radfordc

My Feedback: (14)

I'm not sure what you mean by "belt and suspenders"? If you have one battery (no matter how reliable) then a failure of the switch, a broken wire, or a loose connector will shut down power to the Rx.

Having two of everything absolutely doubles the chance of a single component failure...but any one failure will not shut down power to the Rx. You can check each battery and its associated switch and wiring before each flight. If you're unluckly enough to have failures in both power systems during a flight then it's really not your day.

I'm not sure which "Red" you're referring to? If its this one: http://www.hangtimes.com/parallel_packs.html then he is very much in favor of a dual parallel power system.

"The use of redundant parallel fight packs (packs may be of different capacity but MUST be of an equal number of cells) is an excellent way to increase the available flight time and significantly improve the reliability of the on power system. The simplest means is to run two complete wiring harness, switches and charge jacks from each pack and plug one into the normal battery port and the other into an extra channel on the receiver."

JPMacG

My Feedback: (2)

If the failure occurs as an open circuit then redundancy works. If the failure occurs as a short circuit then redundancy just makes you more likely to crash. Most failures are open circuits, so the duplicate pack and wiring harness is usually a benefit. At least that is true with NiXX. I'm not sure if lithium packs tend to fail as opens or not.

radfordc

My Feedback: (14)

I add diodes to my circuits to protect in the event a battery fails "shorted". Yes, this adds another component that could possible fail...but diodes are extremely reliable and, again, one failure doesn't cause a problem.

BarracudaHockey

My Feedback: (11)

How many batteries fail shorted?

radfordc

My Feedback: (14)

http://www.hangtimes.com/parallel_packs.html

According to Red's Battery Clinic: (He is talking about NiCd/NiMh batteries)
While it is a fact that the typical failure mode of a battery is for a cell to fail shorted there are some subtleties here that escape many people.


Perhaps the following discussion on the nature of shorts will better help the modeler understand.
While it is agreed that shorts are the failure mode in Ni-Cds batteries one has to look further into the "when" of the failure.
A short develops in a Ni-Cd 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.

Rodney

Radfordc has it correct. NiCad and NiMh almost always fail as a shorted cell. In some 60 years of using NiCad's (and later NiMh's) every battery failure I've had has been a shorted cell, fortunately very few and only one ever caused a crash. As Red (in Hangtimes) has stated, even if a cell shorts out and you have a parallel system, you can almost always make a safe landing as there is still enough power to drive most servos and keep the receiver alive as that short is a high impedance short that limits current drain from the still full voltage cells.

Jim Branaum

My Feedback: (3)

And THAT position of Red's is why I now prefer dual Eneloops. I do not have that assurance with the A123 (or counterfeit) nor do I wish to run an expensive research project finding out.

The real question has to do with the counterfeit stuff. How do you tell? All I have ever heard was how wonderful these were and my experience does not track the tales of greatness so I suspect they may have been counterfeits. That does not mean I am open to experimenting with expensive airframes which is what I was about to do when the first failure was noted.

Radforc, "belt and suspenders" is a euphemism meaning equipment for the basic task AND active back up for that equipment. Sort of like wearing a belt to hold your pants up and having suspenders incase the belt buckle fails.

By the way, having a 4 cell pack fail on landing roll out a few years ago caused me to move to 5 cell packs and that decision has paid for itself more than once. You can tell when 1 cell drops and then it is time to land.


YMMV

richrd

+1+1+1

I concur 2 parallel systems with 5 AA eneloops is my preferred method on all my best stuff,, if one batt goes it still works and still one back system to cover that
BTW been using eneloops in planes since 2008 and my camera has a 2007 in it. Have not replaced one yet!!!
Rich

JPMacG

My Feedback: (2)

So I am concluding that parallel systems are only beneficial if there is a diode in series with each pack to prevent the good pack from discharging into the bad pack, like Radforc said earlier.

But regarding Eneloops versus NiCd, how do we know that Eneloops don't have the same failure mechanisim, or worse?

topspin

Cure it with this: http://www.hobbico.com/radioaccys/hcam6415.html

Also, diodes will not only prevent reverse discharge but can act as a fuse as well if the short exceeds the diodes forward or reverse current ratings. Also, Peak Inverse Voltage rating has no impact on the diodes current handling capability so 1,000 volt PIV diodes are just fine.

BarracudaHockey

My Feedback: (11)

One more time on this then I'll shut up.

