Li-Po's in the cold

[David Ashby - Moderator]

Had a good fly this weekend but my Li-Po's seemed to be suffering. I took six packs along which was just as well as three just didn't generate sufficient power. Was this the cold?

I've been thinking and can only put this down to

1. The temperature was about 0-2 degrees C

2. The newer packs were ok but the older ones suffered. Two of the older ones were given to me and may have been (how shall we say) very well used in the past.

 Has anyone else experience of this? 

[scott cuppello]

For sure, flight times are shorter regardless, but "well worn" packs will really show their age in these conditions, I keep my flight times short, and keep them warm in the car.

[Tim Mackey]

Yep, almost certainly down to low temperature. I did post some info on this somewhere, including warning about full charge levels  in the cold -but blowed if I remember which thread.

I will try and recall.

[David Ashby - Moderator]

Yup I do recall seing Tim but I thought a cold Li-Po thread topical so we can share stories - probably wont be the last time the packs suffer, I might pop a warm hottie bootle in the car next time

[Tim Mackey]

The following is all “recommended” treatment, from various sources.

As we mostly know, the normal maximum permissible voltage with Li-pos is 4.2 per cell.
Below 10 degrees C that falls to 4.15V - so if you charge up to 4.2 in the warm and then let the battery get cold out on the field or when stored in a cold workshop - you effectively have an overcharged battery. EG: If they lose say  .75 volt when they are cooled, then they likely will gain .75 if they are warmed. Some say that a pack charged to 100%  should not be cooled below room temperature.  Therefore if planning to take a pack out into the very cold weather, a charge to perhaps just 90% or so of capacity is advisable.  Cells at 90% or less capacity can be safely cooled below 32F (0C).

Cell temperature during use is proportional to current draw. The pack starts the flight at a particular temperature and this rises throughout the flight.

If you have  to store them for more than a month or so,  it is also advisable to discharge them to 3.7 - 3.9 volts per cell and then put them in the fridge or freezer, - protecting from moisture. Lipo capacity drops about 10% a year at room temp but only 2% a year when discharged and cold.

If you do store cold,  then do NOT store fully charged. Remember the maximum safe voltage for a lipo decreases with lower temp, so a pack fully charged at room temp, stuck into cold storage will be "overcharged" when it has cooled down.

Storage alone at low temperature does not reduce cell life, however charging at normal rates when the cell is cold can harm the cells. The FMA CellPro charger actually senses ambient temperature and adjusts the charge  to protect the cells. See link here.
http://www.fmadirect.com/detail.htm...2193&section=45
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Bet you wish you hadn’t asked now don’t ya !

[Tim Mackey]

I tend to charge them at home before setting off, then keep them in an insulated cool bag ( working as a warm bag ) and keep the pack I am about to use in my pocket until I reach the strip. The others remain in the comparative warmth of the car.

I think they are slightly more sensitive to the cold than older metal canned cells like nickel, after all, they only have a thin plasticky bag to keep them warm poor things.... 

[David Ashby - Moderator]

Bet you wish you hadn’t asked now don’t ya !

Not a bit of it mate, good stuff. Now I know. I'll keep the packs warm at the field I think - not that I took mine out of the car yesterday but the boot was open so they were as cold as if they were on the flight line. 

[Tim Mackey]

Incidentally....the guys have still not fixed my stats

Sorry....slightly off topic

[David Ashby - Moderator]

No I know, don't hold your breath..

[jez]

keeping warm is the answer used 2 identical packs last sat and the cold one charged first was down from 9 mins to 6mins but the one just off the charger did 9+ roll on the summer bbbbrrrrrr

jez

[Tim Mackey]

Below is a reply I received from Fred Marks esq of FMA, in response to some points I discussed with him a few years back. The guidelines hold true today.

