Faulty Battery Packs

[Peter Beeney]

PB1

I met a fellow modeller at the field on Wednesday afternoon, he’d asked me if I could give him a hand as he is a beginner and he’d not done any flying since last October. He has a slab type electric foam model, with ailerons half the size of the wings, and a rudder twice as big as the fin. Also a FF6 tx, I switched it on and the battery voltage was 9.2 volts. After some conversation, we established it was a brand new 8 cell battery and had been on charge all night. A short flight, model fine but voltage now down to 9.1V. We need to check this out. I put it on charge, on auto, and within a few minutes it’s full. So, a test discharge, at 1 amp. Within 6 minutes the voltage is down to 8.6V. A quick recharge to full and again discharge, this time with the addition of an accurate voltmeter checking the voltage. Exactly the same result, the charger’s graphical readout giving the perfect example of the ‘vertical step down’, which is a faulty collapsing cell. Thus within about an hour of switching on, and thus discovering this, he was able to ring the model shop, describe the symptoms exactly and they agreed to replace it. I’m not into mobile phones, I’ve never owned one, but it struck me, with the camera facility, if necessary the model shop may have been able to watch it failing as it happened!

It’s a 2200 mAh, pattern type, Ni-MH pack, in a clear plastic wrapping. Price - £20. (From this particular model shop). The Futaba, (Ripmax) version they are going to replace it with is £34. I’ve been in this faulty new battery situation a few times, but that’s far too many in fact; and I only get to notice a tiny minuscule of a teeny-weeny nano-fraction, (that’s small!), of the packs that are in circulation. Not good! His charging technique was not quite correct, so for anyone who is still not quite sure here’s my version assuming, of course, you only have the standard plug-in-the-wall socket charger. Ideally you need to charge at 0.1C, or one tenth of the capacity, but this is not possible with higher capacity packs and a fixed rate charger so we have to compromise. Industry would call a full charge 100% of capacity plus 60%, but I’ve settled for 50% overcharge. This then translates, using this pack, into 2200 mAh + 1100 mAh = 3300 mAh. Divide this by the rate of your charger, say 70mA. So, 3300/70 = 48 hours charging. This is provided the rate is actually 70 mA, it might well be nearer to 50 mA, in which case the charge time could be up to 66 hours. At this low rate, 0.03C, the battery can stand long periods being overcharged, this just turns into low heat, but the pack can safely dissipate this. Some batteries, which are on permanent charge, in standby power situations for instance, have a supply of 0.045C down to around 0.02C.
Having said all this, in practise I’m sure your batteries are never totally flat, frequently hardly dented, and a good overnight charge is generally easily sufficient. When I top mine up with the fast charger they usually only want a small amount. It’s just a case of charging more rather than less!
There are tx safeguards, low volt alarms etc., but what if this had been in a rx pack? A wrecked model is the result, I’ve seen it happen. Once again, may I repeat what I’ve said many times, it’s a very good idea to check out your new packs before you get airborne. With respect to my friend, he said he would not have taken any notice of this low voltage, so in which case he might have had a disaster very quickly. And it was not down to his negligence, simply faulty (new) equipment.

Collating some of this evidence, and I’m sure there’s plenty, would be interesting.

 

Edited By Timbo - Administrator on 10/05/2010 13:04:16

[Chris Card]

Hi Peter, 

 

An interesting description and it's lucky your fellow flyer had your experience on tap.  I must be living a charmed life as I cannot remember ever having a faulty cell in a pack , either NiCd, NiMh or Lipo.  Or perhaps that just indicates that I'm not as good at finding them as you!  I guess the "bathtub curve" applies to batteries, whereby most failures will happen when very new or at the end of the normal lifespan.  For sure I've worn out lipos after 2-3 years use, but they generally just lose their punch rather than fail completely.  Once there's significant volt-drop under load, it's time for new ones.

 

 

[Phil Claridge]

i had a case of vanishing volts, the rx battery pack ejected at 50ft whilst inverted, due to a loose cockpit window. plane destroyed but battery was recovered and lived to fly again

[Pete B]

Bet that was a relief, Phil - I suppose you thought you'd lost everything..................

