PeterNzen

Yep, same applies with a 4 cell pack.

If you think about 'detecting' a short with a meter before flying, what will the meter show if a cell is shorted?

A shorted cell is the same as simply ripping a cell out of the bat, and replacing it with a wire. So if a cell shorts you've basically got a 3 cell pack (with a bit of extra resistance etc.). Try your setup (and the cross-charge experiment) with one 3 cell pack and one 4 cell pack if you are concerned.

stek79

Yes Peter,

a short can be detected using a volt meter.

The problem, I think, is that a short will tear down the batteries, since it becomes a wire in parallel with other cells... it's a real short! This is the reason why Red told me that the setup is not designed against shorts...

stek79

Clarification:

If the bad cell is not completely shorted, we will see a rapid self discharge (as Red says on rcbatteryclinic site) - this is the good situation.Here our approach might work, since we have another good pack that will feed the receiver.

When a real short is present in a cell... I think nothing could be done: a short is when + and - touch, so the cell becomes a wire. It follows that all the other good cells are torn down by the short! Good pack includeded!

To resist to that event the only approach I see is using two smart regulators, that can isolate shorts.

Anyhow, I think the setup is good - as Red says, shorts don't occur in charged cells, because the charged cell will blow it away.

The Raven

Personal opinion here based on one real life situation....

If a pack shorts then don't bother about having a redundant one. I had a freshly charged pack short on the bench. By the time I got it outside the covering had burnt off and the cells were venting and smoke was pouring out. If I hadn't cut the cells apart immediately (not knowing where the short was) I would have expected a fire or something very close too it.

Now, if that shorted pack had occured in flight the plane would have never survived even with a redundant pack. It would have burnt badly or caught on fire entirely before it got to land.

Dual switches, diodes or whatever would not be of any value with one pack having a dead short. As for a pack having a bad cell or whatever, then it's going to help you but then you have to be checking voltages between each flight to pick up on the failure.

stek79

Well said Raven, I totally agree with you.

NJRCFLYER2

My Feedback: (42)

There's shorts and then there's shorts - As Stek relayed from Red's Battery Clinic a few posts back, what happens with an internal cell short (one cell, not the whole pack) is that something fails in the insulating barrier at a confined point and it tends to blow itself out. Bad for the cell in that it isn't going to function right anymore, but it's very unlikely to get excessively hot before the shorted path opens up by itself. That scenario is harmless in a parallel pack config, since it's going to only draw an insignificant amount of current - it's OPEN after the initial failure.

Then there's the barn burner variety of battery PACK short like Raven had, which can happen because of failed insulation of external wiring. Could be wire-wire, wire-cell case, improperly secured battery tabs etc. Some packs aren't made very well and are more susceptible to this kind of failure. It's quite possibly going to go up in flames. How to prevent that? Buy a well made pack. Protect it and all wiring from chafing. Route wiring safely away from everything else that could pinch it, rub it etc. Use a softer material than the insulation is made of to retian the wiring, something like the little triangle holders that JTEC makes are a good example. Take battery packs out of service if they have been involved a crash.

stek79

NJ,
thanks for the infos. I'm not expert, but I remember I saw a pack with a bad cell, but without a short in the way we are used to think of it. Perhaps, this is not a true short, I don't know.

Anyhow, the various experts seems to agree that shorts are very very rare: I asked to Mike McConville too, he told me that he saw only one short occur in his career!

NJRCFLYER2

My Feedback: (42)

Hi Stek:
You could probably damage a cell by dropping it on a hard surface, or maybe crushing it a bit and have a more severe short inside. That is speculation and Red would be the guy to say if that would be more likely to make a severe cell short. Nothing is impossible, at least that's what our buddy Murphy says. I remember when I was about 8 years old, I dropped a dime on a deli counter and it bounced, spun and stopped still standing on its edge. Haven't seen that happen again in 42 more years. Maybe I'll see a really bad NiCad cell internal short first!

stek79

Really good example!!!

I agree with you!

klhoard

My Feedback: (10)

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This guy I know. . . (ahem. . ) . . . was trying to save some time removing the connector from a fully charged NiCd SCR pack . . . . so he clipped both red and black wires simultaneously . . . . with metal tipped dykes. . . . . he quickly learned what "low internal resistance" means. . . .
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Remember folks, it's not the Volts that kill ya, it's the Amps. . . .
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BTW - I have used the two parallel packs plugged directly (no diodes or other junk) into different channels on the receiver and it works GREAT. In addition to redundancy, the parallel packs give your servos twice the Amps to work with - especially important when using multiple digital servos.
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stek79

Thank you Keith.

So, based on various people (including top pilots) experience the dual pack & switch is a really good solution, with both redundancy and more current, as Keith noted.

It is interesting the fact that regulators are not needed, actually: in case of a "bad" short, even that setup will not be able to land the plane... or at least, land an unburned plane!

Did I forgot one situation in which regulators can be useful, with respect to reliability of radio system?

stek79

I mean: a situation in which regulators can save a plane while the dual batt approach would fail...

KeithB

My Feedback: (1)

I don't think so, however, if you want to go with a primary/backup setup using a smaller backup battery the regulators (or at least one on the smaller battery) are necessary. Recall that the regulator for the primary battery feeds a higher voltage than the backup thus causing the draw from the primary.


