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

Todd Burch · Mar 6, 2013

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?

Elshout · Mar 6, 2013

I've seen power output limited on a few really cold days but goes away quickly.

Todd Burch · Mar 6, 2013

Elshout said:
I've seen power output limited on a few really cold days but goes away quickly.

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?

Chas F · Mar 7, 2013

Todd Burch said:
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.

Zextraterrestrial · Mar 7, 2013

electricity is pretty similar to fluid(, or is it a fluid?)

Robert.Boston · Mar 7, 2013

Zextraterrestrial said:
electricity is pretty similar to fluid(, or is it a fluid?)

<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.

Zextraterrestrial · Mar 7, 2013

Robert.Boston said:
<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 · Mar 7, 2013

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.

strider · Mar 7, 2013

Chas is correct. It's a feature of Li-Ion chemistry. It can be safely discharged at lower temperatures than it can be charged. Or put another way, for a given (low) temperature, it can be discharged faster than it can be recharged.

100thMonkey · Mar 7, 2013

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 · Mar 8, 2013

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. 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/

Puyallup Bill · Mar 8, 2013

montgom626 said:
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. 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.

montgom626 · Mar 8, 2013

Puyallup Bill said:
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.

I did not know that. Thanks for the clarification.

Todd Burch · Mar 8, 2013

Thanks for the responses. Curious about the finer details on a more molecular scale as to why this is.

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

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