Does the data presented apply to Li-Fe-PO4 or only to Li-Ion metal
oxide? I know that they have a distinctly different chemistry than metal
oxide cells, and I know that they have different charging
characteristics. It is likely they have different cold weather charging
behavior as well.
I looked up a few references on the web and you seem to be able to
charge LiFePO4 cells down to -10 C quite normally with no caution
mentioned about going to lower temperatures. An example is Powerstream
Batteries <http://www.powerstream.com/LLLF.htm> show in their
specifications charge down to -40 C. The A123 cells spec sheet show a
temperature range of 55 to -30 C. I don't see any reference to lithium
plating out or anything drastic when I restrict my search to LiFePO4. (
I also ignored blogs and forums. I just looked up papers and
manufacturer specs.) I think that cold weather effect may be restricted
to lithium-ion "metal oxide" cells.
That reflects my personal experience with a pack of LiFePO4 ThunderSky
cells in freezing Colorado weather with outdoor storage. The vehicle
took a few miles to warm up the pack, and range suffered a bit because
of voltage sag due to cold weather, but that was about the extent of it.
I think it is important to not group all Li-ion in the same basket. The
LiFePO4 cells share a few of the characteristics with metal-oxide cells,
but are quite distinct in many ways. Also, there are a great variety of
metal oxide li-ion cells, which are distinct from each other.
Bill Dube'
On 2/23/2015 5:19 PM, Michael Ross via EV wrote:
Discharging pulls lithium off of the cathode side SEI (solid electrolyte
interface) if it has been plated there, but according to the
electrochemists other bad stuff goes on so the capacity lost to plating the
cathode is not reversible even if the plating itself is reversible.
I wish I understood this better, but the science of it may be uncertain
beyond knowing that it is bad.
So when you charge, the ions move towards the cathode, and when it is cold
the motion into the cathode is too slow and the lithium piles up on the SEI
as a metallic lithium - it is plated.
When you discharge the metallic lithium diffuses into the electrolyte and
the motion of ions is towards the anode.
I am not aware that discharging is a problem, I have seen nothing in the
literature about it; and high currents will generate heat right where the
action is.
On Mon, Feb 23, 2015 at 2:11 PM, Rick Beebe via EV <ev@lists.evdl.org>
wrote:
That was one of my big concerns when I built my truck so I tried to fix it
from the beginning. I put 1" of foam insulation in all my battery boxes. I
installed battery warmers under the batteries. I used 35 watt Farnum
heaters I got from KTA-EV. And I built a controller to turn the heaters on
below 15C and to disable charging below 3C. All of that runs only when the
truck is plugged in.
I've been leaving the truck plugged in at home and it's keeping the
batteries at 15C (60F) despite temps to -20C. CALB allows the cells to be
discharged at a much lower temperature than charging so I'm less worried
about the cells cooling off with the truck unplugged at work. That said,
the insulation really helps slow down that process. I've found that on a
20F day the cells have dropped to about 45F after 8 hours at work. The
other advantage to the cells being warmer, of course, is much better
performance.
My understanding is that temperature of the anode is the critical piece. I
don't know if your BMS is measuring that or simply the air above the cell.
--Rick
On 02/20/2015 03:25 PM, Danpatgal via EV wrote:
Reviving this thread as we're having another very cold stretch here in the
Eastern US.
My batteries (SE130 CALBs) are still going, but boy do they sag when it's
cold like this. It's annoying.
I've been charging when my BMS sensors (atop each cell) are over 0C, which
they have generally remained over the past few weeks despite the cold
(thankfully my garage generally stays above 0C).
But, my follow-up question on all this is if the BMS measurement is good
enough. For example, I guess there is resistive heat that gets generated
upon charge/discharge/shunting that probably make the sensors read higher
than the cells themselves. How much, I don't know.
Does anybody have any thoughts, experience on this? We've gotten down to
-20C with a HIGH today of only -10C ... yet I'm charging. I didn't really
want to do it, but I was dragging so much on the road with low SOC%, I had
to, or risk having to really limp home.
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