Mistake with Lipo discharging - Advice needed

[Clive Moore]

Hi all,

 

Firstly I am a Lipo newby.  I've been using NIxx for years but have taken the plunge into Lipo's and I think I have made a mistake.

 

I  decided to run my Sullivan starter on 24v as I now have a number of large petrol engines.

 

I did the conversion using a 6s pack. Today was the first time I have used the starter since the conversion and it worked well. As I was setting up a new 40cc MVVS I had the need to do about 35 / 40 starts in fairly quick succession. 

 

The outcome of all this is that I let the battery fall below 3v per cell. The actual volts per cell were 2.3v / 2.4v. I put the pack on a balanced charge at 1c and the volts per cell now read 4.2.

 

My questions are:-

 

1) Have I damaged the pack and how do I tell if I have?

2) Should I be doing anything specific with the pack now that its been below 3v per cell.

3) Is there anything I can fit between the starter and the pack that will shut off the supply when it gets down to 3v per cell?

 

Any help would be much appreciated.

 

Clive

 

 

[Bob Cotsford]

Interesting as I've just ordered some A123 LiFe cells with the intention of replacing the motorcycle battery in my flight box with them.  I was wary of using LiPos for fear of doing as you have, and running them flat.   I would have thought you should get away with it once at around 2.5v and if they didn't puff up and took a charge it sounds hopefull.   As regards damage done, only time will tell.  Did they accept the full rated capacity, that is a fair indicator?   They may die prematurely, or they may not, do ya feel lucky?

I'm sure there are low voltage warning devices, I'm sure a search of the web will locate something.  Alternately you could hook up a cheap voltmeter and keep an eye on that.

[Tim Mackey]

Clive.

1) The minimum 3V per cell is - in reality - for most cells an unloaded figure, and under heavy load, I set my cut off point to 2.8V per cell when under load.

 

Therefore I suggest your cells are fine, especially as they seem to be reading the normal "full" voltage of 4.2V. The best method of preventing this again is to fit a lipo voltage warning monitor which are available really cheaply these days for packs of all sizes - they usually work by either illuminating/flashing an led or /  a "beeper" sounds when the pack voltage drops to a reset level. Some of the smarter ones simply plug into the balance connector and will monitor each cell individually.

If your OK with a bit of simple electronics, then making a suitable device would be simple enough too.

[Tim Mackey]

PS here is a link to the type of thing I mean..... UK suppliers such as GC etc might have similar devices.

[Tim Mackey]

And a DIY version

 

There are other simpler DIY versions around, I have made many of them and they are easy and cheap.  This is the one I usually make, and a variation of this monitors the LiPo voltage of my DX7 transmitter.

[John Gallard]

Whilst there are battery monitors designed for pre flight check I have yet to find an 'airborne' version' that emits  a loud enough audible warning of impending doom! Bright leds are never bright enough. Since converting to electric I am happy to have left the oily rag brigade (although recharging a 5000mah at the field takes much longer than another guzzle of fuel) however some of the advantage is outweighed by the worry of 'have I pushed it too far? I would welcome an in flight monitor (not telemetry) that would provide an earpiercing warning, anyone know of one?

[Tim Mackey]

Hi John - I understand that problem, and I too have never found one that is loud enough unless its in very quiet model, in a close flyby with no other models in the vicinity etc.

The best suggestion I can make is to simply use a timer to ensure you don't fly too long. The LVC on most speed controllers should ensure that power is cut before the cells drop too low, and again some of them have a version which monitors individual cells not just whole pack voltage.  However, best practice really is to time the flight, and land before the LVC even cuts in, this ensures a good safety margin, as well as allowing time for a couple more landing circuits if needed.  HTH

[Clive Moore]

Hi Tim, thank you for your reply. I am reassured as I have now cycled the pack twice with good results.

 

Re the battery checker device that you gave me the link to, a very nice man at BRC suggested I get one when I purchased the pack from him and it works by plugging into the balance lead, it then cycles through each cell showing its voltage and then gives the total pack voltage, is this the device you mean?

