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Old 15-02-2016, 16:32   #1
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Battery Selector Switches Questions

I have a diagram for a battery Selector switch. The positive of battery 1 goes to one input, positive of battery 2 goes to second input, and "common" output of switch takes flow of current from either battery or both to incoming/input/positive terminal on the starter. Then engine is depicted with two separate output terminals returning flow of current to either negative terminal of batteries.

I have of couple of questions about this:

1. Can negative terminals of either battery be connected to same negative terminal position on motor or would there be two separate return locations leaving motor back to either battery?

2. Is switch is selected to battery 1, what dictates that flow of electrical current returns to negative terminal on battery 1 and will not flow or affect negative terminal of battery 2?

3. If switch is selected to both batteries, what dictates return flow of current will be evenly distributed to either negative terminal evenly?

Im trying to get a practical as well as theoretical grasp of marine electrical systems.

A related but different question:

In jumping a car or combing batteries inna marine system, it would seem you in both cases link positives in common and negatives. The appears to be in contrast to say connecting batteries in a flashlight in with the positive of one battery is linked to the negative of another in series with return current flowing back to the first battery. Both are direct current but work together differently in these different applications. Could anyone explain what's going on there or what I'm missing?

Thanks.
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Old 15-02-2016, 16:58   #2
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Re: Battery Selector Switches Questions

Great questions! I'll try to keep it short but complete.


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Originally Posted by MrAntigone View Post
I have a diagram for a battery Selector switch. The positive of battery 1 goes to one input, positive of battery 2 goes to second input, and "common" output of switch takes flow of current from either battery or both to incoming/input/positive terminal on the starter. Then engine is depicted with two separate output terminals returning flow of current to either negative terminal of batteries.

I have of couple of questions about this:

1. Can negative terminals of either battery be connected to same negative terminal position on motor or would there be two separate return locations leaving motor back to either battery?

Yes, both negative cables can be attached to the same point, it makes no difference as long as the attachment point is clean, free of paint and offers a good connection = low resistance.

2. Is switch is selected to battery 1, what dictates that flow of electrical current returns to negative terminal on battery 1 and will not flow or affect negative terminal of battery 2?

Electrons need a complete circuit to flow, it's actually a round trip. They flow from the negative terminal of the power source (battery) through the conductor (cable) through the load (starter or lights, etc) back through another conductor and back to the battery positive terminal. Since the switch is making a connection only to battery 1, the electrons will only flow from that battery through the circuit.

3. If switch is selected to both batteries, what dictates return flow of current will be evenly distributed to either negative terminal evenly?

If both sets of conductors are equally low resistance and both batteries are equally charged and same voltage, the current flow from each battery should be virtually the same. If one battery is higher voltage than the other, it will provide a little more of the current. If one battery has a higher resistance conductor than the other, it will provide slightly less than half of the current. These types of systems adhere strictly to the laws of physics, like conservation of energy, Ohm's Law, etc. If you could precisely measure the resistance of each cable, you could calculate exactly what percentage of current would come from each battery. Like my high school chemistry class, the equation has to balance (including conversion to heat) right down to the last electron.

Im trying to get a practical as well as theoretical grasp of marine electrical systems.

A practical concern of marine electrical systems is also corrosion and the resultant increase in resistance in connections, especially high current connections, like a windlass or starter. Using good crimpers, good connectors, adhesive lined heat shrink, boat cable, etc are all part of ensuring you have a low loss, low corrosion connection for years.

A related but different question:

In jumping a car or combing batteries inna marine system, it would seem you in both cases link positives in common and negatives. The appears to be in contrast to say connecting batteries in a flashlight in with the positive of one battery is linked to the negative of another in series with return current flowing back to the first battery. Both are direct current but work together differently in these different applications. Could anyone explain what's going on there or what I'm missing?

You're not missing anything. When jumper cables or multiple batteries are used, they are being connected in parallel. So all positive leads are tied together and all negative are tied together. This way the voltage remains 12v, but the current available is the total of all batteries.

When you see batteries in a flashlight, they are being connected in series. This ties positive of one to the negative of the next, up to 4 or 5 batteries in the bigger flashlights. In this case, you add up the voltage (1.5 x 4 = 6v) but the current capability of the set is still equal to only 1 battery.

Thanks.

