Exploding Ni-MH pack?

[Pete B]

Hi all,

I'm just returning to the hobby after a long break. Things have changed, haven't they!?

I live in deepest France, where clubs and model shops are rather sparse, and having about 6 acres of meadow behind me, I thought I'd make a sensible return with an electric soarer - I really enjoyed the 100" class back in the '70's and I'm happy with something similar.

Anyway, to the problem. Last year I bought a Ripmax Nebula in the UK, together with Jeti 30 ESC and a Kan Cells 2000maH 8.4v 7-cell pack. I bought my Hitec Optic 6 here and am on 41mHz.

Having finally found time to put it all together, I thought I'd give the 600 stock motor a test-run to check for interference, run-time etc. After about 30 seconds, there was a big bang from within the fus and I found that one of the cells had exploded. It was a very respectable bang, too! Can anyone suggest a cause for this, please?

The battery pack was purchased about a year ago and has remained uncharged until the weekend, when I gave it a trickle charge overnight on my Pro Peak Varipulse charger. I didn't measure the voltage after charge but after the bang, it is stilI showing7.6v. I was hoping to patiently cycle the pack up to full capacity before going to fast-charging, but now......

Have I done anything wrong or could it just be a defective pack? Has leaving the pack uncharged for so long contributed? Would it be possible/advisable to replace the damaged cell or better to write off the entire pack? Any advioce would be gratefully received, thanks.

[001]

Pete, Possibly a short circuit where the strap connection on the 'end' (middle) cell crosses the insulation of one of the other cells. I have found that 7 cell packs will not stand any pressure or slight impact at that end. Personally, if you want to stay with NiMh, I would dispose of that pack and go for eight cells. You could test the four unaffected cells of that pack and make them into a useful 4.8v. receiver pack if they are ok!

[Erfolg]

Richard

You mentioned disposal of the pack, I think he has done that, not in an approved manner though.

In these days of  the Health and Safety, the should H&S lads should have written in by now. An area of 3m must be cleared around the site of the explosion, in preparation of a thorough clean up operation. The ground excavated to a depth of 1m is a recomended minimuim (by me). The model with its toxic dust, must be double bagged and sealed and disposed of through an approved disposal route. Pete must be immediatly be taken to an isolation hospital to be treated with the apropriate drugs, they can be fatal in 90% of usage, but this is the only way to save Pete.

Yes, I also reckon a dead short somewhere, not to be recommended with any electical power source, one of lives no no's.

Erfolg

[Tim Mackey]

It is almost impossible to tell "if you did anything wrong" as you put it, because we were not there ! From what you describe, it appears not, and you have suffered a rare failure of the cell.Several things could have happened. Having left them discharged for a long period could have allowed the pack to get way out of balance, and the one exploding cell remained with a higher charge than the others. When you then charged the pack, the charger read the whole pack voltage ( which would be low ) and peaked it up - but the one cell received a higher dose than the others, the internals got too hot ( which is basically what happes when any cell is charged...they warm up ) and bang. Or as explained, a shorting  connector tab, however I would have expected there to be circuit break as a result of the bang, and yet you say the pack still reads 7.6V ?

Personally, I would dispose of the lot, after all, its not big bucks to replace. Then, seriously consider getting modern, and replacing it with a 3 s LiPo  and a simple charger /balancer. If you have several nickel based batteries still ( and I suspect you dont ) several multipurpose chargers will do all types ( pro-peak prodigy to name but x1 )

I hope Myron is reading this...... exploding batteries that aren't Lithium based huh Myron

Using a balance charger which is recommended these days, arguably makes the charging process of LiPo safer than charging Nickels, which have no such balance adaptor fitted. 

[Andy Sayle]

Most Nimh cells have a vent built in near the positive "pip", so that if conditions exist where the internal pressure of the cell should rise (I.e. a severe over charge, or dead short) the pressure can be vented with no bang.  They work a bit like a pressure cooker, in that the cell is sealed until the vent activates, and releases the excess pressure.

 However, these vents can get blocked, especially if the packs have been soldered together, or severely overheated, or the vent is bypassed by an extra tight seal over the top (like a good tight layer or two of heatshrink!)  What happens then, is that the pressure would build up until something else, other than the vent lets go, usually at a much higher pressure than designed, and with a rather large pop!

 Looking at your picture, your pack is of welded construction, so if it has never been severely overheated, the vent should work properly.  However, looking at the damage, you can just about see the insulating ring (a fibre washer) that has been pushed out of place.  It doesn't look like the actual metal cell case has split. (I could be wrong though?).  for that reason, it looks like the vent worked properly, it was just further restrained by the carboard cell casing, and heatshrink.  I think that the bang was actually the cardboard cell covering/fibre washer and heatshrink splitting under pressure (think firecracker!). 

