Looking after your LiPo cells

[John Cole]

I used a different chart, from an online book on all kinds of cells and batteries:

http://www.buchmann.ca/default.asp

The Li (Ion, not LiPo  but it's similar chemistry) capacity chart is at:

 http://www.buchmann.ca/chap2-page6.asp    Click on the link to see the chart, then page down.

It says that almost all the useable capacity is between 4.0 and 3.5 V.

Loads of other interesting stuff in this book.

[Erfolg]

John

The linked site is extremely interesting.

Erfolg

[iawnski]

ok tim got myself a couple of lipos ,now i read you must check each individual cell how do you do that ??.at the mo only have 2 cell 2000 mah 7.4  lipo , as for the balancing leads  totally out of my depth hear. any info of chargeing set up would be great,bit confusing this lark

[Tim Mackey]

kevin, as you have now entered the dark side, you would be best to read through the many topics here in the electric flight section - yes it is abig subject, and there is a lot to it. part 2 of my "electrickery" article ( both parts are still on here somewhere ) also deals with Lipos and charging balancing etc. If you are going to get the best and SAFEST use out of these lipo packs then you better gen up on 'em my friend -being "totally out of my depth" is not an option really. If you havent already got one, then you MUST get a proper dedicated LiPo charger and balancer unit - again, lots of info in the relevant forum sections, and I have an article in next months RCME special all about how to choose a charger. I will try and post some links to some articles on here.... but as I say, there are so many i forget where they all are !!

[iawnski]

cheers timbo

[Peter Ford]

Go buy a Cellmeter from hobby City.

They're not exactly cheap, but I wouldn't be without mine ever again. And judging by the rave reports by others, neither would they.

It has a ' Fuel guage' bargraph which shows what is left in the battery. Gives various readouts to display each individual cell voltage.

Also shows the overall pack balance and whether your balance charger is doing its job.

This is achieved safely and easily by connecting its multi pin connector to the balance plug. The CELLMETER gets its operating supply from the Lipo under test.

Handles up to 8S packs. Also gives a lot of other stuff to do with prop sizes etc.

It is possible to get a 'sort of ' Chinglish translation via Google.

For further info look here.

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6232&Product_Name=Cellmeter_8_for_Lithium_Polymer_Packs

Its a helluva useful thing for £50.00

[Tim Mackey]

Just added to the original info in the header of this thread....with data on charging and general care etc.

[Chris Card]

Very handy chart Timbo.  I intend to condense it slightly and print onto sticky labels which I can put onto each of my lipo packs.  Very handy for interpreting the voltage reading as the pack is checked.

[Tim Mackey]

I too do this sort of thing....all my packs are dated with purchase date.

[Dave Rose]

 I have printed this chart and use it on the field. I seem to err on the side of caution ,as the packs are reading 3.86 or 3.90  or so after use. A couple of chaps asay I can get a lot longer flights, and they back it up by saying how litle I put in when i charge them again.

I mainly use  3 s 2200 and 3S 4200

Just now I recharged the 4200 and i put in 1277 to get a full charge. I presume I had plenty left in this pack after the last flight.and could have gone on longer

Are  there optimum figures i should aim for when recharging, if so what are they for

2200   and 4200  please

TIA 

[John Cole]

If you want them full to capacity, charge to the 4.2V per cell limit.  Use a balancer to make sure no cell exceeds 4.2V.  If you're only coming down to 3.86V you are only taking 30% or so out of them.

[Tim Mackey]

Discharging down to around 2.8V UNDER LOAD is actually OK, but many people believe that stopping around the 3V mark will prolong cell life.

As JC says 4.2V absolutele full. So for ANY 3s pack ( the capacity does not matter ) 12.6V is FULL.

....and around 8.5V minimum empty again under load. 

[Maurice Harvey]

Has anyone an idea of what the life cycle (not cycle life) of  a li-po is? I have batteries that are now 2 years old that have been recharged about 40 times that have "gone off the boil". The amp reading starts off well when running the motor/prop then quickly degenerates to which is best described as a half throttle setting. These batteries are the "superior quality" type, expensive, whereas I have batteries of less than a year old that have had as many charges but still behave as if they are new plus the fact, that  one could consider them of "inferior quality".  By the way, C ratings are the same on all the batteries. Perhaps someone could be kind enough to shed some light on my problem. 

[Carl Muff]

thanks very helpful ,but one of my lipo on my 3 cell is expanding its not been damaged .is it safe to use ?