Hope that helped!
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Old 15-02-2016, 17:16   #3
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Re: Battery Selector Switches Questions

^^ wot socaldmax said
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Old 15-02-2016, 18:15   #4
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Re: Battery Selector Switches Questions

Very helpful! Thank you.

Two additional questions given those:

1. Electrical current is said to "flow" from the negatively charged terminal with an overabundance of electrons to the positively charged terminal with a depletion of electrons. But in the above referenced wiring diagram, the logic of the conceptualization begins with the positive terminal and ends with the negative. Is this difference only a matter of convention - to put it another way, why in the diagram is flow not depicted as coming from the negative terminal first to the switch, then the the load, and then returning to the positive terminal rather than vuce versa? Would running the negative to the switch first, then to the load, and returning to the positive still function properly or would this cause problems? If so, why does logic of diagrams and"beggininngs andcends" oppose conceptualization of electrons?

2. With regard for 2, and on a similar note - thinking if the logic of diagrams from "beginning to end" beginning at positive and returning to negative - if the return points ofctwo batteries on motor make no difference and can be attached at the same location, the difference between the positive terminal of battery 1 would still be shared with difference in charge of negative terminal of battery 2. The positive terminal of battery 2 seems to be the only thing cut out by the battery Selector switch. And assuming the negatively charged terminals of both batteries 1 and 2 to be equally negatively charged, I still don't see why flow is restricted to a single battery when only one is selected on Selector switch but output returns from motor to negative terminals may be shared? This may not be worded as clearly as I would like...
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Old 15-02-2016, 18:42   #5
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Re: Battery Selector Switches Questions

1. Yep, purely a matter of conventions. You can thank Ben Franklin for it
Actually charge flows in different directions in different materials. We are used to thinking in terms of metallic wires where the charge carriers are electrons and the flow is from negative to positive, but in many cased charge flows in the opposite direction and in other cases in both directions simultaneously.

2. Don't even think about the direction of flow - it is irrelevant. You need a complete circuit for current to flow. If the "line" between the positive and negative terminals is cut anywhere i.e. there is no continuous connection between them, then no current will flow through that line. If there is a continuous connection, current will flow.
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Old 15-02-2016, 18:53   #6
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Re: Battery Selector Switches Questions

Okay, on 2, continuous connection would appear to exist between one positive and two negative locations. Even disregarding direction if flow, given 2 batteries, one positive terminal connected to two possivkecoaths, what dictates one path over the other? Or is one positive not continuously in contact with 2 negative terminals? If location of "negative" connections "motor-side" can be the same from both batteries - isn't one positive shared with two negatives? Still confused in this point...
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Old 15-02-2016, 18:59   #7
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Re: Battery Selector Switches Questions

All good answers above.

HOWEVER, it is unusual to run the negative cable from each battery -- or battery bank -- all the way to the engine or to a negative buss. Rather, it is more common to run a large cable between the negative poles of each battery -- or battery bank.

Then, only one negative cable goes to the engine ground, perhaps thru a positive buss, and only one positive cable goes from the switch to the load (in this case the engine starter).

Bill
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Old 15-02-2016, 19:07   #8
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Re: Battery Selector Switches Questions

If that's the case, in either case - whether one cable links both negatives or whether two separate cables link to two batteries from same location motor side, it would seem in both cases there are a single positive with two negatives connected to it through equally conductive conductors if battery Selector switch is set to only one battery - ?
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Old 15-02-2016, 19:09   #9
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Re: Battery Selector Switches Questions

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Originally Posted by MrAntigone View Post
Very helpful! Thank you.

Two additional questions given those:

1. Electrical current is said to "flow" from the negatively charged terminal with an overabundance of electrons to the positively charged terminal with a depletion of electrons. But in the above referenced wiring diagram, the logic of the conceptualization begins with the positive terminal and ends with the negative. Is this difference only a matter of convention - to put it another way, why in the diagram is flow not depicted as coming from the negative terminal first to the switch, then the the load, and then returning to the positive terminal rather than vuce versa?

I don't see any diagram in your first post, you might not get to post pics until after 5 posts or something. I think I understand what you're asking. I've seen people draw diagrams any way they like, but I grew up with the convention that the power source was on the left, fed various loads drawn on the page from left to right and circuits were drawn to the right and downward with ground connections on the bottom or towards the bottom. You're right, this depiction has nothing or very little to do with actual direction of the flow of current. You just have to keep in mind electrons are negatively charged and the current flow and circuit design will always make sense. At least it does to me.