Tim, the low voltage cut off is based on normal applications in which a cell is slowly depleted and the voltage drops along, e.g., the 0.5C line.  Under these circumstances, the 2.5 V cut off is mandatory.   In R/C or other high drain applications, the situation is more dynamic.  The cells could actually be dropped below 2.5V momentarily without harm.  However, that is not desirable and is unnecessary.  Below about 3.0 V, less than 1% of capacity is left and it doesn't make sense to take it all out.   Further, if cells are cut off at no lower than 80% depth of discharge and never charged above 90 % of full capacity, life cycle may be almost doubled.  Life is maximized if you do this and  do not discharge continuously at current that causes the battery to go above about 150 Deg F.

Our recommendation:  Cut off at no lower than 2.85 V and that only if you anticipate very high drain.   Certain cells exhibit a dip at the start of discharge until they warm up and a high LVC (3v or 3.1V )  may cause a cut during launch.”

Fred Marks.  FMA.

[flytilbroke]

I am not really a leccy flier. I have had a fair practical experience of various types of battery including LiPo. I have never found any which did not have a reduced performance in near, or below freezing temperature. In very cold weather one trick I find very helpfull is to give a boost charge shortly before use, this can be done whether following Timbo's advice re 90% charge or not. In field conditions in another venture we had to ensure our Lead Acid car/Leisure batteries were kept warm as the voltage drop was not acceptable by our equipment.

 A well insulated box with some form of heat (water bottle?) in it helps, another possibility is the reversible cool/warm "12v" box things so popular for the summer picnics, don't flatten the car battery with long use tho'. More,,, make up a box using the thick insulating foam popular for making wings 'n things. Make it with slots to take your packs and put'em in while nicely warm before leaving the house.

[Edmund Comber]

Please forgive what may be a foolish question, but with this talk of charging to 90% of full capacity, what does this represent - how do you calculate % of capacity?  What is 10%? 

I am new to this electric malarky, having just returned to the hobby after a 20 year absence (to fly full size).  I recently re-aquainted myself with RC flying and on going to the model shop to completely re-kit myself out, was persuaded the electric is the way to go.  I got a Spectrum DX7, Multiplex Acromaster ARTF, KMS Quantum 2820/05, Impulse Power 3200 11.1v 3S1P 20C LiPos, Tornado 40Amp ESC, a JP Energy JP LiPo Plus 5 in 1 Inteligent DC Balancing charger and a Pro Peak 20Amp 240v-12V Power Supply.

I flew the Acromaster for the first time last Sunday, having balance charged two LiPos to 4.20 V per cell and had two 5 minute flights off each battery, taking them down to 9.9v and 10.01v checked immediately at the end of the each flight pair.  When I got home and re-checked each LiPo I had 10.01v and 10.03v reported.  It was cold at the airfield.

[Tim Mackey]

2 x 5 min flights off each battery = 10 min flights off each battery. Nothing wrong with that Edmund.

Incidentally, did you stop flying after 5 mins, stop everything, switch off / disconnect etc and then go off again for another 5 mins? If so, the ESC could be fooled into thinking that on the second flight you were using a smaller battery, and the LVC could kick in too late. It is generally best to connect a freshly charged pack, and "fly it out" until your timer sounds, or the LVC cuts in. The pack should then be re-charged before use again.

If full charge per cell is 4.2v ( and it is ) then 90% is ....3.78v isnt it? If your charger does not have the "cold weather charge" facility, then in order to achieve this, simply disconnect the 3s battery from charger when the indicated voltage is 11.34V

Personally, I never bother and do a full charge just before flight. 

[G]

I've been flying in the cold and haven't noticed too much reduction in capacity.  Maybe newish batteries cope better with cold than well-used ones?

[Brian Tootell]

Hello everyone,

I am new to flying RC and I am trying to get my head around the terminology and general technology of Li-Po batteries. Can anyone point me towards just one article of explanation of how they are put together for the right voltage / current comination, balancing?, charging? HELP!

Brian

[Tim Mackey]

You ask a lot my friend ! There are several articles in the mags currently ( pun intended ) and these will explain most of what you ask.

If a LiPo battery is described as 3s2p this means there are 3 cells connected in Series (  which gives 11.1V ) and that this is connected in Parallel to another identical pack - giving 3 in series, and 2 in parallel, or…3s2p.