A bad back won't drag down a good pack in the time it takes to complete a flight.

radfordc

My Feedback: (14)

I've been misquoted! I don't think diodes are needed in order for dual parallel packs to be beneficial. I choose to add diodes primarily to drop the voltage of the 6.6v LiFe down to under 6v. Why....some servo manufactures recommend that their servos not be used above 6v for maximum reliability and lifetime. Will 6.6v hurt...probably not.

Jim Branaum

My Feedback: (3)

Re - the diode issue. That is not necessarily accurate and Radfordc thinks you misquoted him. There was a dual battery system built 15 or 20 years ago that used diodes to isolate the dual battery packs, but most discovered it was a waste of complexity, weight, money, and voltage (dropped across the diodes and led's involved in the monitoring). I have 2 in a drawer..1 brand new never opened and the other with the case cut to copy the circuit used in the planned duplicate idea I had and never implemented due to other knowledge coming available.

Re the Eneloops VS Nicads failure mechanism, I don't care. I have a dual system so if one fails the other will carry the load until I land. The reason I like the Eneloops is the very slow self discharge (will keep 75% for over a year). This tends to suggest they won't grow the shorts NiCads are known for. But it wouldn't be the first time I got shot in the foot by hype, just the most recent.

RC Flier

My Feedback: (9)

Try NOBS batteries

Rodney

No, you do not need a diode in series with each pack but each pack should be on a separate switch and each pack must be charged separately; i.e. not in parallel for charging.

topspin

I never saw the system you described but multiple diodes and LED's are not necessary. All Silicon diodes drop about .6 to .7 volts but LED's can have a voltage drop between 1.2 to 5 volts depending on the chemistry and output power so they are completely unecessary in such a circuit other than bling.

A single silicon diode in series with each pack simply protects the pack from being drained should one of the packs short out. The 6 to seven tenths of a volt diode drop will vever be missed by the receiver or other components.

JohnB96041

Anyone got advice as to using a 4000mah lipo 7.4v battery as a receiver battery using a good regulator? This is an extra HH DX8 transmitter battery that I have and was thinking of using it as the receiver battery in conjunction with a regulator and setting the output of the regulator to 5.0v. Any comments or help?

topspin

No problem John, just use a BEC, that will reduce the output of your transmitter pack to the correct voltage. You can find them here: http://www.ebay.com/sch/i.html?_trks...=220&_from=R40

Jim Branaum

My Feedback: (3)

I am sure there are lots of things in life and this hobby you have never seen. Have a couple of pictures of the real thing to get an idea of what it was and how it worked.



This item dates to the early to mid 1990's when most folks did not know it all and the vendor assumed the user would want to know when either pack failed. If I remember correctly, it only switched to the back up at that point - but I might be making that up too.

RCdude13

My Feedback: (6)

Getting back to the original thread topic, the two things I don't like about A123's are;

1) You need to balance charge them. My Rx packs are buried deep in my warbirds, making getting to a balance connection impossible. With NiMh packs I charge at the on/off switch which is easily accessible.
2) With A123's there is no way to moniter your batteries throughout a day of flying. I use 2 NiMh packs in all my warbirds. Before EVERY flight I check voltages on those, and the ignition pack. Any imbalances in voltage in the Rx packs alerts me that something is up. It could be a bad switch, bad cell, week pack, etc. With A123's the voltages are the same all day...how do you know if something is going bad?

I keep 18 packs in 7 planes ready to go by having them all on trickle charge using three of the old Ace Digipulse multichargers. Works great for me.

init4fun

Um , , Actually , The Original thread topic was Plane Jim asking where to get a good deal on NIMH , and NOT a pro vs con comparason of NIMH to lithium . In fact , he again makes that clear in post #3 where he specifically states he'd "rather stick with NIMH , Thanks" ..... So , to continue the ORIGINAL , original thread topic , got any good suggestions for low priced vendors for Ol Jim's NIMH packs ?

init4fun

Yeppers , sounds like Plane Jim knows exactly what he wants ......

Jim Branaum

My Feedback: (3)

http://batteriesamerica.com/newpage8.htm and you get to pick your poison. I bought Eneloops, but I am . . er . . ah . . um . . oh yeah, special!

Ken Erickson

Well, not on the original topic, but, yes folks, he is.

Ken

chuckk2

"making getting to a balance connection impossible"
You might consider buying balance connector extender cables.
Generally, the balance current carried is small, and the major concern is a few mv induced noise.
If the pack is way out of balance, and the balancing currents are higher than "normal",
there is room for concern.