Flyfast1

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I am nearing completion of a new large warbird project for which I plan to use two LiFe packs from Hangtime hobbies to supply power to my receiver. I have always used NiCd and NiMh battery packs and this will be my first plane with LiFe battery packs. With traditional 5 cell NiCd and NiMh battery packs, if a cell in one of the battery packs shorts, then the nominal voltage of the battery pack with the shorted cell drops to 4.8 volts, while the good battery pack remains at a nominal 6.0 volts. According to various accounts I have seen, the draining the of good battery pack (the one at 6.0v) by the bad battery pack (the one at 4.8v) is fairly slow, presumably because of the small difference in voltage and the internal resistance of the cells.

If I want to test the scenario where two LiFe battery packs are used and a cell in one of the battery packs shorts, is the following a good way to do it? I am thinking of purchasing two inexpensive two cell (6.6v) LiFe battery packs, making one into a single cell pack (3.3v), charging them both and then connecting them together with a multimeter in between the battery packs to measure the current draw. I would also measure the voltage at specified times over one hour to see what happens. One hour is several times longer than I would ever fly in a single flight and I always check my battery packs before each flight, so I don't I would need to test longer than one hour.

Since I don't know the internal resistance of the cells, what would be the expected current draw? Is there anything for me to be concerned about or any precautions to take for the experiment? What is the best way to do this?

Thanks,

-Ed B.

guver

My Feedback: (325)

I think that is a good test. You may be surprised to see the current and it might just be over 10 amps (max on some cheap multitesters)

Duplicating the leads and connection resistance is probably a good idea too and perhaps using a dc current clampmeter would be a better choice? Maybe not. I'm familiar with lipos and have tested pack to pack currents of over 50 amps and have heard of 100+ amps , but these are for bigger packs with a voltage differential of maybe only 1-2 volts. I'm not familiar with life cells very much , but you might want to do a clamp test first so you don't burn up the dvm. Hopefully it won't burn up any leads or anything else in the circuit.

On a side note I have not experienced any single cell short like you describe and can't help but think the possibility of a single cell shorting and staying connected is unlikely inmop. I'm holding my breath though for your results. It is an interesting test because you have about 3 volts differential.

P-40 DRIVER

LiFe cells must all be balanced to each other. I suspect bad things will happen if you exceed the never exceed voltage of your single LiFe cell by connecting it to a two cell pack. Why do you think an Life cell would short when it fails, it may fail open. The standard wisdom of Nicads does not apply to LiFe. Somebody who has done this needs to jump in here.

guver

My Feedback: (325)

Good point you don't want much over 3.7 in a single cell , right? I doubt anyone has done this before.

Flyfast1

My Feedback: (1)

Hi P-40,

I am thinking of testing the shorted cell scenario because in the situation where one cell in a packs fails open, then that battery pack will not conduct any current, so I would basically be flying on one pack. It would be the same as turning off the switch for that pack, or that pack or simply disconnecting the lead for that pack. Of the various failure modes I can think of, e.g., switch failure, cell failing open, cell tab failing open, battery lead disconnecting, etc., the only mode that seems harmful is if one cell in one pack fails short, causing the nominal voltage of that pack to drop to 3.3 volts. Then I have one pack at 6.6 volts and one pack at 3.3 volts connected to my receiver.

-Ed B.

P-40 DRIVER

I gotcha, sounds like you will blow that other cell sky high putting that much voltage to it, let me know how it turns out. The Hangtime site seems to think its perfectly okay to run two packs in parallel (spelling). I have my doubts what will happen if one shorts though. Interesting topic though, you have peaked my interest. I still like the idea that a 5 cell nicad pack will still get you home even with a cell failure.

Flyfast1

My Feedback: (1)

Hi P-40,

I have seen some tests done with two NiCd or NiMh packs and even if a cell in one of the packs shorts, leaving one pack at 6.0 volts and the other at 4.8 volts, the discharging of the good pack into the bad pack is very slow and there are no major events, such as melting of wires, fires, etc. I think the small difference in voltage of 1.2 volts and the higher internal resistance of the cells and the lower C rating of the packs means that not much current flows from the good pack to the bad pack. I believe that Red Schoefield (spelling?) talks about this on his Website and he believes that there is no need for diodes or other power management systems when using dual NiCd or NiMh packs with separate switches into a receiver. But, with LiFe packs, the situation may be very different because of the greater difference in voltage and the construction and chemistry of the cells that allow them to deliver much higher current. This is the one scenario I am curious about before I start using them in my big birds and I haven't seen anyone address this particular issue with LiFe battery packs. Perhaps using diodes would be the way to go?

-Ed B.

P-40 DRIVER

I was referring to running 2 LiFe packs in parallel, Nicad or NiMh not an issue. Diodes are another failure point to contend with. I think I remember there being a big discussion of diodes at some point and the lithium pack issue did come up with dramatic results as I remember. I really am beginning to wonder if a dual battery system is all that important. I see alot of people running high dollar planes without them with good success. I think alot of it, is good battery management. I can definitely see the dual battery leads, dual switches thing though.

guver

My Feedback: (325)

For anyone to give an estimate or guess the current would you please share at least the size/capacity of the cells? Thanks.

Some EE can probably figure this out on paper if the capacity and resistance of the circuit and voltage differential is known. I'll bet one can guess pretty close. I'd be willing to do it with a pair of lipos. That's as close as I can get.

