BarracudaHockey

My Feedback: (11)

When is the last time you saw pack "short" ?

Luchnia

My Feedback: (21)

Good catch. This is just like saying, "Anything can happen" which could also mean that a transmitter could blow up. I see guys come out to the field with all amounts of $$ of redundancy and I ask them, can they run two radios for redundancy?

They look at me sort of odd, but truth is you only have one connection and batter pack in your radio. Granted it is rare that a radio will fail, but I saw it happen and the plane came down to become pieces. They found out the internal antenna wire came apart somehow. Redundancy is only as strong as the weakest link, right? If you cannot change a single point of failure it is still a single point of failure no matter how many other duplicate items you have in the chain.

John_M_

Actually in all the years I've been flying RC, I've never experienced a battery short... but I have had a battery reverse polarity on me, but that was my fault, while I was cycling a NiCad pack I brought the voltage down too far and one of the cell reversed polarity.

But that doesn't mean a battery can never short, it all depends on how much current is being drawn from the battery pack... I've always been a sport flyer, never had an aircraft like these giant scale 3D /aerobatic planes that require those ultra fast / torque digital servos... the more demanding the current draw from these kinds of setups, could push a cell into an overload condition and damage the cell... "could"... that's the unknown that these redundant battery systems are there to try an ovoid the what if's, or could's.

I for one think the more devices you daisy chain together, you're increasing the chances of failure, as electronics do fail,and especially the cheap china stuff... even those wolverine / badger digital "soft" switches can fail in a way that the switch WON'T pass current, they even make sure they state that on their site to protect their liability as nothing is 100% fail-safe.

I just keep it simple, with some redundancy (separate battery packs for the servos and receivers) and then just replace the batteries every flying season, or if they have been unused / dormant for long periods.


John M,

flyguy888

My Feedback: (63)

I have had two battery incidents that brought down planes. Both could have been avoided by better attention to battery life and diagnostics.
Still, batteries give me the willies. Mechanical stuff I can see and handle. Who knows what the heck goes on inside a battery? They are the most mysterious part of my airplanes, probably because I am not an electronics guy.

I do know that cells can short internally. It is rare. But that is kinda what I'm trying to protect myself against. Rare events.
This is what I will have to struggle with. How little is too little, and how much is too much. I dunno the answer to that. This has been a good discussion to look at those questions.

oliveDrab

Hi Dan - I wanted to send you a PM but it's not showing so I'll just ask in this thread. You fly for 90 minutes? Do you fly multiple flights for a cumulative time of 90 minutes ...... or does one flight last 90 minutes?

Thanks
-oliveDrab

Boomerang1

Packs don't usually ''short'', in the EXTREMELY unlikely event of a battery short it's usually
just one cell.

I read an article in an early RC jet magazine discussing & actually CONDUCTING all the possible
battery catastrophes using dual batteries & no fancy back up gismos.

Bottom line? If you conduct basic control checks before each flight on each battery individually you
will pick up a failure. The rate at which the shorted battery will drain the good battery is low & would
take hours. - John.

Charlie P.

I have nine servos in my 27% Ulimate 300 and I use a Batt Share.


But this one was cobbled together three years ago. Since then I have converted two of my other giant scale to LiFe (6.6v) 2100 mAh packs and have had no worries with a single battery and single heavy-duty switch . . . so far.

vertical grimmace

My Feedback: (1)

In fact, if ports to the RX are in short supply, you can use a Y harness, sharing with a servo. The only time I have had a problem here is when I used the Y to my ignition kill. When charging the battery, the charge would kill my ignition kill it would seem!

All Day Dan

My Feedback: (5)

Olive, this new RCU still sucks. Just sending a PM is a PIA. I never got it. At least the forums contents are still as interesting as ever. I fly eight ten minute flights. Total on time for the transmitter is around ninety. I use that time and the capacity of the battery used to calculate average current.