As for why the switch is on the positive side, I think a lot of times it's because the chassis of a car or electronic device like a charger or inverter is always at ground. If a wire comes loose, you can shut off the switch on the positive side of the load and now the loose wire and anything it could touch would be at the same ground potential. Many devices are grounded via a separate ground wire, so if the switch interrupted the negative side of the circuit, a loose cable would still have 12v of potential (or whatever the power source was) to ground against the device cases. There might also be other reasons, but I can't think of them at the moment.


Would running the negative to the switch first, then to the load, and returning to the positive still function properly or would this cause problems? If so, why does logic of diagrams and"beggininngs andcends" oppose conceptualization of electrons?

2. With regard for 2, and on a similar note - thinking if the logic of diagrams from "beginning to end" beginning at positive and returning to negative - if the return points ofctwo batteries on motor make no difference and can be attached at the same location, the difference between the positive terminal of battery 1 would still be shared with difference in charge of negative terminal of battery 2. The positive terminal of battery 2 seems to be the only thing cut out by the battery Selector switch. And assuming the negatively charged terminals of both batteries 1 and 2 to be equally negatively charged, I still don't see why flow is restricted to a single battery when only one is selected on Selector switch but output returns from motor to negative terminals may be shared? This may not be worded as clearly as I would like...
I guess you've got the switch selected to battery 1, and battery 2 is disconnected. In this case, the voltage, or potential energy, at the engine ground is 0v, as well as at both battery grounds. Positive terminal of batteries 1 and 2 would both be about 12v, but are independent of each other. If you turned on a motor or other high draw device, you would see the voltage of battery 1 start to drop lower than battery 2 because current is flowing out of battery 1 and not battery 2, since 2 is not connected.

If battery 2 is at 13v and battery 1 is at 12v, you would see 0v at both negative terminals, 12v on battery 1 positive, 13v on battery 2 positive and a 1v difference between the positive posts of battery 1 and 2. Again, current is flowing through battery 1 because it has a complete circuit, it is not flowing through battery 2 because the circuit is open at the switch.

I hope I understood your question and answered it properly. What you might be confused about is the difference between voltage, which is potential, and current which is the actual flow of energy.

A fairly accurate analogy is to compare it to water.

Wires are like pipes.
Water is like electricity, it flows through wires like water through pipes.
Water always flows from high to low ground, or high pressure to low pressure.
Electricity always flows from high electron concentrations to low.
Voltage is potential energy, like water pressure in a pipe with the valve shut. The pressure is built up in the pipe.
Voltage is like pressure in the wire, current is like water flow, it's a measure of energy flowing through the wire or device.
If you have high water pressure and a small pipe, you can still get a lot of water flow because of all of the pressure. Or you could have a larger pipe with lower pressure and a lot of water could flow nice and slow.
Higher voltage(like 110v) can be carried on thinner wires due to the higher potential energy, just like a thin pipe and high pressure. Lower voltage (like 12v)requires a thicker wire to carry current safely.
A switch is just like a valve, it shuts off current.
A resistor is a lot like a kink in the hose, it restricts the flow of electrons proportionally to it's resistance.
A capacitor is a lot like a water balloon or rubber reservoir, you can store a little power in it and it can smooth out pulses in the current flow, like a reservoir smooths out pulsations in the water pipes.
A battery is like a large water tank with a little leak in it. Sure, you can store 100 gal of water in it for future use, but if you use it, you will have to put back about 120 gal because of the 20 gal that leaked out. The longer the battery sits waiting, the more leaks out (self discharge.)
If you think about loads as being like water pistons and water wheels and other stuff that water can move or power, then the water flowing from the battery through wires through various loads and back to the battery through other wires will all make sense.

The analogy isn't perfect, but it's pretty close.
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Old 15-02-2016, 19:44   #10
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Re: Battery Selector Switches Questions

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Originally Posted by MrAntigone View Post
Okay, on 2, continuous connection would appear to exist between one positive and two negative locations. Even disregarding direction if flow, given 2 batteries, one positive terminal connected to two possivkecoaths, what dictates one path over the other? Or is one positive not continuously in contact with 2 negative terminals? If location of "negative" connections "motor-side" can be the same from both batteries - isn't one positive shared with two negatives? Still confused in this point...
All battery negative terminals should be connected to a common ground, so you can think of them as all just being "one negative". Each battery has a voltage potential between that common ground and its own positive terminal.