 As for why it happened, then that is open to more speculation.  It could have been a dead short, then again, it could have been a duff cell that couldn't cope with the discharge rate you were using...

 If it were me, i would be disposing of the pack, and replacing it with a Lipo pack (more capacity, less weight! very low self discharge rate!).

 Cheers

Andy

[Former Member]

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[Tom Darville]

Are you sure it was Ni-MH and not Ni-cad as Ni-MH require a higher voltage to charge than Ni-cads.

This would explain the possibility of it being caused by overcharge.

Tom

[Former Member]

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[Erfolg]

Eric

Have you died from cadmium poisoning?

Is the house a hole in the ground from the extensive decontaminatioon that was necessary under H&S regulations and most importantly have all the landfill taxes been payed (to HM Goverment) for the disposal of heavy metals.

Which reminds me it is time for a shot of Ramstein and Lincon park. I once took a trip in a silver machine and it is still realy me....Ive got a silver machine. Another shot of whisky, whilst I still ahve a mind.

Erfolg

[Myron Beaumont]

Erfolg  Steady as you go or you'll end up like me

Gumyps Norym

[Former Member]

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[Myron Beaumont]

I RECKON YOU'RE ALL ON A DIFFERENT PLANET FROM ME

Norym

[Pete B]

Thanks, gents, for your very prompt replies - and the amusing anecdotes! I've been shifting topsoil today (oh, joy) so it's taken a while to get back to important things....By the way, thanks to Erfold, the house is now buried and the topsoil, which was intended for flower beds, is now covering the house. Can't wait to see how big the carrots will be!
Having operated on the offending item, I've managed to re-solder the pack as a 6-cell, charged it and, much to my surprise, found it working and giving 8.45v - I was under the impression that one multiplies the number of cells by the nominal voltage, to give me 6 x 1.2v = 7.2v. Have I misunderstood something?
At least it means that, weather permitting, I may get a test flight in tomorrow.
Within the end cap of the cell was a plastic insulating disc. As you may be able see from the photo, the insulator is creased over on one side, and I'm thinking this may have been a manufacturing fault and the cell shorted internally. Perhaps that would explain the bang?

Given that the risks appear to be similar whatever the battery (!), I shall accept the advice to go to LiPo's.
Right now, I'd be happy to get the beast in the air and I can get modern soon.
I have to admit I'm struggling to get my head around this. It's spinning a bit from all I've tried to take in from the various sources. At present, I'm trying to keep costs down, for now at least, and performance is not the priority. Presumably, with LiPo's, I can continue with the stock 600 motor and the Jeti 30 ESC. Timbo suggests a 3S1P battery, which I understand to be 11.1v. Can the 600 motor run with this voltage? Would a 2S1P at 7.4v be adequate for the Nebula? What sort of maH capacity should I need?
I don't want to be seen as lazy about these questions - it's more a case of information overload at present and I need to clear the fog.
In terms of chargers, and it's a pity my nice new Pro-Peak is like to gather dust, are the chargers such as this:

link deleted !

adequate? It is described as a balance charger but the price looks considerably less than many others.

link deleted !

Would I do better with this?
Do you have any recommendations? Do I need any other equipment such as a digital Lipo Indicator, connector leads etc? As you'll understand, living in France requires an internet order and I'd hate to overlook an essential part of the kit.

Thanks for your patience, your help is appreciated,

Buenos Dias,
Pete

[Tim Mackey]

Peter...please read this ...your links made the page unreadable

inserting links in posts !

[Former Member]

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[Tim Mackey]

Presumably, with LiPo's, I can continue with the stock 600 motor and the Jeti 30 ESC. Timbo suggests a 3S1P battery, which I understand to be 11.1v. Can the 600 motor run with this voltage? Would a 2S1P at 7.4v be adequate for the Nebula? What sort of maH capacity should I need?
I don't want to be seen as lazy about these questions - it's more a case of information overload at present and I need to clear the fog.

A speed 600 motor will be fine on 11V - its the current that you need to control, and this wil be down to the prop size you use. 12 - 14A is fine, and they have a stall current rating of over 30A. The capacity you need will depend on how many amps the rig will pull, and for how long you want to fly. Personally, I would ditch the old speed 600 brushed motor and fit a nice cheap low kv outrunner brushless, witha decent 11 / 12" diameter prop, using a 3s 2000mahr LipO battery

[Pete B]

Suitably clothed in sack-cloth and ashes, Timbo, now I can see the chain symbol. I won't make that mistake again, sorry!