If you are using a four cell pack of NiCads and one battery fails as a short circuit you wind up with a three cell pack at 3.6 volts nominal instead of the four cell 4.8 volts. With two packs in parallel the voltage difference is 1.2 volts that is going to force a large amount of current, in the amps range, out of the good pack. The moral of the story is do not put battery packs in parallel. Use a pack with a higher capacity instead if you need it. Dan.

thepamster

Oh Dan, still using 4.8v nicad receiver packs. Not that there is anything wrong with that. Lol.
How many times Dan have you had a shorted cell, "IN FLIGHT" with one of your nicad packs?
Your airplane is more likely to get taken out by falling space debris than a "shorted" cell.
Two batteries, two switches, one receiver. Works for me.

Boomerang1

Sorry Dan, what you are saying is just plain wrong.

Part of my job involves the care & feeding of all types of batteries, nicads included & I have lots laying around.

So no theories, no rumours, just facts.

Just came up from the workshop after confirming the views put forward in the RC Jet magazine.

4 cell NiCad - 5.17 volts.

3 cell NiCad - 3.69 volts.

Packs connected in parallel - 4.99 volts.

Current flow from the 4 cell pack to the 3 cell pack (the higher voltage pack is trying to ''charge'' the lower voltage pack) approx 130 mA.

Eventually the two packs will settle at a lower voltage, my guess, about 4 hours for a 500 mAh pack working on the current flow.

John.

BarracudaHockey

My Feedback: (11)

Alas, common sense

Thank you!

The other flaw is that NiCad cells don't short, they fail open, unless, as someone already said you previously discharged them so far they reverse polarity.

All Day Dan

My Feedback: (5)

Good test Boomerang. Thanks for the updated information. I was using some old circuit analysis stored in my mind. I did have a cell short in flight once. The Futaba battery failsafe mode kicked in and I landed right away. Definitely not a comfortable feeling when the engine goes to idle unexpectedly. Dan.

speedracerntrixie

My Feedback: (29)

John good info, however my guess would be that 75% of us do not use NiCad. My airplane is equipped with dual 7.4V 5000mah 45C discharge LiPo batteries. They are actually R/C car hard case batteries. We know about the catastrophic failures of LiPo so what do you think if I lost a cell ( 3.7V ) while it was hooked up in parallel with the second battery?


Andy, I know you want to be right here bud but John just put up numbers for a 4.8V 500 mah NiCad. When was the last time you saw one of those installed in a gasser?

vertical grimmace

My Feedback: (1)

The best thing going as far as RX batts are concerned now are the A 123's. Especially with the higher power requirements we have now. This is not 1980! I have been so happy with the performance of them and am slowly switching all of my planes over to them.

zx32tt

My Feedback: (9)

Your whole system is overkill for a 30cc plane, but if that makes you warm and fuzzy, go for it. I only use 2 A 123 packs on 35% and larger planes. I feed them into a SmartFly power board. 1 A123 pack (2300mah) will fly a 30cc plane all day.

Boomerang1

Yes, of course you are right, battery technology has moved on & there are many more options available to us now.
The batteries used in the example are just what I had laying around.

As for losing 1 cell from an A123 or any other 2 cell pack, same scenario. The higher voltage pack will try to charge the lower voltage pack.

In this case the current transfer and voltage drop of the two paralleled packs WILL be higher as the difference in voltage is now 50%.

But you are 50% less likely to have a cell failure as you have 50% less number of cells!
Believe it or not this is the same logic used by the manufacturers of large airliners when they were trying to get approval for long overwater
flights by the new, big twin engine airliners (ETOPS). A twin engine aircraft is 50% less likely to have an engine failure as a 4 engine aircraft!

Ahhhh ........ yes, I think ......

What do I use? Currently (sorry!) using 4 cell 2000 mAh Eneloop cells x 2 via 2 switches direct into the receiver.
Yes, I'm aware of their limitations & will probably go something else when it's time to change. But, countless flights with
no problems whatsoever in heaps of different models.

