Dual Battery Switching

[Jerry 5TJ]

My Ovation 2, like all (?) long-body Mooney planes, has a pair of 24V batteries in the tail cone. 

At a MAPA training session, I was told that switching between batteries while the Master Switch was on was a bad idea as "a fuse could blow."  The speaker was an experienced Mooney CFI and long-time Mooney owner.   At the time I had about 4 hours of Ovation time so I just filed that away for future thought.  

I don't see any such warning in the POH.  

I've looked through the schematics and can't figure out where this "prohibition" comes from.   There are two "Master" relays in the plane, and the Master Switch wire is routed to the battery selector switch which controls which of the two is connected when the Master Switch is on.   The alternators, when on-line, charge the active battery directly.   The non-selected battery is charged through a diode D2 so it is "trickle charged" at one diode forward voltage drop below bus voltage.  At least that's I how read it.   Typical M20M schematic detail attached. 

Question:  Does anyone know of a problem with switching batteries when the Master Switch is on, with or without an alternator on-line?    Or is this warning a remnant of some other model aircraft? 

Battery Detail Schematic M20M crop.pdf

[carusoam]

Jerry,

The system may have some momentary overlap of both batteries during the switch-over...(?)

I have found that when you slowly activate the battery switch you can get a dead response from the battery solenoid... (Don't be slow)

the dead zone was large enough that electricity stopped flowing momentarily.

Since it's a mechanical system, it is best to follow a good switching procedure. Check the POH to see if it has specific instructions(?).

This is my experience while on the ground.  I've done a fair amount of hangar flying my instruments...

Best regards,

-a-

[buddy]

I switch my battery's in flight on each leg before I land so I don't forget and I have not had any problems. I have 2005 with G1000's.

[Bravoman]

 I am no mechanic or electrician, but it just does not feel right to me to switch between batteries while the master is on or in flight. I do alternate  between the batteries for each flight so as to keep an optimal charge on each. I do know they are supposed to trickle between each other, but that is what I have been told to do by an MSC. 

Regards, Frank

[M20S Driver]

Don Maxwell recommends not switching the batteries with engine running.  If my memory serves me right, it was due to relay issues causing a voltage spike as carusoam mentioned above.

[M20R]

I switched mine several times with the power on. The last one caused the relay to fail. The relay was close to $300 plus installation!

[mooneyflyer]

If the battery you are switching to his dead, or nearly dead, it could cause a fire... low probability... but conditions exist

[Jeff_S]

The OP is right...there's no prohibition in the POH from switching the batteries with the Master on. In fact, the official checklist calls for it! However, I was doing this one time as a test (just on battery power, no engine) and I also experienced a momentary power outage on the G1000 system and it reset itself...poorly, in fact. I ended up having to do a cold reboot to get it back in operation again. So from that point on, I leave that switch alone once the key is turned.

[carusoam]

Just a note...

The O1 tests which battery is stronger for the start.  Master is on, check the voltage of each battery, then select strongest battery for start.  The nav Coms and JPI are not on until the instrument buss is turned on.

Firmly activating the battery selector switch is better than slowly activating the switch....

Best regards,

-a-

 

[Awful_Charlie]

I try to avoid switching batteries when in flight or when there is significant load, but if the conditions demand it then I will do so (not happened yet)

My SOP is to prepare to start (inc prime), swap the battery, start (followed by alternators, one on, both on, first off, both on - can check they are both working that way, and avoids larger voltage swings on the bus), and then maintain the same battery for the flight. If the previous sector was short (say less than 30 mins) and the same day then I'll skip the battery swap before start.

Switching a high current will cause some arcing, and I don't see the point in wearing out the relays unnecessarily

[PMcClure]

My procedure is to switch batteries as the first item on my checklist before anything else. I have switched them in flight before, but generally don't see the need. 

 

[ArtVandelay]

There is no "both" position?

There should be very little current coming from the battery, the alternator(s) should be supplying the power, assuming the batteries are charged.

