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Cold Pack: Limited Regen and (A)symmetric Power Limits

Todd Burch

12-Year Member
Nov 3, 2009
8,488
37,781
Smithfield, VA
I haven't seen a thread on this specifically...although there are threads discussing the regen limit that I've seen. I hope this doesn't rehash an existing topic.

I understand why regen is limited when the pack is cold. It is not healthy for batteries to take a significant charge when cold. I get that.

Today, my pack was a little chilly (37°F) and regen was limited to about 30 kW for a few miles. But power output was not limited. Why is motor power not limited similarly?

I would've guessed that with a cold pack, it's equally unhealthy to suck 50kW FROM the pack than it is to pump 50 kW INTO the pack. Why is it not symmetric?
 

Chas F

Model S 60kWh #P6396
Aug 22, 2012
220
4
Orlando, FL
Yep...regen limits are much more common. Which begs the question: why is regen limited more frequently and by a greater amount than power output?

Todd,
I am by no means a battery expert (nor have a played one on television), but I would guess this asymmetry is inherent in the physics of the battery's internal structures. In other words, perhaps the micro fractures produced by charging are more pronounced than those of battery depletion. I think you and i both are mechanical engineers and you may be able to relate that my thinking tends toward an analogy of a high pressure fluid reservoir that is stronger when stressed in contraction than when in expansion.
 
<wince> No, if it were a fluid, you'd have to put stoppers in all your wall sockets to keep the power from flowing out. Comparing electricity to fluids generally leads to more confusion than comprehension.

I thought power does flow out when I pull the stoppers from my wall outlets? :tongue:

ok fine, electricity is similar to fluids in some respects, not all, and maybe not most
but confusion, compression, and comprehension are all similar
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,030
1,026
I'm a fan of Battery University. I don't pretend to understand all the details of battery charge/discharge, but the site helps me to understand:

The performance of all battery chemistries drops drastically at low temperatures. At –20°C (–4°F) most nickel-, lead- and lithium-based batteries stop functioning. Although NiCd can go down to –40°C (-40°F), the permissible discharge is only 0.2C (5-hour rate). Specially built Li- ion brings the operating temperature down to –40°C, but only on discharge and at a reduced discharge.

Discharging at High and Low Temperatures Battery University

I suspect you're seeing a "curve" effect of that "Specially-built Li-ion" clause.
 
I've been charging to a bit shy of standard each night and then, stop charging via the app, about an hour before I leave in the morning I start charging again from the app, about a half hour before I leave I also turn on the climate control remotely. This combo conditions the battery enough to eliminate the regen limits in the upper 30's low 40's. compared to the Leaf, which has no battery warmer, its much easier to get the S up to ideal operating temps. on the Leaf the regen is limited and stays that way for quite a while on a cold soaked pack.
 

montgom626

Active Member
Dec 15, 2012
1,328
1
USA
I thought the leaf could heat the battery, but not cool it?????

- - - Updated - - -

1. Keep your LEAF plugged-in and out of the weather until you need to drive it.leafcold7-150x150.jpg In extremely cold weather the LEAF has a battery warming system that is designed to keep the battery pack at an optimum temperature. This system does require some energy, so if the car is plugged in to an EVSE. the car will replace the battery power used to keep itself warm.


Link

http://sfbayleafs.org/ev-resources/leaf-tips-tricks/maximizing-your-leafs-range-in-cold-weather/
 
I thought the leaf could heat the battery, but not cool it?????

- - - Updated - - -

1. Keep your LEAF plugged-in and out of the weather until you need to drive it.leafcold7-150x150.jpg In extremely cold weather the LEAF has a battery warming system that is designed to keep the battery pack at an optimum temperature. This system does require some energy, so if the car is plugged in to an EVSE. the car will replace the battery power used to keep itself warm.


Link

Maximizing your LEAFs range in cold weather - SF BayLEAFs
That passage is somewhat in error.

The LEAF battery heater is only to protect the battery from freezing, not to optimize the temperature. The heater turns on at -4°F, and turns off at 14°F, hardly a good operating temperature.
 

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