 

Pete

[Peter Beeney]

PB2

Chris, Yes, generally speaking, these things must be pretty rare. I’ve only seen this happen probably because I’ve tampered with so many batteries. Over the years, I’ve bought two personal packs with one rubbish cell. This particular one was a bit unusual in that one cell was completely duff. Normally it would have some life in it, and would fail further down the discharge curve. Unless you were looking for it, you wouldn’t spot it, it might find you though, with, at the very least, an incident. I would say, as far as tx and rx packs go, it’s perfectly possible to have a dodgy pack and not realise it, simply because they don’t do enough work. I would estimate that the average modeller charges his battery, then goes to the field and has perhaps three or four flights, which probably the battery doesn’t even notice! And then charges again before the next time out. Just guesstimating this, I reckon the average model, say a trainer or a sport model, will use an average of 100/150mA. The 4/5 servo airborne pack’s idling current is around 40/60 mA. If this is the case, then 4 ten minute flights, 40 minutes, at 150mA, will be: 150/60 = 2.5 × 40 = 100mAh. If the rx pack is the average 1000mAh capacity then we’ve used 10% of the total. The faulty cell may have say, 4/5ths of the capacity, 800mAh, so in which case here we would have to fly for about five hours before it caught us out. So, in which case then, this dodgy cell will maintain it’s voltage right down to the 800mAh discharge point, where it will suddenly collapse to zero, within the time frame of just a few seconds, even if it’s being discharged at the very sedate rate of say, 0.1C. Nicad and hydride cells can have a very flat and extended longevity curve, whereas lithium cells officially start to fade from the first discharge. Probably not very noticeable, particularly when they are new, but they are considered exhausted when the capacity is down by 20%. But with careful monitoring, this might not be an issue with some Li-Po packs used as the rx supply. Phil, I saw this happen, too, (not my model!) circa nineteen seventies, a biplane with an open cockpit. Also heard exactly the same story from another club. Battery plugs and sockets have also parted company within the model, at very embarrassing moments, inevitably to the chagrin of the pilot when he finds that he doesn’t have black bags left. Hence one of my very long standing ‘rules for beginners’. Mandatory for youngsters, advisory for the slightly more senior hooligans. A piece of lacing twine through the wires and round the length of the plug and socket. Even if the battery does somehow escape the plane, it will have to stay attached to the switch. And while we’re at it, for the halcyon days of 35MHz, (remember those?), a couple of turns of insulation tape round the crystal in the receiver. For the same reason. Was your battery put on a charge for going AWOL?

Edited By Timbo - Administrator on 10/05/2010 13:04:59

[Former Member]

[This posting has been removed]

[Peter 'Ivanna Crashalot' Savage]

For something that is £20-£34 you'd expect it to be tested etc

[Peter Beeney]

   Firstly, my humble apologies folks, my post seems to have zapped the tread again. I think there must be something about my writing the site doesn’t like! I’m not posting any links, I’m using plain text and also posting from the ‘paste plain text’ insert box. Difficult to see what I can do differently, other than not post at all! Or maybe the technical division can find an answer.

   Tom, If this is indeed the case, then the tx will surely have it’s own dedicated six cell charger? It might not be wise to use the old eight cell one. Also you would need to know the capacity, in amperes, or fractions thereof, in order to base the charging time on. This would be a new ball game, so you would need to know all the details.

   Peter, Testing all the packs would be a very time consuming process, for any other than the manufacturer. And, as I’ve remarked more than one or three times before, I’ve now grown very old and a trifle cynical, and I’m sure the model industry is not going to lose any sleep over a few faulty batteries! It’s a commercial competition out there!       PB

Edited By Timbo - Administrator on 10/05/2010 12:56:11

[John Privett]

Posted by Peter Beeney on 10/05/2010 09:44:13:

   Firstly, my humble apologies folks, my post seems to have zapped the tread again. I think there must be something about my writing the site doesn’t like! I’m not posting any links, I’m using plain text and also posting from the ‘paste plain text’ insert box. Difficult to see what I can do differently, other than not post at all! Or maybe the technical division can find an answer.

 

Peter,  I don't know why this would be,  but your post that

"zapped the thread" has a lot of spaces (strictly speaking

they're non-breaking spaces ie. &nbsp after your initials

at the end of the post.  This is what's causing the text to

expand and creep under the adverts.  Any idea if you did

anything different on that post that would create the extra

spaces? 

 

Perhaps a mod can edit Peter's earlier post and delete

the spaces after "PB" at the end.

 

(BTW, I've put carriage returns at intervals in my post to make it

readable.  And that smiley after &nbsp is meant to be a

semi-colon!)

Edited By John Privett on 10/05/2010 11:01:51

Edited By John Privett on 10/05/2010 11:02:52

Edited By John Privett on 10/05/2010 11:11:02

[Former Member]

[This posting has been removed]

[Tim Mackey]

Sorted your postings Peter, but only by copying everything in your two postings, into word, changing font to arial 11, losing the odd symbols like your division symbol (  which my keyboard does not even have ) and replacing it with forward slash etc. Then I paste via the "Paste from Word" button, NOT plain text.

I suggest you do likewise, and refrainfrom using other formatting and inserting symbols etcplease, as this is very time consuming for the mods to keep sorting. Ta.

Edited By Timbo - Administrator on 10/05/2010 13:11:03

[Peter 'Ivanna Crashalot' Savage]

Could always get more mods *hand up* :D

 

I think especially as this part is such an important one, you would expect it to atleast be partly tested, I guess you just test it thoroughly yourself but that could be time consuming  :s