IMAA's Marez speaks out on parallel systems  
View related threads: (in this forum | in all forums)

Tower Hobbies
Enter up to 4 keywords or Tower stock numbers
Logged in as Guest



Users viewing this topic: none
  Printable Version
       

All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> IMAA's Marez speaks out on parallel systems
Page: [1] 2   next >   >>  

Login
Message << Older Topic   Newer Topic >>
IMAA's Marez speaks out on parallel systems - 3/25/2005 3:10:20 PM   
Red Scholefield



Posts: 4902
Joined: 12/8/2001
From: Newberry, FL, USA
Status: offline 		Eloy Marez writes in High Flight, Spring 2005

“ON THE SAME SUBJECT, somewhat anyway, is the use of a backup battery system. I know it is commonly thought and done that the best (read: cheapest!) way is simply to plug another battery into an unused servo socket in the receiver. That is fine - but only a long as everything is fine! In the event of a discharged or defective cell in one of the packs, the good battery will attempt to charge it. That is the way battery charging is done my friend, a higher voltage is applied and charging current flows.

I have tested this and, using a professional quality ammeter, found that a good 4-cell NiCd battery will charge a defunct one at three amperes. Right three amps. This rate will deplete the good one very rapidly, enough so that it too will good [go] bad before the end of your flight if the first one fails soon after takeoff.

The Cermark Company to the rescue - it has recently introduced its new 'Power Backer’, which takes a secondary normally less capacity battery along for a ride and switches it in if the primary one goes low. It will then light a red LED so you can know that you landed on the backup battery – the important thing is that you were able to get back under full control.

The rest of the good news is that Cermark's unit priced at only $19.95 - about halt of all other similar devices, around.”

With all the modelers successfully employing the simple parallel pack/dual
switch system over the last decade, Mr. Marez wants to complicate the system
by adding more complexity. One has to wonder if he ever did a thorough
failure analysis comparing the two options for reliability. We won't get
into the probability of a pack failing in flight after a good preflight ESV
check or the reliability of a redundant path vs. single path, or the
charge/discharge characteristics of Ni-Cds . . .or the hundreds of flight
hours of giant scale models with no failures using the parallel pack/dual
switch system.

< Message edited by Red Scholefield -- 3/25/2005 3:28:56 PM >

_____________________________

Red S.
AMA 951 Leader Member
       Post #: 1

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 3:14:38 PM   
famousdave


 

Posts: 2263
Joined: 9/16/2003
From: Bradenton , FL, USA
Status: offline 		I would think that 99.9% of battery failures in flight are open packs (i.e. vibration breaks the tabs) anyway so basically the backer device is useless for that scenario. I have never seen a shorted cell in flight, only during charging.

I have been using the simple setup and it has served me very well.
KISS prevails.
DP



(in reply to Red Scholefield)
       Post #: 2

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 3:39:49 PM   
Red Scholefield



Posts: 4902
Joined: 12/8/2001
From: Newberry, FL, USA
Status: offline
quote:

ORIGINAL: desertpig

I would think that 99.9% of battery failures in flight are open packs (i.e. vibration breaks the tabs) anyway so basically the backer device is useless for that scenario. I have never seen a shorted cell in flight, only during charging.

I have been using the simple setup and it has served me very well.
KISS prevails.
DP


I think we would have to take a careful look at just how the backer device is employed in the system before a blanket statement can be made. Regardless, it does add more complexity which usually compromises reliability. I think most will agree that an ESV check on the pack(s) prior to flight eliminates a significant number of "in flight" power failures. This, in conjunction with using the time proven parallel/dual switch scheme reduces the chance of system power failure to acceptable levels for most.

_____________________________

Red S.
AMA 951 Leader Member

(in reply to famousdave)
       Post #: 3

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 3:43:35 PM   
branded



Posts: 984
Joined: 7/16/2002
From: cary, IL, USA
Status: offline 		I happen to agree with Mr. Marez. The only way to reliably provide for backup is to isolate the packs.

Contrary to what Desertpig says the most prevalent mode of failure is a shorted cell, not open. I'm not saying that it never happens but rather packs do fail in the shorted cell mode more often than the open cell (broken tabs) mode.

Red, one could argue that adding the additional pack, without regard to the most prevalent mode of failure, increases the chance of experiencing the most prevalent mode of failure by a factor of two times.......in other words you've decresed the reliabilty of the system by adding components that could concievably cause an electrical contention if one or the other fails. That's not what you're looking for, is it?

When designing reliabilty into a system one must always consider all modes of failure and then assure that the most likely is covered. Simply plugging another pack, via a second switch is not covering the most prevalent failure mode.

I'm an EE and have much experience with designing redundancy into a system.

(in reply to famousdave)
       Post #: 4

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 3:46:51 PM   
exeter_acres



Posts: 6025
Joined: 9/22/2004
From: Johns Creek, GA, USA
Status: offline 		Interesting read......but to me it sounded like a paid Cermark ad

_____________________________

This plane has no limits as to performance with this engine except vertical,..