What is the capacity and size and brand of cells?
What is the length and size of leads?
How many connectors? type? ect?

Flyfast1

My Feedback: (1)

Hi P-40,

Yes, I am thinking the same thing. Like you said, if using one receiver battery on a large gas airplane, at a minimum I would want two leads and two switches with good battery management.

-Ed B.

Flyfast1

My Feedback: (1)

Hi Guver,

I am thinking of using the Zippy Flightmax 2100mAh 2S1P 30C LiFePo4 Pack from Hobby King (http://www.hobbyking.com/hobbyking/s...arehouse_.html), since they seem to have similar specifications to the Hangtime hobbies LiFe packs that I have and are only $9.52 each. For leads and connectors I would use 18" 22 gauge servo extension wires and a heavy duty ultra switch from Hangtime Hobbies, since that's what I plan to use in my plane.

-Ed B.

richrd

?? What iam i missing ?? 2 cells in series is app. 6.6 volts one cell charges to 3.6 v put 6.6 into 3.6 what do you think will happen
it will draw what ever current it wants to in overcharge, i would not use this battery again after this test [:@]

BTW iam the guy saying if a doide fails YOU did not solder it correctly they have over 100 years of mean time between failure it will not be the problem,,
it is the best solution to this problem, just use 3 times the current needed for diode an never think about it again.
Rich

guver

My Feedback: (325)

I think I can easily do this with some similar cells. I'm no EE , so is the actual voltage important or would it be ok to just match the voltage difference?

In otherwords could I use 20 volts and 23 volts with some similar cells (30C means fairly low IR)

I'd only be able to get you the first inrush of current reading , but not any readings much over a few seconds and won't be able time it in any way because I will not allow any of my cells to go much overcharged. Speaking of that overcharge: If we had a 7 volt 2s hooked to a 3.5 volt 1s of identical capacity would the voltage settle to an exact average of 5.25 volts?

P-40 DRIVER

The cell construction and durabilty of the A123 cells and hobbico/zippy cells are completely different. The A123 will take alot more abuse in all forms such as vibration, overcharging etc. I would pay the extra money and get the A123 batteries. I would be happier running a single A123 battery setup then dual Zippie/hobbico setup. The A123 is a much better battery. Just my opinion

richrd

No the 2S is a voltage source Ri in milli ohms it will dump into the overcharge at a non repeatable rate depentent on seperator failure rate then gasses will build to ??? again each battery will fail different i would guess each test would have different out come an each manufacture battery would change out come greatly.
a) the 1 cell outgass an opens
b) the 1 cell shorts then 6V dumps until it overheats or goes dead

But we are discussing what if a cell shorts or tabs short or wires short a failure is possable so !!!! even if 1 in 1000 odds still ?

Flyfast1

My Feedback: (1)

Hi P-40,

The packs I have from Hangtime hobbies have the A123 cells and those are the packs I plan to use in my plane. For purposes of the experiment, I wanted to save some money and use the less expensive Hobbyking packs.

-Ed

Flyfast1

My Feedback: (1)

Hi Rich,

I don't think you are missing anything. The purpose of the test is for me to see for myself what happens if one of the cells shorts and as originally stated, the test would be done with battery packs for test purposes only that I would not use in a plane after the test. The theory is that the good pack at 6.6 volts will charge the bad back at 3.3 volts, but I would like to run the test to actually see the state of the good pack after one minute, five minutes, ten minutes, etc.

In your packs where you use diodes, is there a particular diode that you use and have been happy with?

-Ed B.

richrd

I don,t use LIxx as any source in plane's but motor so no experience other than that. I work as electronic instructor, an ex engner so got tons of diodes i use all the time. MR751 series is great 6 amp. So many to chose from PM me just let me know the setup an i'll tell u what need then go to jamco an get one. I do it to isolate two packs an use MR751 all the time 6 cell NiMH can charge either pack no separate switches. But have since gone to separate switches with built in charge plugs an no diodes with 4 cell using eneloops too. In your case it is a close call using Lixx in parallel does have this concern that NiXX does not have. If it was me an they were in parallel for long time no isolation switches yea I would use diodes because no reason not too.
Rich

Goldenduff

Hello, a little off topic but I hope to get a simple answer so we can get back the Op, just taking advantage of the minds assembled....

chuckk2

Look at the receiver for a moment.
The black - lead is common, and a reference return for the servo control signals.
The red + lead can be supplied from a junction board that splits the servos and receivers.
between different power sources, as long as the red leads from different power sources are not cross connected.
In the dark ages, with the larger receivers, there was room to parallel solder a wire across the + terminals and the - terminals.
This allowed the receiver pins to carry as much current as the pins can handle.

Goldenduff

Thanks- pretty sure I can connect many batteries.

richrd

Yes to Futaba R617 if you look thur the plastic you can see the center red is all bussed "connected together"
No to different pack chemistries just a very bad settup to chargeing etc. and never different voltages.
It's been discussed over an over about packs in parallel we seem to agree on NI xx chemistries same aged packs identical as possable.
Just put in open slot in rec. or Y as you are asking is ok, always two is better than one.
A sujjestion maybe to have some 5 cell sub "C's" made up then canfly forever best weight is batteries
Rich

Goldenduff

Cheers- why no to different chemistries and voltage- or maybe easier to give me the link to the other discussions....

P-40 DRIVER

Google Hangtime Hobbies, all the answers you are looking for are there.