Just to practice a little, a battery marked as 5s3p would consist of 3 batteries, each of 5 cells in series ( 17V ), - connected in parallel with each other.

So if we want to increase the volts, we join in series, and if we want to increase the duration or current ( Amps ) available, we join in parallel.

Combining both increases both.  Easy huh

Balancing simply means ensuring that each cell in a battery pack is maintained at the same voltage as each other, using either a standalone balancer, or a combo balance+charger.

LiPos should be charged at 1C which means 1 X the capacity of the pack. A 2000 m/ah pack should be charged at 2000 m/a ( 2A), which will take around an hour or so.  If this battery is marked as a 10C pack  then it can be dischrged at up to 10 times the capacity ( 2000m/a ) therfore max discharge is 10 X 2A = 20A.

They can be discharged ...or "used" at whatever the maximum C rate states on the pack. 

[Brian Tootell]

Cheers Timbo! It's simple when you know how. I shall go away and mull over your response. Have a nice Christmas everyone!

Brian

[Grant Darnley]

Took the wee edge out yesterday, was -2 so left it in the car until just before I flew it, even then by the time I'd connected up the batt and got to the runway the canopy had misted over.  Vertical performance was down, wouldn't pull from a hover, and duration was down to around 5mins(33% reduction).  I'm sticking to I.C. in the winter from now on

[Edmund Comber]

Thank you Timbo. 

Yes I flew for 5 mins, landed, switched off (disconnected LiPo) and then checked the voltage,  reconnected and flew again for 5 mins.  I did this because I did not want to discharge below 10v and had no idea how long I could fly for before I got to that stage.  The LVC is set to 2.8v per cell on the Tornado ESC, which I understand is too low, and it should ideally be 3v or even 3.3 v per cell.  However this cannot be changed using Tx programming, it needs a programming gizmo which I do not have.

I note your advice re switching off, thank you.

[Tim Mackey]

2.8V is fine for a lipo under load, which it is when flying

Once you have discovered how long you get from a battery in normal use, use a timer and plan to land just before the LVC cuts in anyway. 

[Edmund Comber]

OK than is re-assuring - I am reading so many different 'minimums'.

[Tim Mackey]

Yes I know what you mean, however Fred Marks himself (of FMA fame ) states that even down to 2.75v was ok for a cell under load, and it almost immediately recovers to 2.9 or more as soon as load removed. There is evidence that overall life may be increased if they are not dropped below 3.0v.

See Fred's note below to me from a while back.

“ Tim,  the low voltage cut off is based on normal applications in which a cell is slowly depleted and the voltage drops along, e.g., the 0.5C line.  Under these circumstances, the 2.5 V cut off is mandatory.   In RC or other high drain applications, the situation is more dynamic.   The cells could actually be dropped below 2.5V momentarily without harm.  However, that is not desirable and is unnecessary.  Below about 3.0 V, less than 1% of capacity is left and it doesn't make sense to take it all out.   Further, if cells are cut off at no lower than 80% depth of discharge and never charged above 90 % of full capacity, life cycle may be almost doubled.  Life is maximized if you do this and  do not discharge continuously at current that causes the battery to go above about 150 Deg F.

Our recommendation:  Cut off at no lower than 2.85 V and that only if you anticipate very high drain.   Certain SHD cells exhibit a dip at the start of discharge until they warm up and a 3v or 3.1V LVC may cause a cut during launch.”

 

Fred Marks.  FMA.

[Tim Mackey]

oops....just realised I had already posted this earlier.

[Russell Mexome]

Has the COLD WEATHER  damaged my Li-Po's? I kept them in the garage over the last 2 weeks fully charged (and its been very cold) went flying today with 3 battery packs, I only got about 4 mins flight out of each battery pack. When I got home I put them on charge the problen is after 10 mins the charger bleeps to say that the batt is charged. I put it back in the model and bench run it lasted only 1 min, and it was the same for the other 3. The Li-Po's are Rhino2150  3cell 11.1volt  and I have had no problems up to this point. The charger is a Multiplex LN3008 EQU.