[Tim Mackey]

Probably not - it means its vented , probably due to excessive current draw, or overcharging /overheating.

When ever I have had that happen, I usually stripped the pack and retained just the 2 good cells. 

[John Cole]

I would like to raise a different aspect of LiPo care: does anyone KNOW why we are advised to store LiPos at about 50% charged (that is, not close to either the low or high voltage limits).  I've seen this advice in several places, and my new LiPos always arrive in that state.

 I can make several GUESSES as to why this might be best practice, but I'm after the real gen.

[Tim Mackey]

The temperature changes which can and do occur in the areas where the lipo is stored, can vary, which in turn alter the internal resistance of the cells. This changing IR allows the cell to either increase or decrease its stored voltage slightly, and this could allow the cell to become either over or under discharged.

Part charging stops this occuring.

My best guess rather than scientific fact...I am not a chemist /scientist.

However I am not  a doctor or surgeon, but I do know its not good to smoke....... dont really know why or care or need to know...but thats what the experts tell me.

Not being sarccy John....

[John Cole]

Thanks. That's exactly what I thought, so at least we're making the same (educated) guesses.  So if we are right then it's OK to keep them charged up, provided they are kept at (relatively) constant temperature. More particularly, there's no real need to part-discharge them for storage.  Mine are kept indoors in a heated area so I would not expect them to get significantly colder over winter.

[Tim Mackey]

Well the best practice recommendations are still that they should be stored ( even in a refrigerator or near freezing ) at a reduced voltage, so I guess it is a bit more "chemical" than my simple theory alone. All the acknowldeged experts including Mr Li-Po himself Fred Marks of FMA suggest partial charge for long term ( say 2 weeks plus ) will preserve overall lifespan. High temperature and very low voltage dropping is certainly the biggest killer of the lithium cell ( any type ) and it also increases the cell imbalance.

[John Cole]

Thanks.  I had a look at the FMA site.  They give a link to Cadex, who recommend storing at 40% capacity and 15 degrees C, saying that this minimises loss of usable capacity.  So i'm going to run all mine down to that point (about 3.75 volts per cell) and leave them like that for the winter until I want to use them again.

[John Cole]

I have some more data.  Flightpower recommend storage at a 3.75 volts.  I have found a link which explains why LiPos deteriorate in storage and recommends storage at 60% discharged: about 3.80 volts:

http://www.buchmann.ca/article23-page1.asp

 

Here's the text giving the technical explanation and table:

 

I've taken the data from this US Army chart (top line of graph):

 

and put it in a spreadsheet:

 

 

These results are slightly different from Timbo's.  Both lots of data say that Tim Mackey (RCM&E Feb p. 85)has given the wrong voltage for safe storage.  He quotes 40% (i.e. 60% discharged) as 3.9 Volts.  3.9 Volts is 70% charged / 30% discharged.

 

Edited By David Ashby - RCME Administrator on 17/12/2009 20:35:16

[Tim Mackey]

Thanks for spotting that John - I havent actually received my copy yet - but will check against my original text here and make any amendments necessary in part two of the article - could have been just a brain freeze moment

Anyway, I have temporarily removed the original capacity chart so that I can double check it

PS I did notice that some of the data you included above was not for common or garden variety lipo cells - EG: the "army data" is for LiMn204 cells which may well have different characteristics..

[Tim Mackey]

OK, new capacity versus voltage chart now produced and entered on front page of this thread - this reflects later technology batteries and cells, and contains extra information.

Of course, all these figures are variable with different cells, age, and so on... but the general guidelines are about right

[IanN]

Having my first go at electric, using the set up recommended by Nigel H for Hepcat in July/Aug RCM&E. The bits have arrived.

 

My pair of 2s 1300 lipos both measure 8.9v on my dvm. Surely according to the info in here they should be 8.4v?

 

Have they been overcharged? What, if any, consequences would that have, and should that worry me?

[Tom Foreman]

Hi Ian, did they arrive in this state or have you charged them? 8.4V is what they should be. I suggest you discharge them sharpish, and then try charging them up again slowly, say 1/2 C and make sure the charger terminates at 4.2v per cell or 8.4v for the pack.

 

I would recommend a balance charger that monitors individual cell voltage, this is the best way to charge as it won't allow any cells to go over 4.2v regardless of how many cells in the pack.

 

Cheers,

 

Tom