and you are making a lot of wiring changes that the next person who works on your boat may or may not understand.

What you describe should work. As far as the details:
-Wire size from the alternator to the battery sounds good.
- It sounds like you have the alternator output wire protected by an existing fuse and will keep it in modified system. That is a good thing.
- You didn't mention the wire size between the ACR and the batteries (size for alternator output and provide fuses).
- You didn't mention if there is a battery switch(es) involved.
- You are 100% counting on the operator not incorrectly using the switch in the alternator output wire.
 
Single engine single alternator right?
This device made by Victron does exactly what you need and eliminates the risk of unloading the alternator. If you don't have three battery banks but rather two then only two of the outputs need to be connected. If you are worried the alternator is limited in output and further want to prioritize then upsize the alternator to a 120 or 150 amp.
https://www.victronenergy.com/battery-isolators-and-combiners/argo-fet-battery-isolators#manual
 
jgoedd99 said:
I think the key is that if you have similar (i.e. the same) house and start battery battery size and chemistry, you are not gaining much by doing this, and you are making a lot of wiring changes that the next person who works on your boat may or may not understand.

What you describe should work. As far as the details:
-Wire size from the alternator to the battery sounds good.
- It sounds like you have the alternator output wire protected by an existing fuse and will keep it in modified system. That is a good thing.
- You didn't mention the wire size between the ACR and the batteries (size for alternator output and provide fuses).
- You didn't mention if there is a battery switch(es) involved.
- You are 100% counting on the operator not incorrectly using the switch in the alternator output wire.
Click to expand...
All the power wires I added are the same size, 4.0. I bought a roll of 4.0 wielding cable, crimp connectors, hydraulic crimper and made my own wires. There is the Blue Sea battery switch installed before the ARC as directed by Blue Sea.
Down the line, Sea Ray divided the total electrical load between the 2 batteries. What I did is moved all the electrical items from the starting battery and placed them on the house bank. Everything still has it's original breaker and label. The starting battery has the ignition switch and engine. I will add more batteries to the house bank as needed when I replace them.
 
Last edited:
ttmott said:
Single engine single alternator right?
This device made by Victron does exactly what you need and eliminates the risk of unloading the alternator. If you don't have three battery banks but rather two then only two of the outputs need to be connected. If you are worried the alternator is limited in output and further want to prioritize then upsize the alternator to a 120 or 150 amp.
https://www.victronenergy.com/battery-isolators-and-combiners/argo-fet-battery-isolators#manual
Click to expand...
Thanks for this info. I originally looked at battery isolators but I can't remember why I chose the ACR over them. It does look like it would be easier to install.
 
Southern Pirate said:
Thanks for this info. I originally looked at battery isolators but I can't remember why I chose the ACR over them. It does look like it would be easier to install.
Click to expand...
And since this Victron one is FET based you won't have that voltage drop like the diode based ones have. The problem with the ACR's is they need a differential to connect the two banks and it is then an amperage flow from one bank (the one the alternator is feeding) to the other. I understand you want to priortize the House bank over the other and the reason to connect the alternator to it. The Argofet will charge either / both yet still not have to deal with voltage differentials.
Again, if the desire is to charge quicker the house bank then up-size the alternator with this isolator.
 
ttmott said:
And since this Victron one is FET based you won't have that voltage drop like the diode based ones have. The problem with the ACR's is they need a differential to connect the two banks and it is then an amperage flow from one bank (the one the alternator is feeding) to the other. I understand you want to priortize the House bank over the other and the reason to connect the alternator to it. The Argofet will charge either / both yet still not have to deal with voltage differentials.
Again, if the desire is to charge quicker the house bank then up-size the alternator with this isolator.
Click to expand...
How does it decide which battery to charge?
 
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