Looking after your LiPo cells

[Tim Mackey]

I attach a couple of charts which I made up to help show how much juice is really in your LiPo packs and also the correct terminal voltage levels for different conditions.. The figures are approximate, but compiled from several knowledge bases. I have updated this chart to reflect the latest cells, and correct a few minor % errors in the old chart. This new chart also now shows 1 , 2 , 3 and 4s packs

Clicking the images makes them quite legible

 

Several of the more modern chargers have a "storage level" mode for Lithium cells. The correct storage level is around 3.85V per cell.

At ambient, normal room temperature, a cells optimum fully charged voltage is 4.2V. This optimum fully charged voltage value goes down if charged in lower temperatures - If you charge a lithium cell at below 10degC temperature the charge voltage needs to be about 3.9 volts to 4.15 volts not 4.2V. Remember that at 4.15v or so a cell is over 90% charged and will still give excellent performance.

If you charge to 4.2V at this low temperature you can damage the cell due to overvoltage - at the very least you will probably reduce its overall lifetime, because when the cell is then taken to a normal temperature it will rise in voltage slightly and therefore become effectively slightly overcharged.

If you charge a lithium cell to 4.2V at normal room temp and then put the cell in the fridge and take its temperature down to 10C or less you have effectively overcharged the cell because 4.2 volts at this low temperature is where cell damage is starting. This same effect could be seen by charging at normal temperature, then taking the batteries into a cold outside environment such as a winter flying session.

Storage at low temperature - even in a fridge - is a very good way to preserve the overall life of the cells, providing they are only charged to the levels as shown above.

Using them cold of course is different - they perform very poorly if cold - very noticeably in EDFS etc, where every little bit helps. I always warm them in the car or my inside pocket before fitting and flying.

The following chart gives the various charge level parameters for all common cell types.

The minimum discharge voltage shown here is for an unloaded cell - in use - under load. LiPos can generally brought down to around 2.8V without damage. Longer overall life can be expected if this is kept higher..... but then there is always a compromise between flight time and battery life!

Edited By Timbo - Moderator on 18/08/2009 19:10:15

[Alan B]

Nice one Timbo! useful to have.  Legible in full size. If somebody wants a hard copy ie:  me  If other members feel the same but do not know how to.  Just open microsoft word or compatable program - click on full size Image above - left click to copy - Then paste into your word document expand or crop if required and print.  I presume all copyright is waved Timbo!

Thanks   Al

[Tim Mackey]

Aye....copyright waived Alan, - but I would still like to be able to wave occasionally

Or if that nice simple explanantion of how to get a copy is too much for some peeps,  then email me and I will send an email back with the word doc attached

[Alan B]

Damm stiiicky I key!

[martyn sharp]

Thanks Timbo that will be really useful

Martyn 

[Former Member]

[This posting has been removed]

[Alan B]

  I can tell you are from my neck of the woods you old dog!  Even iiiif I had'nt read your profiiile Eric!!  Wiiill have to check out Rivington flying sites one day.  Was up there in the area recently but never made it past the Black Dog at belmont or Rivington Barn!  Must admit I do miss the area.  The Lincolnshire moutain rescue team dont get alot of work around here!  Sorry Timbo - off thread!

[Former Member]

[This posting has been removed]

[Tim Mackey]

David A...could I suggest this as another sticky please ?

PS big box arrived today

[David Ashby - Moderator]

Ok, no prob Tim. Ah good and the little one I hope?

It looked lovely, nearly forgot to send it on

[Tim Mackey]

Yep

Minor damage from handling though I suspect

[David Ashby - Moderator]

I added some bubble wrap too......

[Tim Mackey]

No worries...it will all be Ok after some minor work

[leccyflyer]

 Thanks for that Timbo 

The voltages shown for 80% and 90% full 3s packs are identical (12.3v) - do you have a  suggested amendment?

 cheers

 Leccy

[Tim Mackey]

I did say they were approximate....but if you insist, how about 12.35v for 90%?

[leccyflyer]

That's close enough for jazz, Timbo..

12.45v for 90% full would be fairly close to a linear relationship, as suggested by most of the other data points.