I'm pleased to say I managed my first flight since about 1982 today, and, with nothing else to compare against, I was very satisfied with the performance of the existing set up. I managed about 15mins flight time overall, with 7.9v left at the end - rain stopped play. CG was probably a bit forward of optimum and I can see that less all up weight with a modern system would make it a bit more of a floater, so I'm heading towards your suggestion of a brushless/LiPo replacement.

Unfortunately, your phrase "fit a nice cheap low kv outrunner brushless, with a decent 11 / 12" diameter prop" exceeds my knowledge base, so I'm going off to do my homework and try and unravel the jungle I seem to be entering!

Thanks, all, for your help so far.

Pete 

[Tim Mackey]

Your welcome Peter, and glad you enjoyed your flight

Please feel free t0o repost your links, and we will be happy to help all we can

For possible motors ...try these. Motor AND brushless Speed controler for less than 34 quid

[Pete B]

I am impressed, Timbo - that clears the fog somewhat! As the motor diameter is 30mm compared to the c.35mm of the existing 600, I presume that I should replace the existing mount with, say a ply bulkhead inside the nose with a cruciform engine mount  such as this

The existing set up weighs c.1300g, so the brushess version should reduce that by at least 250g plus any noseweight needed (!), so would you suggest a prop of 11 x  6 or thereabouts? I haven't yet got all the calculations clear and I'm aware that this can be done mathematically but, in time.......

Turning to the charger questions and links that I screwed up in the earlier post, I understand that you get what you pay for but, as each vendor/manufacturer seems to have a different way with words, this too appears to be a minefield. I see this at the cheap end of the scale

which, on the face of it appears to be able to perform largely as this one which appears more versatile

I see that the Prodigy II still needs a balancer as well and, as I don't envisage needing more than 3 cell charging for the foreseeable future, would the Multiplex be likely to meet my needs? I'll address the plug compatibility later......

 I will try to start thinking for myself soon, honest!

Pete 

[Tim Mackey]

Hi Pete, will post a response here later tonight - watching telly right now

[Tim Mackey]

The mount ? YES.

Prop size is more to do with the flying characteristics you want rather than AUW. Larger diameter will give more thrust and less speed, and vicky verky for small diameter / high pitch. AUW is important in determining power to weight ratio ( which typically would be about 100watts per pound ) so for circa 1050grams - thats around 2lb, so aim for around 200 watts. The motor I suggested is good to around 200watts ( 240 maximum ) so would be OK. Lower Kv motors will swing bigger props slower ( like a 4 stroke IC) and high Kv motors swing little uns faster...it all depends aas I say on what sort of pitch speed and flying you want. The Nebula is a powered glider right? So you want to get it up high and cut the throttle type flying, so you want a lowish Kv motor to swing a biggish prop to gain height via thrust - you dont a high revving small prop which would give speed but litttle climb ability. Again the motor I suggested would be OK as it will swing an 11" prop. However you might also consider an even lower Kv ( higher torque ) to swing the 11" even better...maybe this ? I have been testing this motor and esc combo today with a friend, and it was very nice - comes with an alloy mount too !

Even lower Kv - more "swingability" - this
It also has a mount included, but will go up to 12" prop if you want. Its a bit like any powerplant...it will perform better overall if it is not being run flat out all the time IYSWIM. I also suggest a folding prop would be nice

BRC dont do any decent ones, but these - although pricey are superb quality 5 mm shaft though 
 

Chargers. The first cheapie one is pretty useless because it only outputs at 750m/a maximujm. If you use say a 2000m/ahr battery you need the chrger to output at least 2000m/a also, otherwise you will wait ages to recharge. LiPos should be cooked at 1C ( 1 x the m/ahr rate ) Second one looks a good price for a reasonable unit.

Glad you spotted the balance plug compatibility issue...a real headache for all this particular issue...but adaptor leads are available. HTH

[Pete B]

Many thanks, Timbo - I'm an hour ahead here so it must be bedtime - will absorb this when I'm compos mentis!

Pete 

[Tim Mackey]

Bonne nuit

[Pete B]

Well, suitably refreshed,  I'm about to place the order for the 2814-8T combo as it seems the biz. Good news is the diameter of 36mm means that I may be able to use the existing cradle in the model.

I like the look of the Graupner folders but the 5mm shaft left me searching fruitlessly for info on adapting shaft diameters to accommodate folders. However I have found this spinner

which seems to have the correct spec. I'm tempted to get two sets of 11x6 props (one for spares) but would it be worth going for a 10x8 or 11x8 as well to cover the options? - at less than £3 per set it's not a lot of money.

I'm back in the UK next week so I can collect the bits there. I'll be able to sort out any connection leads then.

Once again, thanks for your help - it is becoming clearer - slowly

Pete 

[Former Member]

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