[Danb]

Jerry I attended a MAPA program this weekend and asked the question in the class and in  private to Jerry J one of the most knowledgeable Mooney guys in the field he answered as I expected, it doesn't matter. It is not placarded in my plane or mentioned in my poh to use the situation mentioned to you. I always monitor the two systems and in flight may switch the batteries for an hour or so and then revert back, just to confirm its working as intended. I reviewed the electrical diagram with JJ and he pointed out it's designed to do both, he's an electrical engineer who has worked along with Mooney for decades, I'm clueless in mechanical stuff.

[Danb]

On August 23, 2016 at 10:51 PM, M20S Driver said:

Don Maxwell recommends not switching the batteries with engine running.  If my memory serves me right, it was due to relay issues causing a voltage spike as carusoam mentioned above.

I asked this question to Don last year and he didn't see a problem with it ?

[mike_elliott]

54 minutes ago, Danb said:

I asked this question to Don last year and he didn't see a problem with it ?

Don Maxwell doesn't have a problem with it if you like buying $400 relays from him. Ask him again in a couple weeks at the Mooney Summit, Dan. He will tell you not to do it for the relay's health sake. Who ever told you it was ok to do it because it wasn't placarded not to switch is in error, and his Mooney knowledge lacks here. Perhaps this is something that was "learned" after the POH, after the checklists were printed etc, but in the field, it has been the cause of failures according to Dmax.

[Danb]

Mike I don't have to I understand, I'd rather not buy anything else, Jerry did not mention the placard he was the Mooney guy I'm sure you know he explained the system to me. The great thing about Mooneyspace is the info gathered in which we may have no clue about, It's real easy I just won't do it.   Thanx

dan

[Jerry 5TJ]

On August 30, 2016 at 11:07 AM, Jeff_S said:

The OP is right...there's no prohibition in the POH from switching the batteries with the Master on. In fact, the official checklist calls for it! However, I was doing this one time as a test (just on battery power, no engine) and I also experienced a momentary power outage....

That's a good thing, from the relay circuit point of view: you want "break before make" relay sequence.  That is, the deselected battery relay should open ("break") before the just-selected battery relay closes ("makes") so that the two batteries are not connected in parallel even momentarily.  

If the alternator were running there should be no interruption of power on the bus.  

[Jerry 5TJ]

7 hours ago, teejayevans said:

There is no "both" position?

There should be very little current coming from the battery, the alternator(s) should be supplying the power, assuming the batteries are charged.

There is no "both" option.  The battery switch connects either battery 1 or 2.  

True, if the two battery voltages are close the charging current from alternator should be similar.  

 

[LANCECASPER]

When I bought a Bravo in '96 Mooney sent me to Flight Safety and I was told to use that switch before starting to see which battery had the most charge and not to change it once the airplane was started

[Danb]

11 minutes ago, LANCECASPER said:

When I bought a Bravo in '96 Mooney sent me to Flight Safety and I was told to use that switch before starting to see which battery had the most charge and not to change it once the airplane was started

I took the Flight Safety Course also and can't remember. I have the big book they gave us I'll grab it and look it up there is a section on electric.

[Mark89114]

I don't have enough experience or thoughts either way on this, but I was told to switch batteries every 1/2 hour.  If the concern is the relay arcing shouldn't this switch be turned with the battery OFF?  With no alternator all ship power is going through the relay, if the alternator is online it is feeding the bus somewhere, thereby reducing current flowing through.  Again I am taking a wild ass guess at this. 

 

[gsengle]

There is absolutely NO reason to switch the batteries back and forth every half hour... You sure they weren't talking fuel? :-o

Sent from my iPhone using Tapatalk

[carusoam]

Mark, check your source on that detail....

Both batteries are being charged at the same time.  The one in use gets the usual charge.   The alternate battery gets a trickle charge.  Switching every 1/2 hour?

pp idea, not a CFI...

Best regards,

-a-