 

If yes then presumably I can cut the return wire on the balance lead extender that I have fitted and put a simple push button switch in line to 'push to check' the state of the cells?

 

The reason for putting the switch in line is that someone told me that if I leave the checker connected all day at the field than it will slowly drain the capacity, or am I worrying about nothing?

[Tim Mackey]

Well it will eventually drain the pack yes - but it would take a heck of a long time!

Ive never checked the current drain of these checkers, but I imagine it would be only a few m/a.

The one I linked too is not only a monitor, but a programmable alarm also - which is what you asked about, and is IMO a better idea.  According to the specs, the total current drain of this device in use is only 8m/a so assuming you are using say a 4000m/a pack, thats err, around 500 hours before its "flat" LOL

Inserting a push to make switch would work, but if this aspect of use worries you, I think I would simply plug the checker in occasionally during the day to see how things are looking.

Edited By Tim Mackey - Administrator on 20/07/2010 13:10:55

[Clive Moore]

Hi, thought I'd give you an update.

 

I have now cycled the pack 5 times. Each time I discharge it gives me 2265 mah from a pack that has a stated capacity of 2500 mah.

 

The discharge is always terminated early due to a "volts mismatch". On inspection of the graph that my charger displays on my PC it is cell 1 that suddenly drops to 2.6v whilst the other cells are all at 3.3v.

 

Each time I charge I have done a balanced charge.

 

 The pack is not "puffy" at all and only gets very slightly warm near the end of the discharge, again according to the PC screen the temperature is rising only about 2 degrees or so.

 

So as this pack is not in an airframe do you think its OK to keep using it in the starter or should I bite the bullet and get a new pack?

 

I've heard so many horror stories about LiPos that I'm a bit nervous.

 

What would you guys do?

 

Clive

 

[Tim Mackey]

First off, under normal circumstances, LiPo packs should not be discharged /cycled as it serves no real purpose and shortens their lifespan.

Are we sure the charger is not at fault? Have you checked the cell voltages individually with a voltmeter ( probing carefully on the balance plug pins ) Its also somewhat worrying that the charger even allows a discharge to continue to the point where a cell falls off to 2.6V.

2265 from 2500 pack is not at all bad, and as there are no other signs of problems with it I would certainly continue using it for the non flight critical operations.

The horror stories are always a result of incorrect charge settings, current draw far exceeding max permissible (such as a dead short across the terminals) or physical damage to the casings allowing oxygen to enter the mix as it were.

I have never heard of any issues of fire or whatever from a pack which has simply been allowed to discharge a little too low,and I suggest this poses no hazard.

Usual disclaimer applies

[Clive Moore]

Tim, thanks for the advice its really helpful..

 

I have discharged the pack partly to understand what's going on. I do understand that in doing so I am using up some of the packs life but I see it as an investment in my own training.

 

I decided to discharge the pack again only this time using a different charger. The results were different from what I got with the first charger. This time cells 5 and 6 discharged down to 3.4 (ish) volts whilst cells 1 to 4 came down to just below 3v and the charger cut off with a "cell volts mismatch charge terminated" message.

 

The good news is that I managed to find that BM6 cell voltage monitor at  Falcons RCS www.falconsrcs.co.uk and it works really well. As well as cycling through each cell voltage and the total pack voltage it emits a loud buzz when a cell or cells get down to 3v so hopefully this will prevent me from doing the same thing again.

 

I have also invested in a second 6s pack of the same type and make as the problem pack. When I put the new pack on to a balanced charge it does the balancing bit very quickly. When the older pack gets to the balancing stage it takes a good hour to do. So I guess that my discharging down below 3v has caused some issues. The packs I'm using are Flighttech EON-X Lite which at £55 are about a 1/3rd of the price of some other 6s packs and I assume that a more expensive pack may have withstood my abuse better?

 

 Clive