(in reply to branded)
       Post #: 5

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 4:03:29 PM   
Red Scholefield



Posts: 4902
Joined: 12/8/2001
From: Newberry, FL, USA
Status: offline 		From one that is also and EE and PE and has 30+ years in the design and application of battery systems . . . .

You need to understand the nature of cell shorts and how they occur in your "reliability analysis".

A short develops in a Ni-Cd or Ni-Mh when conductive particulate bridge the separator or the separator itself deteriorates to the point where it allows the positive and negative plates to touch. Rarely does the short occur all at once but rather building up a very small conductance path termed "soft shorts". In a charged cell the energy in the cell will blow away any short as it tries to develop. You've heard about "zapping" cells. The cell actually zaps itself before the short can develop. Only in cases of severe overcharge at high rates can the separator melt down to the point where the plates contact each other (hard short). In this case the energy in the cell then dumps and we have what is referred to as a hot steamer, the electrolyte boils, nylon in the separator melts down and is forced by the steam through the vent. On some occasions the vent is clogged by the molten nylon separator and becomes inoperative causing the cell to rapidly disassemble. So under normal circumstances a cell maintained at some state of charge is much less likely to short than a cell that is completely discharged. It should be noted however that the self discharge increases rapidly in cells where there is a short building (high resistance -soft short) due to separator deterioration and/or cadmium migration. One other shorting mechanism is a manufacturing defect where the positive or negative collector tab bridges the opposite plate. These usually fall out before the cells are shipped or assembled into batteries.

Bottom line: After a good ESV check, the probablity of a shorted cell occuring during any given flight session is nill. I challenge anyone to present evididence to the contrary. On the other hand, the probablility of a single point failure (ie. open circuit), be it the switch, connector, wire, solder joint, intercell tabs is the thing that the parallel/dual switch system addresses. This is only exceeded by the probability of someone forgetting to charge, or thinking he charged when the charger was plugged into a switched outlet or not functioning itself, and flying without an ESV check.

_____________________________

Red S.
AMA 951 Leader Member

(in reply to branded)
       Post #: 6

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 4:17:48 PM   
branded



Posts: 984
Joined: 7/16/2002
From: cary, IL, USA
Status: offline 		I'm well aware of the mecahnism by which a cell shorts, Red. Given all that you've just written, may I ask exactly what is the point of adding the second battery if not to provide for an "inflight" failure?

Given that you advocate checking each battery with an ESV before flight, what's the supposed inflight failure mechanism you hope to avoid with the second battery?

Obvousely when both switches are in the ON position, the batteries will seek equilliberium and in essence become one source with additive capacities.

(in reply to Red Scholefield)
       Post #: 7

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 4:21:19 PM   
branded



Posts: 984
Joined: 7/16/2002
From: cary, IL, USA
Status: offline
quote:

ORIGINAL: exeter_acres

Interesting read......but to me it sounded like a paid Cermark ad


I have no affiliation with CERMARK. You can easily isolate the packs with a couple of SCHOTTKEY rectifiers in lieu of the CERMARK gadget. The fact is there was once a supplier in the RC world that provided his NICAD packs with such isolation.

If using a SCHOTTKEY rectifier make sure you choose one with a forward voltage drop of .1v or less.......

(in reply to exeter_acres)
       Post #: 8

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 4:43:15 PM   
Rodney



Posts: 4718
Joined: 12/8/2001
From: FL
Status: offline 		If you use two 5 cell packs with any good high current (by high current I mean one which will easily handle the maximum current the battery is required to supply) diode in series with each battery pack, you will have a pretty good insurance of not having an electrical failure due to supply problems whether the failure mode is an open or shorted cell in one of the packs. If one pack fails, the diode isolates the bad battery from the rest of the system. In some 40 plus years of flying RC, my only in flight battery failure was a cell which shorted in flight.

(in reply to branded)
       Post #: 9

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 5:07:11 PM   
famousdave


 

Posts: 2263
Joined: 9/16/2003
From: Bradenton , FL, USA
Status: offline
quote:

ORIGINAL: branded


I happen to agree with Mr. Marez. The only way to reliably provide for backup is to isolate the packs.

Contrary to what Desertpig says the most prevalent mode of failure is a shorted cell, not open. I'm not saying that it never happens but rather packs do fail in the shorted cell mode more often than the open cell (broken tabs) mode.

Red, one could argue that adding the additional pack, without regard to the most prevalent mode of failure, increases the chance of experiencing the most prevalent mode of failure by a factor of two times.......in other words you've decresed the reliabilty of the system by adding components that could concievably cause an electrical contention if one or the other fails. That's not what you're looking for, is it?

When designing reliabilty into a system one must always consider all modes of failure and then assure that the most likely is covered. Simply plugging another pack, via a second switch is not covering the most prevalent failure mode.

I'm an EE and have much experience with designing redundancy into a system.



