LiFeYPo4 capacity in winter?

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LiFeYPo4 capacity in winter?

Kirill Spitzer
Hello list members,

Ive read that these batteries capacity do remain nearly the same at cold temperatures. Is there any truth behind it / who has the necessary experience about this type of battery in winter temperatures? Please tell!

Thanks a lot!

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Re: LiFeYPo4 capacity in winter?

Elithion
To the best of my understanding, the physical capacity is not affected by temperature.
On the other side, the "effective" capacity could be: because of increased internal resistance at cold temperatures, the voltage sag under load is such that you may have to stop using the battery sooner than you would in the summer. That doesn't mean the battery is empty at that point. Oh, no: the charge is still there. It's just that you can't extract it at that same rate (at the same load current). That's how the "effective" capacity is reduced even though the physical capacity has not changed.

So, the real question should be: " LiFeYPo4 resistance in winter?"

Now, for the data.
We hired the service of a testing lab, and measured the resistance vs temperature of CALB (LiFePO4 ) and Thundersky (LiFeYPO4) cells.
Here is a graph showing the results:
http://elithion.com/graphics/ResistanceVsTemperatureCalbThundersky.gif
The performance of both cells is very similar: about -35 mΩAh /°C below 20 °C and about -3 mΩAh /°C from 20 to 60 °C.

Note: this is the *DC resistance*, not the impedance at 1 kHz (which is what cell manufacturers give you, if they give you anything at all). Also, the values given are not actual resistance, but "specific resistance". To get the actual resistance, divide by the capacity of the cell.

How does this help you?

Well, if you have current consumption data about your application, you have a good understanding of electrical concepts and physics, and are adept at calculations using spreadsheets, then you'll be able to calculate the battery efficiency versus temperature, and the point when you'll have to stop using the battery because the cell voltages will drop too low.

Otherwise, at the very least you can take away this qualitative idea: LiFeYPO4 and LiFePO4 cells work pretty much the same from 20 to 60 °C. Below 20 °C, their resistance increases significantly as they get colder, which makes them less efficient and forces you to stop sooner.
Davide Andrea
Elithion
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Re: LiFeYPo4 capacity in winter?

SLPinfo.org
In reply to this post by Kirill Spitzer
If your actual question is whether you will get the same range from them in
cold weather at least one person here in Idaho has told me he loses range
when it's cold.  Not sure how
much.

Peter Flipsen
On Jul 4, 2012 11:48 AM, "Kirill Spitzer" <[hidden email]> wrote:

> Hello list members,
>
> Ive read that these batteries capacity do remain nearly the same at cold
> temperatures. Is there any truth behind it / who has the necessary
> experience about this type of battery in winter temperatures? Please tell!
>
> Thanks a lot!
>
> _______________________________________________
> | Moratorium on drag racing discussion is in effect.
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> | REPLYING: address your message to [hidden email] only.
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Re: LiFeYPo4 capacity in winter?

Mike Nickerson
I have a little experience driving in winter conditions with Thundersky.
The reasons are very similar to those that Davide mentioned.

I was getting pretty significant voltage droop as conditions approached 0C
(32F) ambient.  My batteries weren't insulated and some of them were under
the hood so they saw ambient conditions pretty quickly.

I need 100A at 140V to cruise at 60 mph in the summer.  In the winter, the
additional internal resistance caused the battery voltage to drop to 130V.
The current went to about 120A to compensate (get the same total kW to the
motor).  This additional current caused my range to go down.

Mike

> -----Original Message-----
> From: [hidden email] [mailto:[hidden email]] On
> Behalf Of SLPinfo.org
> Sent: Wednesday, July 04, 2012 2:16 PM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] LiFeYPo4 capacity in winter?
>
> If your actual question is whether you will get the same range from them
in

> cold weather at least one person here in Idaho has told me he loses range
> when it's cold.  Not sure how much.
>
> Peter Flipsen
> On Jul 4, 2012 11:48 AM, "Kirill Spitzer" <[hidden email]> wrote:
>
> > Hello list members,
> >
> > Ive read that these batteries capacity do remain nearly the same at
> > cold temperatures. Is there any truth behind it / who has the
> > necessary experience about this type of battery in winter temperatures?
> Please tell!
> >
> > Thanks a lot!
> >
> > _______________________________________________
> > | Moratorium on drag racing discussion is in effect.
> > | Please take those discussions elsewhere.  Thanks.
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> > | REPLYING: address your message to [hidden email] only.
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Re: LiFeYPo4 capacity in winter?

Martin WINLOW
In reply to this post by Kirill Spitzer

On 4 Jul 2012, at 18:41, Kirill Spitzer wrote:

> Hello list members,
>
> Ive read that these batteries capacity do remain nearly the same at cold temperatures. Is there any truth behind it / who has the necessary experience about this type of battery in winter temperatures? Please tell!
>
> Thanks a lot!
>

Others have explained this effect is basically true and why so I won't go over that too other than to say in my experience I found not only loss of range with my LFP160s (bought 08'08) but also loss of power.  Below 15 deg C  it was significant but not wildly so, below 10 it was robbing me of about 30% of max power and below 0 about 50-60% power loss.  THe other issue is associated voltage sag which limits your top speed.

Of course, some might argue that you wouldn't necessarily want to be able to accelerate rapidly in sub zero road conditions.  So, there's a little serendipity going on there!

So the question is, how to get around the problem?  Simple answer is to heat your batteries.  IMO the simplest way of doing that is to put a domestic-type electric bed heater blanket under them and insulate the pack with foil bubble wrap-typ insulation. Power it from the mains when plugged in and the pack when not but usually the latter is not needed as the cells are kept warm by being used, driving.

 This has all been covered before on the EVDL so I won't bore everyone with it all over again.

 Regards, Martin Winlow
Herts, UK
http://www.evalbum.com/2092
www.winlow.co.uk

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