Two Stupid Battery / Connector Questions

Solstice · Apr 29, 2014

JK_Boston_Catalina310 said:
Interesting setup on the new boat. Looks like you have two 4Ds in parallel. Do you have a reserve/starter battery too?

Jesse,
Yes I have a reserve as well. I actually cut the section out behind the battery box so I could wire the boat the way I wanted...echo charger, buss bars, direct wire to Alt with safety switch.

Solstice · Apr 29, 2014

Ward H said:
Solstice, That would work for me but I looked and could not fine a 4 post 5/16 busbar. BS only has 1/4 post bars listed. Nice setup. Jess, thanks.

I believe I got it mine at Defender. The part number is 2127... 250A with 4 5/16 studs....

JK_Boston_Catalina310 · Apr 29, 2014

Ward H said:
Jesse, Good links. I had read all but the one on fuse voltage drop. (or had and forgotten). Let's start with why I chose the 1/0 wire size. I used the genuinedealz.com voltage drop calculator. I had 7' run to the busbar. The 1/0 gave me something like 1% voltage drop at the busbar for 100 amp current draw. My DC Panel is rated at 100 amp but since I will never get there, I used 80 amp and 3' from the busbar to the panel. #8 wire kept the voltage drop under 3%. A more realistic 20 amp current draw should have a voltage drop of less than 1% while the 80 ampacity of the wire is closer to the panel rating. The #8 wire will allow a little more flexibility for pulling and working on the panel. The battery switch is in a location not easy to get to without taking things apart. I mounted the busbars where I can easily get to them to change fuses and connect/disconnect the OB cable at the end and beginning of each season. If I used just the one fuse at the battery, I would have had to run a much heavier wire to the DC Panel so I put a dual fuse holder on the positive busbar. Considering your point about voltage drop due to being under fused, I should move up to a 250 or 300 fuse at the battery. Then move up to a 80 fuse for the DC panel. Then I need to determine the gage wire of the OB. I know the length is 7'. I will then be able to determine the size fuse for the OB. Finally, all of this is bought and installed. The yard stores the battery and OB and just reinstalled on the boat last week. I told them I would make the connections, which led to the original questions in this thread. So, it sounds like I need higher amp fuses to reduce their voltage drop and find the wire size for the OB cable so I can get the proper size fuse for that cable? Does that work?

Given what MS has added about the voltage drop across the fuse being more of a concern on charging circuits and therefore wouldn't be a significant concern in your setup. Plus that everything is already bought and installed. I would either get a new negative buss bar with a 5/16 post or install a new lug to fit your existing bar.

Sorry to add to any confusion.

LloydB · Apr 29, 2014

I've never solved a voltage drop problem with a bigger fuse.

JK_Boston_Catalina310 · Apr 30, 2014

LloydB said:
I've never solved a voltage drop problem with a bigger fuse.

The article by Maine Sail is here: http://www.pbase.com/mainecruising/fuse_voltage_drop

To quote from that article, "The key take away here is that using the largest fuse that is well within the safe range for the wire results in the least voltage drop in your system wiring."

As he stated in this thread, it is more of an issue on charging circuits.

Ron20324 · Apr 30, 2014

JK_Boston_Catalina310 said:
Given what MS has added about the voltage drop across the fuse ....

Are you sure you don't mean voltage drop across a diode? Makes more sense, and is frequently pointed out as the downfall of charging isolators.

LloydB · Apr 30, 2014

"As he stated in this thread, it is more of an issue on charging circuits."
So fuse size affects voltage drop on charging circuits but differently on lighting circuits is that right?

Maine Sail · Apr 30, 2014

LloydB said:
"As he stated in this thread, it is more of an issue on charging circuits."
So fuse size affects voltage drop on charging circuits but differently on lighting circuits is that right?

Your lighting circuit just gets slightly dimmer and is not noticeable. With charging circuits your voltage at the battery terminals is what determines how much current will flow into the batteries. If the source end thinks it is at absorption voltage but the battery end is only at 13.6V due to drop in the wiring your charging performance will suffer. Keeping voltage drop to the bare minimum in charging circuits is far more critical then in interior lighting circuits. Under USCG regs/CFR and ABYC standards critical circuits, such as navigation lighting, must be kept to a max of 3% voltage drop but interior lights can be higher. Still, I don't personally like to see them over about 5%....

Any voltage drop in a circuit, caused by a fuse, poor termination, corrosion/resistance or wire size is cumulative so the extra .1V caused by the fuse is simply added on top of all the other areas that can create voltage drop... The fuse is but one more area that can create voltage drop but this is really only critical in circuits that really don't like voltage drop such as charging circuits. The fuse won't "solve" a voltage drop issues, it is usually just part of the problem in a bigger puzzle, but one that is very often over looked.....

SeaTR · Apr 30, 2014

RE: Maine Sail's most recent writeup on this topic

....."Your lighting circuit just gets slightly dimmer and is not noticeable. With charging circuits your voltage at the battery terminals is what determines how much current will flow..... The fuse is but one more area that can create voltage drop but this is really only critical in circuits that really don't like voltage drop such as charging circuits. The fuse won't "solve" a voltage drop issues, it is usually just part of the problem in a bigger puzzle, but one that is very often over looked.....

HEAR, HEAR !!
Well done (or is that written), clear, concise yet complete wrt Ward's issue.

10Q Main.