Branded -

If we're going to throw around titles, I'm an EE PhD, electrical and aeronautical engineering with a minor in mechanical. I have extensive experience in power system design, including battery subsystems. I have performed very sophisticated ESS and HALT tests on many types of battery cells. In the majority of "in use" failures cells increase in resistance or fail open - far more often than they short. You see this in reduced capacity or a slow degradation of the batteries ability to deliver current (i.e. hitting the wall) Without getting technical - Red's explanation above explains it perfectly. This holds true in the batteries used in full scale aircraft, automotive, cellular phones, back up power inverters, rechargeable consumer devices and just about everywhere else batteries are used. Perhaps there is something special about the cells in our flight packs? Doubt it.

In mechanical battery banks (i.e. assembled packs) the prevailing failure mode was an open pack (not an open cell) due to failure of the mechanical bond (i.e. weld) of the tabs or wiring.

While I agree I may have used a blanket statement above without offering any data - I have found no merit whatsoever in using a battery backer or sharing device as long as both packs are routinely checked, in the same capacity, and charged in a similar fashion using a gang charger. I have in fact seen them create more problems than they solve, hence my statement.

If you want to use different capacity batteries or different technologies, then maybe I can see using it.

I wasn't trying to start a p'ing match, just trying to offer up a biased opinion - how's that for piling it higher and deeper!

DP






< Message edited by desertpig -- 3/25/2005 5:16:51 PM >

(in reply to branded)
       Post #: 10

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 5:16:06 PM   
Red Scholefield



Posts: 4902
Joined: 12/8/2001
From: Newberry, FL, USA
Status: offline
quote:

ORIGINAL: branded

I'm well aware of the mecahnism by which a cell shorts, Red. Given all that you've just written, may I ask exactly what is the point of adding the second battery if not to provide for an "inflight" failure?

Given that you advocate checking each battery with an ESV before flight, what's the supposed inflight failure mechanism you hope to avoid with the second battery?

Obvousely when both switches are in the ON position, the batteries will seek equilliberium and in essence become one source with additive capacities.


I think the general consensus is that the primary cause of in-flight power failures has been an opening somewhere in the path from the battery to the receiver, either switch, connector, wire, solder joint or welded tab breaking. By using separate paths from the battery to the receiver this single point failure mode is eliminated. One other caveat in parallel packs is that you can easily add capacity using normal packs supplied with radios. A pair of 700 mAh packs takes you to 1400 mAh without the expense of buying a larger capacity pack. Only the cost of an extra switch harness.

My purpose is not to convince those with alternative systems to change to a parallel pack/switch system, but only to present enough information to help others make an intelligent choice.



_____________________________

Red S.
AMA 951 Leader Member

(in reply to branded)
       Post #: 11

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 5:18:03 PM   
branded



Posts: 984
Joined: 7/16/2002
From: cary, IL, USA
Status: offline 		Are we a bit sensitive, Mr. Desert? You made a comment, I rebuked it in a way that wasn't inflammatory. Sorry you feel that way.

I'm impressed with your credentials but the fact is I've met many (and deal with on a daily basis) many supposed engineers that are not practicing engineers and have little or no practical experience. Not saying you fall into this category or not, just surprised that you would go off the way you did.

I only mentioned that I'm an EE because I do indeed design systems and components that are part of todays most complex military fighter aircraft. That's what I do for a living.

When applying reliabilty to a design, particularly one that will be airborne, one does not approach the problem by simply adding components beleiving that having two, three, or fifty of something automatically relates to an increase in reliabilty.

The point is that without looking at the most prevalent mode of failure adding components to system that do not address that particular, in the least, is a waste of time.....

Red seems to suggest that this scheme exists to prevent an open. Adding a switch in parallel with the primary would cover that base without the second battery. The only way to cover all possibilities is to isolate the batteries.

(in reply to famousdave)
       Post #: 12

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 6:02:54 PM   
JNorton



Posts: 3134
Joined: 1/29/2002
From: Montague, MI, USA
Status: offline 		Branded,
Just another EE wading in. Looking at this from a typical users standpoint, assuming the modeler wants redundancy, dual harnesses and battery packs work. The average user is not going to be doing an exhaustive failure mode analysis.

It is easy to put in two complete switch harnesses and battery packs, everything plugs together. I use a voltwatch on all my airplanes. By having 2 switch harnesses I can cycle each pack individually and use the voltwatch to insure both are connected to the receiver buss. Charging is a snap. It's cheap insurance and easier to do than adding parallel switches or soldering Schottky diodes in series with the packs.

Just my 2 cents.

John

_____________________________

Have desk ... Will Fly....
Saito Member #467

(in reply to branded)
       Post #: 13

RE: IMAA's Marez speaks out on parallel systems - 3/25/2005 7:17:19 PM   
SitNFly


 

Posts: 608
Joined: 2/22/2002
From: Four Corners, USA
Status: offline 		I'm not an EE, but I did sleep at a Holiday Inn last night . Anyway, if all you want is redundancy in the path from the battery to the receiver, buy a pack with two leads. Hangtime sells them as do other vendors. Then connect your two switches, etc, to the receiver. I don't know if another pack really buys you all that much. It's the mechanical stuff I worry about.

(in reply to JNorton)