LiFePo4 - reduced cold weather performance?

[GCinDC]

Yesterday I learned the hard way that a Lifepo4 battery can NEVER sit in the cold for any reason. NEVER NEVER NEVER.

Hold on a second... Is this true, or is something else going on? I've never heard such a thing.

In fact, I just went through my first email exchanges with Ping (11/08):

...Thanks Ping. I have two more questions:

1. How much should I drain the battery the first 5 times? only 75%?
2. Is it okay to leave the battery in - 0 degrees celcius temperatures?

Thanks,

Greg

Hi Greg,

1, Just use the battery how you want it to be. LiFePO4 doesn't have memory effect. So, it doesn't matter.

2, Yes, you can leave it in the temperatures. But its performance will be reduced much. On the other hand, the low temperatures are good for storing batteries. But please pay attention do not take the pack from low temperature to higher fast, that will produce water over the pack.

Best regards,

Ping

I'm very interested because my Ping pack has been sitting out in the garage all winter, and the last times I've tried to test it, I'm pretty sure the BMS was cutting out...

 

[LI-ghtcycle]

I think this could be a good cheap solution. There must be oil pan heaters that are limited to a certain temp. How heavy is one of those little guys?

BTW you could stick on the top of the battery as well as the bottom. Doesn't matter which side you stick on, remember this is conduction heat transfer, not convection. wiki the suckers

That pad would burst the second I hit a Chicago pot hole. ..

There is no need to use a heating pad or anything while riding. I just biked 11 miles and it 26 degress outside. The battery worked fine because it was WARM before I put it on the bike. ad started riding.

What do you mean burst? I think all that in there is a heating element. (Oil pan heater, not Oil heater)

I thought the issue here was to use it while charging, not while riding. And warming up batteries before a ride.

Exactly my thoughts, but yes there are hight watt heaters, that is why I suggested the 50watt mini heater. I had one on the oil pan of my Subaru Brat and it was never "hot" it's just to keep things from freezing over-night, it was never too hot to touch, just warm.

 

[LI-ghtcycle]

Whoa! Heard about the storm about to hit ya in DC, how much snow you got so far?

I'd get that battery indoors if I were you, according to Dewalt, A123 batteries (same basic chemistry) don't like to be used in anything cooler than 40F, but I guess if your not going to be snow-plowing on the E-Bike, maybe you won't be using it for a while. :wink:

 

[GCinDC]

Whoa! Heard about the storm about to hit ya in DC, how much snow you got so far?...but I guess if your not going to be snow-plowing on the E-Bike, maybe you won't be using it for a while. :wink:

seems like about a foot right now and still falling hard! it's starting to look like another world. i can't wait to get outside tomorrow and take pictures.

oh, and i will try to get around on the bike. handles beautifully in several inches of snow. i'm sure the derailer will get messy with this much, and i'm not crazy about getting water in the axle..

btw, i charged my 48V12Ah in the garage all winter last year, through several sub-zero stretches, never had a problem until i blew a couple cells overdischarging w/ lipo booster pack.

 

[amberwolf]

Exactly my thoughts, but yes there are hight watt heaters, that is why I suggested the 50watt mini heater. I had one on the oil pan of my Subaru Brat and it was never "hot" it's just to keep things from freezing over-night, it was never too hot to touch, just warm.

And how large is the thermal mass of that engine, plus it's surface area exposed to cooling air, compared to a battery pack inside an insulated box or bag?

 

[LI-ghtcycle]

Exactly my thoughts, but yes there are hight watt heaters, that is why I suggested the 50watt mini heater. I had one on the oil pan of my Subaru Brat and it was never "hot" it's just to keep things from freezing over-night, it was never too hot to touch, just warm.

And how large is the thermal mass of that engine, plus it's surface area exposed to cooling air, compared to a battery pack inside an insulated box or bag?

Yes, but you must realize that the heater I used was MUCH MUCH more than this tiny 50 Watt heater, the other was something like 300+ Watt, but point taken. 8)

 

[The Stig]

Whoa! Heard about the storm about to hit ya in DC, how much snow you got so far?...but I guess if your not going to be snow-plowing on the E-Bike, maybe you won't be using it for a while. :wink:

seems like about a foot right now and still falling hard! it's starting to look like another world. i can't wait to get outside tomorrow and take pictures.

oh, and i will try to get around on the bike. handles beautifully in several inches of snow. i'm sure the derailer will get messy with this much, and i'm not crazy about getting water in the axle..

btw, i charged my 48V12Ah in the garage all winter last year, through several sub-zero stretches, never had a problem until i blew a couple cells overdischarging w/ lipo booster pack.

YEaaahhh ebike ride in the snow, make some vids too!! I think its kinda fun riding in the snow. Esp if you AIR-DOWN to about 5psi!
Your ebike looks like a beast, I would definitely want studded tires at those speeds. Or nice fat tires, 2.5"+ for powder

 

[GCinDC]

...About half way there the BMS cut out and the battery went completely dead.

So just curious, how's your pack now? Does the heating trick work?

 

[DAND214]

Hey Guys, what about the electric heat wrap for water pipes in the cold. How would that do for a battery heater?

http://cgi.ebay.com/WRAP-ON-ELECTRIC-PIPE-HEATING-CABLE-13FT-NIP_W0QQitemZ380205436495QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item5886028e4f
Dan

 

[USAEBIKECO]

The hand warmers had zero infuence on the battery performance.

It was 17 degrees the other day and i biked 10.2 miles with no problems. As long as the lifepo4 battery is warm before you start riding it works fine.

I highly recommend never leaving any battery in temps below 40 degrees.

