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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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Old 08-19-2009
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More Battery Wiring Questions

Hi all, if there was any question about my being an idiot, I will now erase all doubt. I am going back and forth between two good books on boats and 12v systems. Looking at pictures of batteries in series, it appears that my house batteries are not truly in series. My set up has the positive cable from BOTH batteries going to one side of a large fuse. The other side of the fuse has the lead from the battery charger and the main positive cable from bank one that goes to the selector switch.

Apparently, I should have only one positive cable from one of the batteries to the fuse, with a short cable from one battery positive to the other battery positive terminal. Is this correct? If so, when I add a third battery to this bank, I should only have to run a cable from the new battery's positive terminal to the middle battery's positive terminal (which already has a cable to the first battery's positive terminal. Until I hear from you guys I will assume this is correct.

The way my boat has the negative cables set up is they all link with each other and one then attaches to the main ground on the engine. Is this okay even if they are in separate banks?

Going back to my original set up with both house batteries each having their own connection to the fuse, do I get any less power or capacity than if they are correctly wired in series?

One last stupid one, the distance between the new third house battery and the original two house batteries is about 4-5' and will require a cable of about 8-9'. I am planning on going with #1 cable. It seems to follow that the negative cables that tie all the batteries together and to the main ground should also be #1. Am I on the right page?


Thank you in advance, Bill
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Last edited by montenido; 08-19-2009 at 06:22 PM.
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Old 08-19-2009
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When you figure your wire size you want it big enough to carry your max amp load. On the wire size charts you want to figure the load and add both distance's into the equation. If your battery is 8 feet away you have to figure the return so you have to figure the amps in a 16 foot wire X2. This goes for all wiring. You may have to increase your wire size depending on load and protect with a fuse/breaker.
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Bill,

Though you haven't said, I assume that the new batteries are 12V each. If that is true, you DO NOT want to wire them in series.

I believe your confusion comes from the definition of "series". Series connection doesn't mean that the batteries are just strung together. Rather, it means that the positive terminal of one battery is connected to the negative terminal of the second battery, thereby doubling the voltage. If you were to do this with two 12V batteries, you'd wind up with 24V which you don't want!

Parallel connection means that positive terminals are connected to positive terminals, and negative terminals are connected to negative terminals. You have a parallel connection, even though both positive leads from the batteries are going directly to the fuse. They don't have to be wired this way, but there's nothing wrong in doing it that way.

Normally, when you connect batteries in parallel it's good to:

(1) wire all positive battery terminals together; and (2) wire all negative battery terminals together, this using short "jumper cables"; then

(2) you take the positive lead from one of the batteries on the end of the pack and the negative lead from the battery at the other end of the pack.

For example, you have 3 batteries:

Battery #1 next to Battery #2 next to Battery #3.

You'd wire them all together using the short jumper cables, then run a positive cable from, e.g., battery #1 to the fuse. Then, run a negative cable from battery #3 to the engine ground.

In your case, it seems that battery #3 is going to have to be separated by 7 feet or so, in which case you want to be sure that you use heavy enough cable. I'd use 1/0 battery cable for both positive and negative connections.

My setup is similar, with 3 banks of house batteries, each made up of two 6V golf cart batteries in series. The separation between bank #1 and Banks #2 and #3 is about 8'. I run them all as a single house battery bank (675AH total capacity). Ideally, you'd want all batteries co-located to keep the cable runs as short as possible, but in my case -- and, it seems, in yours -- this isn't possible.

Hope this helps a bit.

Bill

Last edited by btrayfors; 08-19-2009 at 08:48 PM.
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Old 08-19-2009
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Bill, thanks for the clarification. Yes, it was parallel that I was trying to describe, not series. So the batteries are still parallel when both + cables go to the same location? Cool, that means something is right.

I guess I have to go to the boat again and re-think some of the cable needs before I place an order.

Thanks for the tips everyone.

