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Dual Circuit battery switch

Old 06-03-2016, 06:59 PM
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Default Dual Circuit battery switch

Am I wrong to assume that when this battery switch is switched to both batteries such as the pic that the engine will also charge both batteries?

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Old 06-03-2016, 07:04 PM
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Yes but by the kit with the acr so you don't have to move the switch. A switch should never be in both except for emergancys
Old 06-03-2016, 07:29 PM
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Agreed. The ACR with the built in charger is really nice too Bluesea 7655.
Old 06-03-2016, 10:43 PM
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Yes it will, it is switched to 'BOTH' AFTER the engine has started in order to charge the house, as well as to prevent the voltage spike from the starter motor when it is stopped after cranking. Depending on engine, you may have a secondary charge output for the house battery which negates using the switch at all. If not, invest in an ACR to avoid voltage spikes being applied to electronics as a consequence of a high resistance house battery connection.
To support that comment...

What happens is that the collapsing magnetic field of the starter motor has no resistance and changes polarity causing a high voltage negative spike to the battery positive. If the battery terminal has resistance due to lack of maintenance, this spike cannot be absorbed by the battery acting as a big capacitor, so the electronics cop it as a reverse polarity high voltage. We all know the consequences of reverse polarity.

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Old 06-04-2016, 06:12 AM
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Originally Posted by isitstuffed View Post
to prevent the voltage spike from the starter motor when it is stopped after cranking.
To support that comment...

What happens is that the collapsing magnetic field of the starter motor has no resistance and changes polarity causing a high voltage negative spike to the battery positive.
I'm trying to grasp this.
Guess I don't understand what "after cranking" means.

"during cranking" the starter motor is connected to the battery positive via the contacts of the starter solenoid.

Isn't what precipitates the "collapsing magnetic field of the starter motor", the opening of the starter solenoid?

hence the starter motor is no longer connected to the battery positive
Old 06-04-2016, 06:42 AM
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After cranking just means once the engine has started the amp draw returns to normal and voltage stabilizes.
Old 06-04-2016, 06:54 AM
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Originally Posted by RogerMurdock View Post
I'm trying to grasp this.
Guess I don't understand what "after cranking" means.

"during cranking" the starter motor is connected to the battery positive via the contacts of the starter solenoid.

Isn't what precipitates the "collapsing magnetic field of the starter motor", the opening of the starter solenoid?

hence the starter motor is no longer connected to the battery positive
Yes that's correct but the collapsing magnetic field after solenoid opens, reverses polarity and the negative cable, which is still connected to the battery negative terminal, becomes positive. So this is normally a short duration short circuit to ground.

If this negative battery terminal exhibits any resistance to this positive spike it superimposes itself on the negative supply to the electronics negative stud.Usually the negative supply for the electronics is connected the same battery terminal.
The electronics is permanently connected to the negative also, even when turned off.
In worst case the polarity protection diodes can rupture as the spike may exceed the peak inverse voltage rating of the diode.

Another example would be the ignition coil on a car engine, supplied with 12 volts but generates thousands of volts from a collapsing magnetic field from the coils windings to create a spark from ground to the positive when the points open. This removal of current by the opening of the points does the same as the starter motor solenoid does in your remarks.

You may aware of the automotive relays in outboards that have a diode inside across the coil...this is to put a short across the coil so that the change in polarity and collapsing field causes the diode to become forward biased and provides a short circuit (to within a harmless 0.7 volts) when it turns off to protect the EMC from damage. Again, the much larger starter motor windings do exactly the same, but relies on the battery to absorb it. Therefore good clean tight terminals on the negative are critical.
Old 06-04-2016, 07:13 AM
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Originally Posted by isitstuffed View Post
collapsing magnetic field of the starter motor has no resistance and changes polarity causing a high voltage negative spike to the battery positive.
Originally Posted by isitstuffed View Post
the collapsing magnetic field after solenoid opens, reverses polarity and the negative cable...connected to the battery negative terminal, becomes positive.
well, I guess its one - or the other.....
Old 06-04-2016, 07:41 AM
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Originally Posted by RogerMurdock View Post
well, I guess its one - or the other.....
Yes its not one but the other. .
Wasn't concentrating on the first post as the subject was 'charging' using a battery switch. So got mixed up..although if the negative was switched instead of the positive, it holds true....

So to satisfy your curiosity it should have read 'positive spike to negative terminal' not 'negative spike to positive terminal'.
My two examples clearly advised that. So do you understand now? or do I need to post pictures.
Old 06-04-2016, 07:53 AM
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The amount of energy in the "negative spike" from a starter motor is minuscule compared to the energy required to run the motor so if the battery connections are sufficient to run the motor they are more than sufficient to absorb the inductive spike. The spike occurs in the starter motor and solenoid AFTER they have been disconnected but arcing across the contacts can transfer some to the battery. The only concern is any instruments or electronics that share the cable between the starter motor and the battery instead of an independent connection to the battery, however it is very rare that anything else is connected to the starter cables.
Old 06-04-2016, 06:50 PM
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Originally Posted by yandina View Post
The amount of energy in the "negative spike" from a starter motor is minuscule compared to the energy required to run the motor so if the battery connections are sufficient to run the motor they are more than sufficient to absorb the inductive spike. The spike occurs in the starter motor and solenoid AFTER they have been disconnected but arcing across the contacts can transfer some to the battery. The only concern is any instruments or electronics that share the cable between the starter motor and the battery instead of an independent connection to the battery, however it is very rare that anything else is connected to the starter cables.
I think some are getting bogged down on conventional resistance theory.
The starter motor is drawing anything from 50 amps to over 300 depending on the engine.
The amount of magnetic flux is huge which is an opposing magnetic force. The number of poles (coils) determines its RPM fed usually via 2 sets of brushes, therefor it has a frequency. Consequently it is a 'reactance' and the current lags the voltage by 90 degrees. Inductive Reactance is Xl=2πfL where f is frequency and L is Henrys and Xl is in Ohms. Then there's Flemings left and right hand magnetic force rules for motors and generators yada yada yada. Suffice to say the usual Ohms Law does not apply.

As soon as the solenoid kills the current there is nothing limiting this magnetic flux from the 2 brush sets, producing a huge back EMF voltage. Same for the solenoid coil.

So as you say, although the battery terminals can deliver and receive the starter motor starting current when cranking, you are referring to terminal resistance. Which also has internal resistance of the battery.

But there is no resistance from the starter motors collapsing 50 to 300 amp field and resulting voltage spike. This reactive power from the starter motor never came from the battery. It is now a generator and it only sees the battery as a capacitor acting to snub the spike. This spike, as advised, will also be present on all negative wires if the battery terminal is corroded enough to allow the cranking current but prevent the snubbing action.

Here is a photo of the automotive relay in my previous post and how the diode prevents damage. The diode conducts the negative spike so it does not superimpose on the negative supply, which you guys seem to think is just ground. The oscilloscope image is the primary of the ignition coil, 12 volts in and 300 out when the points open. These voltages and currents cause electronics damage. Maybe not straightaway but the constant stressing may over time cause failures.
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