 

[monster]

the problem with heathing pads and hand warmers it that its difficult to get even heating. if you make one side of the battery hotter than the other side this probably is bad. you don't want to raise temperatures above 50*C, but if you heat only one side of the pack to 50*C then the opposite side will be cold.

 

[Lock]

the problem with heathing pads and hand warmers it that its difficult to get even heating.

I am *mildly* concerned about this... I have two of these to install under 16 10Ah Headways arranged in a single row:

They are rated as 35W plugged into 120V but plan is to run `em directly offa 48V DC pack voltage where they should provide 14W ea so I'm thinking this'll be mild enough plus good insulation.
tks
loCk

 

[rocwandrer]

I know this is basic stuff, but I couldn't find it in searching here...

When a LiFePo4 gives less ah in the cold, is it inefficiency, or inability to deeply discharge that reduces the available capacity? In other words, when discharging cold,then warming to room temp and charging, will it take more ah to charge than were delivered in discharge? Does discharging while cold shorten the life of the cells?

At what point is it worth running a resistance heater off of the battery to get/keep it warm? I've left my battery disconnected outside tonight. It will be warm-ish in the morning, maybe 28f. I figure I only need 45% of its rated capacity anyway. Bad call?

 

[amberwolf]

some info
http://endless-sphere.com/forums/viewtopic.php?f=7&t=23628

also
http://www.endless-sphere.com/forums/search.php?keywords=cold+LiFePo4&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search
http://www.endless-sphere.com/forums/search.php?keywords=winter+LiFePo4&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search
http://www.endless-sphere.com/forums/search.php?keywords=winter+battery&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search
http://www.endless-sphere.com/forums/search.php?keywords=cold+battery&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search

 

[dnmun]

the rate at which the ions can flow through the electrolyte gel is determined by the temperature. the trans conductance of the electrolyte is the limiting part of the ion mobility from it's hole in the cathodic mixture through the electrolyte and then through the intercalations of the carbon until it can find a spot to precipitate into lithium metal.

so when the pack is cold the ability of the charge to get out of the pack is severely degraded. the lipo packs can be damaged by charging at high rates while the battery is cold.

the lifepo4 also will lose some capacity if the charging is done while cold because the lithium metal will end up plated on the carbon too, just like the lipo. except the lipo can have bad juju later from having the metal all precipitated on the surface of the carbon instead of inside. lifepo4 is not liable to the thermal runaway.

 

[dogman dan]

Clarify that please?

Charging lifepo4 when cold causes this? Or more just a problem with charging at high rates in the cold?

I never did much charging in really cold conditions, and charged at 4 amps or less. The battery would be warm from the ride in, and charged in say 25F weather. But the freezing temps would be over by the time the battery could cool down. So I'd say I never put a charge on it while the pack was actually below freezing.

 

[rocwandrer]

Edit: some of my answers are here, thanks: http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=23893&start=0

A lingering question: is the degree of stress I'm putting on the pack directly correlated with voltage sag? In other words, can I say I'll never run it below 36 v under discharge load on a 12s pack, and know I'm not shortening the pack life as much as any other temperature or SoC or c rate condition where I go below 36v/pk? Is this a valid approach to knowing how stressful a condition is on the cells?

 

[rocwandrer]

So with the battery left out, starting voltage of 39, 12 amp voltage of 36v. With the battery starting warm, at the end of the ride, battery starting voltage of 39, 12 amp voltage of 38.0, 8 amp voltage of 38.5.

Seems clear the battery is much happier starting the trip warm. Can I comfortably say these cells are 3c safe, given even cold, they stay above 3v/cell at 3c?

 

[dnmun]

yes, the sag is different from the charging of lipo at low temps. sag is just a function of the low transconductance because the electrolyte is so cold. high internal resistance.

when charging lipo at high rates when cold. the ions just cannot get through the electrolyte in the pores of the carbon matrix on the electrode so the ions start precipitating into Li metal while still on the surface of the carbon. the ions do not make it into the 'intercalations' or layers inside the carbon and deposit the Li metal inside there. so the Li metal builds up on the surface of the carbon and has a hard time getting off again when you discharge. but i think this is more in the -20o C range.

 

[FLBikeDude]

I know that cold will have the effect of creating more sag in the cells, but does it "hurt" them in any way?

Being in South Florida, its not normally a problem, however its been quite chilly the past couple of days and I've noticed much more sag than usual and was hoping that it wasn't doing any real damage..

Fyi, Its been about 40° F (I know its not THAT bad..)

 

[Ypedal]

40f / 4c

Not that cold compared to our -20f / -20c !!... but below 10 celcius ( 50f ) you will notice a difference.

No harm to the cells as far as i can tell, once the temperatures improve the performance usually shows up along with it.

Video i made in 2010
https://www.youtube.com/watch?v=PzoyfFuwDKE

[youtube]PzoyfFuwDKE[/youtube]

It's on it's last legs now.. but ran for 5 years !

 

[wb9k]

Not sure whose cells you're using, but 40F is not anywhere near dangerously cold for any cells I know about. Discharging at typical rates is not where the problem is anyway...it's in charging cells from a very low SOC at a very low temp. I wouldn't even give this a thought until you are well below freezing unless you typically charge at high rates. Too much charge current in a really cold cell will plate Li onto the cathode, wiping out capacity rather quickly.

 

[Ypedal]

How far below freezing ?.. can you slap a " rough / average " number on it ?

 

[cycborg]

Ah, 40 deg F, I can almost remember what that was like...

 

[wb9k]

40 degrees is still motorcycle riding weather!

YP, I'll try to get some numbers posted later on...