Cheers, Bill
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Old 08-20-2009
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Hi Can anyone of you bright sparks help me?
I have bought a Hardin and the previous owner has taken the 30amp dock connector off and replaced with 50amp I have a 100' 50amp cable. if the dock has only 30amp supply can i get an adapter, also if i want to go from 16amp normal plug can i also jump up?
thanks
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yes wookie they make adapters for almost any direction, 30 to 50, 50 to 30, 30 to a normal plug, normal plug to 30. they are pricey thou
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Thanks Scottyt guessed they would be
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bill (monti) the way your set up is, is okay. its actually nicer in some ways, ie you can disconnect one battery at the bus bar with out shutting the whole thing down. also in your case its nice becuase you can get away with a smaller cable from the battery to the fuse.

look at it this way, if you max draw from one battery is say 50 amps, if you put they in parallel with the wire battery to battery the wire from the second battery to the fuse will be carrying 100 amp. now we dont size wire at actual draw but you are still putting less power thur the one wire so less heat, less voltage drop ( due to heat ), less power going thru the second batteries terminal
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I used to get confused all the time about which was parallel and which was series. Then I created a little mnemonic.

I used to shop at a Pac-N-Sav store, so now I just remember Pac-N-Sav.

Parallel Adds Current - N - Series Adds Voltage.
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Quote:
Originally Posted by montenido View Post
Hi all, if there was any question about my being an idiot, I will now erase all doubt. I am going back and forth between two good books on boats and 12v systems. Looking at pictures of batteries in series, it appears that my house batteries are not truly in series. My set up has the positive cable from BOTH batteries going to one side of a large fuse. The other side of the fuse has the lead from the battery charger and the main positive cable from bank one that goes to the selector switch.

Apparently, I should have only one positive cable from one of the batteries to the fuse, with a short cable from one battery positive to the other battery positive terminal. Is this correct? If so, when I add a third battery to this bank, I should only have to run a cable from the new battery's positive terminal to the middle battery's positive terminal (which already has a cable to the first battery's positive terminal. Until I hear from you guys I will assume this is correct.
That's one way of doing it. The only reason I can see for doing it that way is they can use smaller, and generally cheaper, battery cable. Batteries in parallel double the current (batteries in series double the voltage) so the wire going to the fuse would have to be big enough to handle the current requirements of both batteries.

If you add a third battery, and the fuse post will support it, you can use a separate cable for the positive lead. If you try to cheat the distance and connect it to an existing battery terminal, you'll have to increase the size of the wire going from that battery to the fuse.

Basically, if you're going to connect 3 batteries to one fuse point, the battery closest to the fuse will have the largest wire size (and highest current passing through it), the next battery will have a smaller wire as it's carrying the current of two batteries, the last battery only has to carry the current load it's rated for.

Quote:
The way my boat has the negative cables set up is they all link with each other and one then attaches to the main ground on the engine. Is this okay even if they are in separate banks?
That's one way of doing it, if the runs are short between batteries and the wiring is up to snuff. It sounds like they wanted to save some money and linked them all together. If that's so, then I'd check that the wire going to the main ground is big enough. If it's the same size as the one connecting the negative terminals together, I'd be a bit concerned. Voltage and current go out the positive terminal and back the negative, so I'd want the wiring to be identical in size.

There is some discussion of "star" grounds where each battery has a dedicated wire to a common ground point. This is supposed to help level out current loads.

Speaking of grounds, it's also highly suggested that all grounds for all loads go the the same ground point. This will help prevent ground loops which can be difficult to diagnose and fix. If you absolutely have to have another ground point then you should run the largest possible wire between the ships ground and this new ground. You want to minimize the voltage drops in the wiring as much as possible.

Quote:
Going back to my original set up with both house batteries each having their own connection to the fuse, do I get any less power or capacity than if they are correctly wired in series?
I think you mean parallel if your description is correct. Not enough to worry about I think. Depending on the wire size and load, there can be an additional drop in voltage due to the resistance of the wire especially in high current loads. That's why most folks go for the biggest wire the opening, and wallet, will tolerate.

Quote:
One last stupid one, the distance between the new third house battery and the original two house batteries is about 4-5' and will require a cable of about 8-9'. I am planning on going with #1 cable. It seems to follow that the negative cables that tie all the batteries together and to the main ground should also be #1. Am I on the right page?


Thank you in advance, Bill
I don't know the current capacity of the house or 3rd batteries but if they're the same, I'd probably go one size larger than the original house bank for both the positive and negative terminals.
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