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Old 27-07-2005, 11:54   #1
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3-P Battery Selector Switches

Most Battery Selector Switches are either 2-Pole (On-Off) or 4-Pole (1-Both-2-Off), with (or without) Alternator Field Disconnect contacts.

Recent discussions of Make-Before-Break, Alternator Field Disconnects, & Zap-Stop Diodes suggest (to me) that a better BatterySwitch arrangement might utilize a 3-Pole Selector Switch (1-2-Both) without the off function. A separate “On-Off” Master Disconnect Switch would be wired (series) between the Selector Common & DC Distribution Panel. The Master Disconnect could be “shrouded” to reduce the likelihood of inadvertent switching (“Off”).

Is anyone aware of such a (readily available) 3-Pole Battery Selector Switch c/w AFD ?

Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 27-07-2005, 16:40   #2
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The REAL solution

Hey Gord, this is a subject near to my heart because I have been dealing with it for years. When I originally designed the Alpha (or Incharge) regulator I made provision for the regulator to "observe" both the regulator enable and the +12/24V power in so that if either the "REG ON" voltage sense (ostensibly connected to the battery bank for desired regulation accuracy) or the alternator output voltage exceeded the "commanded" voltage by some margin then the regulator would limit at that value so no damage would occur.

Because management would not tolerate the slight increase in cost that feature was dropped, yet it is one good solution in that it would not matter if one turned a battery selector switch to OFF or not. Regardless, keep in mind that a "Zap Stop" type device (built in to the Alpha) is still required to absorb what is called a "load dump". Load dumping is caused by the back emf caused by the energy stored in the alternator stator winding that must be absorbed by something or else the voltage heads towards infinity at the alternator output should the output current be interrupted quickly.

Now with an internally regulated alternator (usually they also have a load dump device internal to the regulator) it does not matter if one open circuits a battery selector switch or not yet, as we know, intenaly regulated alternators do not properly charge deep discharge batteries if for no other reason than they inherently have poor regulation. Internally regulated alternators actually regulate the ac output of the alternator before the diodes and NOT the battery voltage as most people mistakenly think. As the alternator output current increases the voltage across the diodes increases and the battery voltage decreases. In addition, there is an internal temperature compensation which decreases output voltage with an alternator temperature increase

Back to the battery switches. It is interesting to dissect one to observe the geometry which allows a simple high current switch to be made. It is easy to see that an "OFF" position comes almost for "free" by merely providing a mechanical detent where the sliding contacts are all off of the pads connected to the battery banks. As a result, it is not likely for a manufacturer to make one without an "OFF" position. Cole Hersey switches are easy to take apart, inspect and "rebuild". It is possible to buy a high current switch to do what you describe from Kraus and Naimer yet such a switch is hundreds of dollars and really industrial grade.

As a side note I'v never felt that using a field disconnect on a battey selector switch provided anything other than a patch for the real problem which should have a real solution as outlined above. Running field current all over the place to a remote battery switch merely decreases system reliability and radically adds to radio noise. In addition the switch cost goes up.

One solution is to wire the alternator output so that it is ALWAYS connected to at least one battery and THAT battery is where the voltage sense wire to the regulator goes. Now the reason that this is a workable solution is because of the concept of battery charge acceptance. Most people don't understand (they may even disagree) that it is O.K. to charge batteries having unequal states of charge in parallel. Depending upon the details you can even mix battery chemistries (AGM, GEL, flooded). The battery with the highest charge acceptance takes the most current. When "full" a battery in good condition will not charge accept hardly any current. Yes, one may gas slightly at an acceptance regulation voltage yet a good system does not spend more than two to a maximum of four hours before going to float and that is not sufficient time to be concerned about gassing flooded batteries at least.

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