Here's a possible setup:

Primary: 5 cell NiMH 1650 mah (5 oz), no regulator (assuming your RX can handle a 6 volt pack with no regulator)
Backup: LiPoly 900 mah (0.4 oz) with a 5.1 volt regulator (1 oz.).

Total Weight 6.4 oz.

Since the 5 cell pack should always provide > 5.1 volts the backup will never be pulled from unless a failure on the primary.

*** DANGER ***: Note, the danger here is accidentally charging the LiPoly with the wrong charger. If you us this setup I HIGHLY recommend changing the charging plug on the LiPoly so it won't fit on your standard NiMH charger.

Personally I'd use two lithium batteries.

If you want to avoid lithium and don't mind running 4-cell packs the just run two 4-cell packs without regulators.

Keith

KeithB

My Feedback: (1)

BTW, before I went to Lithium batteries I used a five-cell 750 mah triple A battery as my backup to save weight.


Keith B

iflyrctoo

Hey Kieth,
I think the main advantage of using a regulator is that you will always have the same voltage going to the servos. That means that the servo speed will always be constant and won't change as the voltage on the battery changes. In theory, it should fly the same every time.

Joe Dunnaway

The Raven

Very good summary. With an upcoming gas project I may now consider dual packs. The extra amps are beneficial and I'm hesitant to use high Amp 6V packs.

Why not big mAh 6V packs, you might ask?

From the experience of a friend, his high Amp 5cell pack (in this case 3300mAh) full charge yielded almost 7volts, enough to make the receiver and servos jitter. Yes, I know that the voltage will drop as current is used but that seems to be wasting mAh you wanted in the first place. Additionaly, with regulators costing upwards of $80 locally they are more expensive than the batteries. Looking at alternative regulators there are some simple designs out there but none will really handle loads over 1Amp.

From my perspective, the benefits of twin battery packs becomes clear. You don't end up over volting, you get the extra mAh, don't need an expensive regulator, and get the bonus of some redundancy. Sounds good to me.

OK, 5 cell (6V) battery packs may belong in another thread but dual batteries sounds like a safe way to more power and inherent redundancy.

stek79

Well said Raven! I didn't consider the price factor of a regulator! Anyhow, even if it was cheap, personally I don't like to add other components to a critical system , like the link battery - switch - receiver.

Just for information, I've asked in the Futaba support forum whether the 149DP receiver has a built in voltage regulator, as I read here on RCU...

Well, the reply is that it is! The problem of a higher voltage are definitely servos, not receiver. Link to my question:

http://www.rcuniverse.com/forum/m_3765553/tm.htm

very nice, don't you think?

The Raven

Very nice. The reason it doesn't regulate the servos is because it's easy to implement a very basic (less than 1Amp) regulator to the RX circuit for next to nothing (I could make one for under $2). To build in a regulator that supported the maximum amount and max load of servos the regulator would cost a lot more and that RX would get bulky and hot. All the same, you get a neat feature albeit not all inclusive.

I don't think Hitec gear has that feature, I may ask them.

OhD

There is no question in my mind, and I think most folks would agree; the switch is the weak link in the " battery-switch-receiver" link. There are many solutions, but for pattern where weight is very important, the best solution is battery-solid state switch/regulator-receiver. Take a look at the Jaccio Perfect Switch Harness at Central Hobbies. The solid state switch/regulator is orders of magnitude more reliable than a mechanical switch and the regulator function provides the consistency in servo speed and torque that pattern pilots want. And if it hasn't been established here, the two cell Lithium battery packs are an improvement over NiCds and NiMHs in just about every way I can think of unless you believe adding a regulator is bad. Personally, I used a regulator before I used Lithiums because they improve system performance. As far as reliability, Lithiums seem to have a gradual death that makes it easy to know when to replace them before they will fail in a manner to crash an airplane. My vote; lithium battery-perfect switch harness-receiver.

Jim O

The Raven

Ohd you make very valid points. Of course, not everyone will have the same needs as we don't all fly the same.

Good thread so far: the pros and cons of dual batteries, alternatives, some myth busting etc

stek79

Jim, thanks for the infos, how that perfect switch works?

The Raven

A wild guess to how it works is that the switch is momentary contact which then triggers transistored switching to pass the power through the regulator. In simple terms it's more like a relay, the switch is super low current which then signals the transistored switch in the regulator to do it's stuff. Even if I'm only partly right, that's a pretty clever way to do it.

stek79

Nice idea!

But I still see a switch!!! Ok, it works like an open circuit... but...

OhD

Not sure I understand your concern. The Perfect Switch Harness comes with a switch but it can be replaced by a shorting plug if you don't want a switch. The shorting plug is removed to turn the system on and installed in order to turn it off.

Jim O

The Raven

If I understand the concept correctly (remembering I'm guessing how the solid state system works) the main benefit is that the main load does not pass through the switch at all.

Normal switches are common point of failure, and because they carry ALL the electrical load their failure means no power. With this system the switch can fail, which will limit the ability to turn the power on/off. So, a failure of the switch during flight shouldn't interupt power at all.

Also, no vibration induced failures with the main load circuit.