Battery Selector Switches Questions

[StuM]

> Butbin DC systems ground iuscsame as negative?

Yes. Here's a simplified diagram.

Inside the switch you can complete the circuit to either battery or to both at the same time.

When you complete the circuit by connecting to "1", current can flow from B1 but there is no unbroken line going via B2 so no current flows from there.

When you complete the circuit by connecting to "2", current can flow from B2 but there is no unbroken line going via B1 so no current flows from there.

When you connect to "B" current can flow from both batteries. They are effectively connected "in parallel" so the voltage won't change, but you have twice as much energy available to go to the starter.

[jwcolby54]

Originally Posted by MrAntigone

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?

Think of it this way - if an electron leaves the negative terminal, another electron needs to enter the positive terminal of the same battery.

So if you have two batteries, but one is "disconnected" (by the switch) no electrons can flow onto the positive terminal of the disconnected battery (the switch is in the positive "side"), so all electrons will leave the negative terminal of the connected battery, and all electrons will flow back onto the positive terminal of that same battery.

If you do have two batteries, both connected, then electrons will leave the negative terminals of both batteries, and so however many electrons leave battery A will need to flow back into battery A, and likewise battery B.

BTW it is definitely NOT the "same electron" moving all the way from the negative terminal to the positive terminal. Just "an electron".

Metals such as copper have "loosely bound" electrons, i.e. electrons can flow easily from atom to atom of copper and so electrons "jump" between atoms. That is what makes copper a "conductor", it easily allows electrons to "jump" from atom to atom.

So one electron "jumps" (or is pushed) onto an atom at one end of the copper wire, essentially "pushing" an electron off of that atom onto the next atom, which pushes an electron off of that atom on to the next... and on and on in a chain reaction until one electron "jumps" off the wire attached to the positive terminal of the battery. Notice that the electron that left the negative terminal is at the wire end attached to the negative terminal and a completely different electron jumped off the positive end of the wire onto the positive terminal of the battery.

By the way, insulators are materials which strongly resist the free flow of electrons from atom (or molecule) to atom. Rubber as an example. It is possible for electrons to flow through insulators but it requires high voltages to make it happen. Think lightning.

I am NOT a chemist or physicist so this description is a high level overview of what is actually going on.

[Capt_Flrp]

Perhaps a different perspective might help the great posts so far. Think of the battery as a pump. The chemical composition of a battery provides the pressure to push/pull the electrons through the completed circuit. If the switch is not connected (turned on) to the positive post of the battery, the electrons will not be pulled from the circuit by the battery (pump?). The battery creates a differential by moving electrons from the positive side of the battery to the negative side. Short out a battery and watch the sparks fly. As the battery sits idly by, the chemical activity has created a "potential" difference between the posts. It does not create a significant differential on a separate battery.

[roland stockham]

Bit of a word of caution re battery switches. These are leftover from the days when most boats had simple electrics an small batteries and they have a problem. First thing is that you can't disconnect n alternation when its running or you blow it up. So to prevent that the switches are 'make before break' ie if it is connected to batt' 1 when you switch to batt' 2 it connects to both momentarily then disconnects from batt' 1. If there is a different level of charge in the two batteries then, just for a moment, they are trying to balance the charge between them. In the days when batteries where typically 1-200A/h that was OK but if you do the to a modern high power start battery and a 500A/h house bank the current is enough to weld with! The result is that the contacts quickly get burn and pitted causing higher resistance and lots of heat. Eventually the whole thing melts and starts a fire. The 'proper use' instructions that come with the switch say to stop the engine every time which helps but often people don't do it and anyway you still get some flashing of the contacts. Diode blocks or solid state relays (VCR's) are much safer.

[Sailorbob8599]

I recall many, many years ago, I was on a small ship about 60' - 80' ... I believe it was an ex-Royal Navy ship ... I noticed the wiring was done differently. The power or positive insulated wire was inside copper conduit tubing which became the negative.
The copper tube was attached with small brass clamps to the surface of the cabin liner wherever power was required; Being a navy boat, all was polished bright.
The end of the copper tube was cut back and flattened with a hole in it to enable attaching to the light base, while the internal wire was attached to the terminal contacting the base of the bulb.
I've never seen this method of wiring since ... has anyone else?
I considered using this system when building my boat but a marine electrician talked me out of it. Now the wiring is buried behind the ceiling and un-accessible unless ceiling is removed.

[tinkrman69]

get the book "sailboat electrics simplified" by Don Casey. All your questions will be answered and then some.

[topmast]

Bob ,I remember that tube wiring.the positive conductor was insulated by a filling of some moderately flexible ceramic (the white stuff) The invention of cheap durable insulating plastic coatings ended that .and not a mistake either. Hiding the ' modern' wiring behind the liner is easily rectified by timely deletion and replacement with accessible stuff well clipped on to the liner.

[Stu Jackson]

This is only true if the 1-2-B switch is wired improperly. The way they do it from the factory. While this isn't directly to the OP's question, he might be interested in these. Stu's diagram was very helpful, these include some more.

OEM 1-2-B Switch Wiring History Alternator/Batteries & "The Basic" 1-2-B Switch BEST Wiring Diagrams

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
1/BOTH/2/OFF Switches Thoughts & Musings | SailboatOwners.com Forums

Originally Posted by roland stockham

Bit of a word of caution re battery switches. These are leftover from the days when most boats had simple electrics an small batteries and they have a problem. First thing is that you can't disconnect n alternation when its running or you blow it up. So to prevent that the switches are 'make before break' ie if it is connected to batt' 1 when you switch to batt' 2 it connects to both momentarily then disconnects from batt' 1. If there is a different level of charge in the two batteries then, just for a moment, they are trying to balance the charge between them. In the days when batteries where typically 1-200A/h that was OK but if you do the to a modern high power start battery and a 500A/h house bank the current is enough to weld with! The result is that the contacts quickly get burn and pitted causing higher resistance and lots of heat. Eventually the whole thing melts and starts a fire. The 'proper use' instructions that come with the switch say to stop the engine every time which helps but often people don't do it and anyway you still get some flashing of the contacts. Diode blocks or solid state relays (VCR's) are much safer.

[MrAntigone]

Thanks for all the great posts and book recommendations!