[kenneth gustavson]

Good work!!    Kenny

[Erfolg]

Thanks Timbo

Your chart has now been pasted into my  reference book, over the rough pencilled figures you gave some time ago. I make reference to the chart frequently, to access how much capacity was left after flying.

Much smarter.

One question remains, in your opinion have deep can you sensibly discharge to. I note that you have indicated about 20%. I have gone as far as 3.86, without the BEC coming in.

As a point of interest I plotted out the graph (some time ago), and found it is " Directly Proportonal" or "Linear" or of the type Y=mx+c (c=0), I am having such fun. Given the normal accuracy for experimental observations.

By the way, My Imax6  works far better than booth my  Perkins Lipo 2000 with Overlander cell balancer, far quicker and balances the cells far more accurately. 

Keep on working Timbo, your reward will come in heaven.

Erfolg 

[Tim Mackey]

Ah but thats based on the assumption that I go there, some people I am sure are expecting me to end up in the other place where its a bit hotter

I regularly allow my cells to get down to 2.75 under load   and have on occasion seen some ESCs set a 2.65 cut-off.....in fact the 80A one I am currently testing on the EDF setup does that. I immediately put the pack onto balance charge, and 2 x cells show below 3V but quickly recover to just over 3V and charge back to 4.2 as normal.

Remember the chart is really based on very lightly / no - load figures ( as it says on the heading of the chart )

Knew you would like the Imax6 - for the money, I reckon it takes some beating. 

[kenneth gustavson]

Just give me the facts-------  Thats good

[Tom Doyle]

Your chart is very enlightening and a copy is now pinned up in my workshop. As a result of my other Airpower lipo problems which you have kindly responded to in another thread, and your chart, I decided to take a closer look at the capacity of my other lipo packs. For the record I'm using a Maplin regulated voltage supply, a Prodigy 11 charger and a Poly Pro balancer, perhaps not the most sophisticated equipment in your terms but adequate for my needs - I think!

On checking three particular packs of the same make and capacity, they were coming off a balanced charge reading around 12.35v, the individual cell voltages were within 0.01v. With my Prodigy 11 set at 3.7v and using a made up charging lead, I plugged directly into each individual cell through the balance lead. Each cell was 'topped up' in turn until the Prodigy 11 terminated the charge. To my surprise, on 2200 mAh packs, this procedure put in, on average, another 90 mAh per cell and the new pack voltage was 12.64v. Each cell was within 0.01v. Questions: Am I doing anything dangerous here? Is my Poly Pro balancer suspect? Or is it time I invested in better equipment?

[Tim Mackey]

Well the maximum voltage allowable for an individual cell ( LiPo ) is 4.2 and therefore a 3s pack showing 12.64 is OVERCHARGED!  As each cell was within 1/100th of a volt of each other then each cell may well have been at 4.21 which is slightly higher than it should be. Remember that at the figure of 12.3 your pack was over 90% full and any difference in performance would be negligble.

However, more relevant is the fact that slightly undercharging the packs will increase their life span.

I would certainly leave them at what the pro peak ( which is a good charger ) cooks them to. Another thing to consider is that the terminal voltage of the pack will drop ever so slightly as soon as the charge stops - many chargers actually display the maximum voltage attained at the final CV stages and that figure is retained on the chargers display - others give an actual real time display which may well account for your readings. This slight drop off is common to most batteries. If cells come off the cooker within .01V of each other it seems to me that your balancer is doing a good job - as for whether its time to buy more toys...well thats up to you and your bank

...but we can ever have enough chargers

[Tom Doyle]

Thanks for your informative reply, I'm reassured things are OK. The old addage, 'too much knowledge can be dangerous' doesn't apply here, on the contrary, the more knowledge we have on electric flight the safer we will be.

[IanN]

Timbo, re charging LiPOs, I have a pro peak Prodigy 1 (the one that states its for LiON, but doesn't claim to be for LiPO). I'm sure I read on here somewhere that you'd used this charger ok for LiPOs

Am considering having a go at electric via Nigel's Fling Wing as that loooks like a great low cost intro. However, the cost factor goes up considerably if i need to shell out on a new charger.

Could you advise - obviously with all necessary disclaimers  - what the score is re charging liPOs with the Prodigy ?

[Tim Mackey]

Its Fine.