If there is no "uncut line" between the battery's positive terminal and that common ground/negative terminal then no current will flow.

If there are any "uncut lines" between that positive terminal and the common ground, current will flow through those lines. How much current will be determined basically by the resistance in that line (which is essentially the "wattage" of the electrical equipment in that line).
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Old 15-02-2016, 19:57   #11
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Re: Battery Selector Switches Questions

I didn't draw or show a sketch in my first post. I just described one - and your initial report actually responded to that description which is the source of my confusion. I described a sketch in which two negative terminals of two separate batteries are connected to two different spots on the motor. You said they could be the same spot so long as connections were clear of corrosion. If two negative cables are connected to the precise same spot motor-side, and battery Selector switch is only selected to battery one or two but not all - it seems that in either case one positive of one battery is conductively connected to two negative terminals of two batteries. Without going further - am I mistaken so far? And if so, if both negative cables can be connected to same spot, how so?
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Old 15-02-2016, 20:01   #12
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Re: Battery Selector Switches Questions

If the battery switch is in position #1, then current will flow from battery #1, but NOT from battery #2 because there is no return circuit.

For current to flow you need a complete connection from negative to positive on the same battery. Yes, the "other" black wire is there leading from the engine ground to the negative post of battery #2. But the positive side of that battery is broken by the switch.

And, if you're going to run two ground wires directly to the engine, then you DO want to connect them at the same point on the engine.

You really need to get a primer on marine electrical circuits, especially DC circuits. I recommend Charlie Wing's book.

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Old 15-02-2016, 20:10   #13
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Re: Battery Selector Switches Questions

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If the battery switch is in position #1, then current will flow from battery #1, but NOT from battery #2 because there is no return circuit.

For current to flow you need a complete connection from negative to positive on the same battery. Yes, the "other" black wire is there leading from the engine ground to the negative post of battery #2. But the positive side of that battery is broken by the switch.

And, if you're going to run two ground wires directly to the engine, then you DO want to connect them at the same point on the engine.

You really need to get a primer on marine electrical circuits, especially DC circuits. I recommend Charlie Wing's book.

Bill
I probably need a book, but from another post in the thread "flow" is up for grabs. If flow is from an overabundance of electrons to a deficiency, it would seem two superabundances of electrons connected to the same point (two black cables leading fromntwo separate negative posts) connected to one deficiency of electrons or one positive post would flow to a single location of deficiency. No case above suggests single circuit. And language of "ground" further confuses things as I first thnk of homes and AC systems in which a ground would beva third location outside incoming and outgoing flows from power plants. Butbin DC systems ground iuscsame as negative?
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Old 15-02-2016, 20:56   #14
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Re: Battery Selector Switches Questions

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I probably need a book, but from another post in the thread "flow" is up for grabs. If flow is from an overabundance of electrons to a deficiency, it would seem two superabundances of electrons connected to the same point (two black cables leading fromntwo separate negative posts) connected to one deficiency of electrons or one positive post would flow to a single location of deficiency. No case above suggests single circuit. And language of "ground" further confuses things as I first thnk of homes and AC systems in which a ground would beva third location outside incoming and outgoing flows from power plants. Butbin DC systems ground iuscsame as negative?
Perhaps you aren't aware that the current flow doesn't so much flow from one point to another as in just from a negative battery post to a positive battery post on a different battery. The must be continuous circuit (as others have explained above).

The internal chemistry of the battery provides for the continuous circuit (or loop if you like). Therefore both battery posts of any battery must be part of the circuit.
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Old 15-02-2016, 21:03   #15
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Re: Battery Selector Switches Questions

You seem to be confused because of Mr Franklin. Don't worry about electrons, just think of your battery having a Guzouta (POS) terminal and a Guzinta (NEG) terminal. Also, since boats are non-conductive, there is no Ground, and all circuits must have a return wire to a common NEG.
When we got our boat, it o had a copy of "The 12 Volt Bible" on board. Looks to be a pretty good "intro to electricity" type book.
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