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Old 20-03-2011, 17:15   #16
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Quote:
Originally Posted by witzgall

I imagine that putting in such a switch, along with turning off the bms (it has it own switch) would accomplish the same thing. I was hoping to not add to the complexity with yet another switch.

I am aware of the potential damage to be done to flat lining LIFEPO4 cells, but in an emergency, it of course would be worth it.

Chris
I would have thought that if you can switch off the BMS that would take the disconnect out of the circuit

Note the standard 1-2-both switch will actually do what you want and the " both" setting is the same as one of the "1" or "2" positions , but really just disabling the BMS. Should be sufficient.

Anyway as I've said before unless you are charging at C or more a BMS on these batteries isn't needed. HVC events can't happen and in reality a simple low voltage alarm, rather then a disconnect will handle LVC,s

Dave
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Old 20-03-2011, 18:50   #17
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Re: Batt Selector with 1, 2 and NO Both ?

I was wondering the same thing, it didn't make sense.

We have a new boat to us, with 3 banks all 32 volt, I had to replace 2 banks as they were toast. It took awhile to understand how it was wired but here it is, also 2 inverter/chargers wired in parallel, 2 battery switches 1 or 2 both or off. House system is 2 banks charged by engine alt. start battery is charged by separate ac charger. both pos. and neg. have shunts, as well there is a breaker on each bank house and start, as both house sets can be used together.

I was also wondering about adding the auto sensor to charge start battery once house is done.
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Old 20-03-2011, 19:30   #18
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Remember listees the OP is talking about a very specific setup using lifepo4 batteries and a bms

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Old 20-03-2011, 23:44   #19
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Re: Batt Selector with 1, 2 and NO Both ?

Quote:
Originally Posted by osirissail View Post
Only one problem with that diagram - if you look closely the output of the alternator is going to the "other" side of the right hand "on/off" switch and the alternator has no direct connection to a battery except through the switch. This is a recipe for a burnt out alternator should that switch be off or turned off while the alternator is operating. Alternators need a good load for the output or they can go to infinite voltage and burn themselves out.

No different than using the 1-2-n-b switch and switching it to N while running. Cept with the 3 switch solution.. you never need to touch the switch.. ever. You CAN, but you never NEED to like with a conventional switch.

My personal setup uses the BEP switch using an outboard. slightly different wiring, but same concept. http://www.bepmarine.com/media/produ...73148082df.pdf

I would assume you would want it wired like they have it, so you can take power away from the alternator so you can service it(The 'N' position on a traditional switch). If it were on the hot side of the switch..the alternator would get energized all the time..now way to shut it off. Logical to me.

Still shouldn't be messing with switches while the motor is running.. but with a traditional switch, you have to..and that induces potential human error, which is significantly reduced by the 3 switch solution.
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Old 21-03-2011, 06:44   #20
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Re: Batt Selector with 1, 2 and NO Both ?

Quote:
Originally Posted by phantomracer View Post
. . . I would assume you would want it wired like they have it, so you can take power away from the alternator so you can service it(The 'N' position on a traditional switch). If it were on the hot side of the switch..the alternator would get energized all the time..now way to shut it off. Logical to me. . . .
An alternator is not a generator. Rotating an alternator without a load battery attached is a sure recipe to wipe out a $400 alternator. "Taking power away from the alternator" ???? huh? The alternator converts mechanical energy from the engine into electrical power and supplies it to the batteries. The battery connection is important to provide a place/sink for the rotating alternator to send its power so that it does not "run away" and fry itself. Externally regulated alternators are better as the external field circuit keeps the alternator "off" until you wish to turn it on. But the source of the power for the external regulator must be examined and considered.
- - There is nothing to "service" on an alternator other than to clean the screens, tighten the belt or replace it. So to "service" an alternator you physically remove the battery lead and the other wires and unbolt it from the engine.
- - The better battery switches have a continuous connection - except for a micro-second interruption - between selections of position 1 and 2. If you have an external regulator type alternator the better battery switches have a field interrupt switch contained inside the battery switch.
- - Turning simple "on-off" switches with an rotating alternator is a sure recipe for an expensive replacement. Burning out $400 alternators gets to be a bummer real quick.
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Old 21-03-2011, 08:47   #21
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Re: Batt Selector with 1, 2 and NO Both ?

"Rotating an alternator without a load battery attached "
An excellent point, but you've left out a critical part of the picture. Every alternator has a battery sense lead, and how that is connected is critical. In the diagrams shown here--it is not shown at all! And if you have more than one battery, in theory that sense lead MUST also be switched (make before break) whenever the battery is also switched.
The traditional kludge around this is to convert a "3wire" alternator to a "1wire" setup where the sense lead is taken a foot or two away from the alternator and then joined to the alternator output lead on the starter, or whatever the first connection is. This kludge means the alternator will always see it's own output voltage and be fooled into thinking that is the battery voltage. And it will also prevent a runaway alternator, because as soon as the battery is disconnected, the sense wire sees...the full alternator output, at the output, so it throttles back instead of racing away.
Some battery switches have a second set of contacts for the field lead, which does essentially the same thing, but if you've got an external voltage sense lead you can use that instead.
Some alternators have internal protection against runaway, some have the equivalent of a zapstop built in...it pays to find out what your alternator really has.
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Old 21-03-2011, 11:25   #22
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Re: Batt Selector with 1, 2 and NO Both ?

Quote:
An alternator is not a generator. Rotating an alternator without a load battery attached is a sure recipe to wipe out a $400 alternator. "Taking power away from the alternator" ???? huh? The alternator converts mechanical energy from the engine into electrical power and supplies it to the batteries. The battery connection is important to provide a place/sink for the rotating alternator to send its power so that it does not "run away" and fry itself.
Not really its to do with inductance, , firstly an alternator is primarily an inductor, in as an inductor,this rules applies V = L * dI/dt , hence if the current is interrupted , ie di/dt approaches infinity, the voltage produced can be infinite, in practice, load dump spikes of 100 volts are typical.

Most modern regulators actually have protection of the diode bridge built in and can survive a disconnect transient( load dumping. Not reccomended, but its not the same as the old regulated units. In cars this is done to ensure that the cars electronics arnt fried if the battery is disconnected.

Dave
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