What I tend to do is wait until technology matures, the necessary accessories are developed & available at reasonable prices (competition)
and everyone else has learnt the hard way through experience.
(Remember when lipo's & chargers DIDN'T have balance leads, remember when Spektrum arrived & didn't have satellite recievers & took ages to reboot when they inevitably had low voltage drop out on 4 cells? - hey that was our fault because we later found out we needed 5 cells ).

An early adopter I'm not, more a Luddite!


John.

vertical grimmace

My Feedback: (1)

I do not subject myself to needing Satelite RXers. Again, another failure point. Not sure why they even need them when other manufacturers do not.

The Eneloops are great, but I think the key to the new 2.4 gig systems is to have batteries with low internal resistance. Hence the benefits of the A 123. Also, they only use 2 cells in series to get 6.6. At this point, the Eneloops that I have left are being used on ignitions.

I am of the opinion that if you have to use old school batteries, you are better off with Nicads on 2.4 than NiMh. The needs of the 2.4 are more than 72, so as our RXer needs increase, the batts need to move up to current technology as well. Especially considering we use a lot more servos now than we did.

Boomerang1

Yes, like I said, as long as you are sure of their limitations they are fine. The discharge curves are the guide to this.
They cannot handle high current draws & their voltage drops off if you overload them.


But check the current draw for a stalled servo, our worst case scenario. For the modest servos I use it's less than 1 amp.

I can understand large 3D gassers with large, ganged servos & large jets with monster servos on all-moving tailplanes
are a different situation entirely, Eneloops not suitable at all for that.

Great for transmitters though!

John.

vertical grimmace

My Feedback: (1)

Interesting that the RX voltage requirements ar or have gone up, and the TX requirements are going down.

Truckracer

My Feedback: (19)

That is correct but the chances of the second battery going short are extremely low. Extremely low! While NiCd and NiMh batteries had an occasional failure mode of going short (single cell) that is just extremely rare for A123 batteries.

flyguy888

My Feedback: (63)

The only way I can make sense of this reply is if batteries never fail on any airplane under 50cc.

thepamster

The important thing to have here isn't so much dual batteries but rather dual power paths. A failed switch or battery connection is much more likely than a failed battery. It is quite easy to just run one battery with two switches going to your receiver.
Two batteries on a 30cc plane may be overkill but so what.

speedracerntrixie

My Feedback: (29)

In reality battery failures are quite rare. For airplanes under 50cc there are fewer servos, less current draw and ideally less vibration. Being physically smaller, a single battery makes more sense due to room, weight and complexity. For most a 20cc or 30cc airplane is their introduction into gassers so keeping things simple is the smart route.

Flyfast1

My Feedback: (1)

Some battery manufacturers sell battery packs with two power leads. I think Hangtime hobbies offers this. If I were to ever fly a plane with a single battery pack, I would use a battery pack with two leads, connected through two separate switches into the receiver. In my personal opinion and experience, in a gas engine plane, a switch is more likely to fail than a properly maintained and pre-flight checked battery pack.

I have now converted two of my gas engine-powered warbirds from NiCd/NiMh to A123 packs and I really like them. So far, they have held up fine and require less "maintenance" than NiCd or NiMH battery packs. I am also using a Tech Aero IBEC on both planes, which eliminates the need for a separate ignition battery and switch, and also provides an ignition kill without a separate optical kill switch.

There was once a test conducted and documented somewhere on RCUniverse that showed the discharge scenario for two NiCd battery packs where one of the packs had a shorted cell and the results were similar to those posted by John in this thread. I think in that test they used two five cell packs with a nominal voltage of 6.0volts and the discharge rate to the battery pack with one shorted cell was a few percent per hour, so it was easily discovered in time with a pre-flight check. It would be interesting to run a test with two A123 packs. Since the voltage difference would be larger and the internal resistance of LiFe packs is lower than that of NiCd or NiMh packs, the discharge rate would be higher, but it would be nice is someone would conduct and document a test, since I know many pilots who use a two pack LiFe (A123) setup. Perhaps I will purchase some cheap Chinese LiFe packs and do it myself.

Cheers and happy flying,

-Ed B.