Why Volt requires active thermal management, but not Leaf

Leaf has a 80KW motor that is supplied by the 24 kwh battery (likely more).

That equates to slightly above 3C at max power.

Volt has a 111KW traction motor + a 55KW moto/generator. It has a battery of 16 kwh capacity.

Just for the traction motor, at max power, the battery needs to pump out 8C. No wonder they need active thermal cooling. Ofcourse the chemistry is a little different too ...

BTW, Fisker Karma has 2 motors with 300 KW power and a 20kwh battery. That would be 15C and thus unlikely. In EV mode Karma is limited to 90 mph but can go to 125 mph in EREV mode.

So....

Let's say you live in Phoenix. On a typical July afternoon you are going to drive around the 'burbs' for an hour in a Volt/Leaf.

What would be the typical/max battery temps you'd see in each car?

temp should be outside temp + cooled by inside AC.

You can probably work out the temperature from the average load of the motor.. for the Volt it will do 35 miles from about 10.5kwh, the Leaf about 75 miles from 25kwh.. about the same energy consumption of 3-3.3 miles per kwh but the Volt will stop using its pack much quicker (and heating up).. in the end both cars will generate about the same amount of heat but the Leaf has more thermal mass to dissipate that temperature. They both use a similar cell chemistry.. My guess that without active cooling both packs will end up at the same temperature.. and probably not that high since neither pack will be abused and lithum-ion cells used under these conditions are about 99% efficient. Thus 1% of the stored energy will be converted to heat.

So assuming the Leafs pack is 270kg, has a specific heat similar to water, and is 99% efficient while being discharged under a conservative SOC, and assuming the pack is insulated so that the heat created will not escape then the Leaf's pack should increase about 0.8° C during the time it takes to discharge.. note that losses increase rapidly once you go past 80% soc. Heat gains from the hot asphalt pavement will probably be higher.

Just curious - has anyone reached under the seat and touched their batteries after a long commute? How much insulation or space is between the battery and where you can touch? Can you tell whether or not the batteries are heating up? Any correlation between your battery temperature gauge and what you can feel with your hand?

Yes, I was thinking the duration for the Volt's battery discharge duration will be less than 1/2 that of the Leaf. But then the ICE fires up and all that heat will have to be shielded away from the battery. And then there's the "mini-cycling" of the battery during CS operation which would cause some battery heating as well.


It's going to be fun watching the air cooled vs liquid cooled battery battles as the technology develops.

I guess you could say there has been a similar battle in motorcycle engines. Water cooled eventually won out ,, .. but then again, Honda still puts some fine air cooled engines on some of their motorcycles.

LakeLeaf said:

...... reached under the seat and ... touched their batteries .....

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Aren't those the lyrics to a Van Halen song?

(maybe that's the "new" Van Halen)

Herm said:

You can probably work out the temperature from the average load of the motor.. for the Volt it will do 35 miles from about 10.5kwh, the Leaf about 75 miles from 25kwh.. about the same energy consumption of 3-3.3 miles per kwh but the Volt will stop using its pack much quicker (and heating up).. in the end both cars will generate about the same amount of heat but the Leaf has more thermal mass to dissipate that temperature.

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You need to check the amount of current out of a cell and how much that would heat it given the internal resistance, which may be different, since the chemistries are somewhat different - and it would also change a bit depending on current drawn.

If we just take the current, 3C for Leaf & 8C for Volt, means a Volt cell will have nearly 200% more heating than Leaf.

So, if Leaf battery goes from 100 Deg to 120 Deg, Volt's would go to 160 Deg. That is why they need liquid cooling.

evnow said:

You need to check the amount of current out of a cell and how much that would heat it given the internal resistance, which may be different, since the chemistries are somewhat different - and it would also change a bit depending on current drawn.

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The internal resistance is most definitely different - the specificiations for batteries for EV, PHEV and hybrid use are drastically different each requiring vastly different maximum charge/discharge rates on a cellular level as you imply.

The 24 kWh pack in the Leaf only has to sustain 90 kW or 3.75C discharge rates. The 16 kWh Volt pack has to sustain 120 kW or 7.5C discharge rates. The Prius has about 1.5 kWh battery pack and has to sustain 30 kW or 15C discharge rates.

If the Volt used batteries with the same internal resistance of the Leaf it's EPA tested efficiency in EV mode would be significantly different than the Leaf, but it's not despite it's heavier curb weight.

Ya know, after 1000 miles of driving I have never seen my batt temp reading go up or down at all. Maybe mine is broken or something.

TRONZ said:

Ya know, after 1000 miles of driving I have never seen my batt temp reading go up or down at all. Maybe mine is broken or something.

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The gauge is fine - there will be no significant heat.

Sorry for those wanting there to be heat.

drees said:

The internal resistance is most definitely different - the specificiations for batteries for EV, PHEV and hybrid use are drastically different each requiring vastly different maximum charge/discharge rates on a cellular level as you imply.

The 24 kWh pack in the Leaf only has to sustain 90 kW or 3.75C discharge rates. The 16 kWh Volt pack has to sustain 120 kW or 7.5C discharge rates. The Prius has about 1.5 kWh battery pack and has to sustain 30 kW or 15C discharge rates.

If the Volt used batteries with the same internal resistance of the Leaf it's EPA tested efficiency in EV mode would be significantly different than the Leaf, but it's not despite it's heavier curb weight.

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Not really.

Just because Volt or Prius "requires" better batteries doesn't mean they will magically get that. LG batteries are a little older technology than Leaf's and definitely we don't have any reason to think they have better internal resistance. Volt will have to do something else about the heat - like active thermal management - which unfortunately increases the weight, cost and bulk of the batteries.

If the energy loss because of int resistance is small - the overall efficiency - will not get affected much. But the temperatures will. That is the point of this thread.

evnow said:

That is the point of this thread.

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I though the point of the thread was to discuss possible reasons that GM decided they needed thermal management.

What makes you think that the LG batteries are "older" technology than the AESC batteries in the Leaf? As far as I can tell the Leaf technology is also fairly "old" as AESC has been producing similar batteries for some years now. One might call the batteries "different" but not necessarily "older". After all, tried and true NiMH batteries have very low internal resistance and can handle very high discharge rates without issue. Those batteries might be "older" technology, but they do quite well with very minimal thermal management under high loads. Most of the thermal management requirements of NiMH cells are because they don't like cold/hot temps, not because they generate a lot of heat. Perhaps the LG batteries are simply less tolerant of extreme heat/cold than the AESC batteries due to their chemistry.

evnow said:

If we just take the current, 3C for Leaf & 8C for Volt, means a Volt cell will have nearly 200% more heating than Leaf.

So, if Leaf battery goes from 100 Deg to 120 Deg, Volt's would go to 160 Deg. That is why they need liquid cooling.

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Perhaps under racetrack conditions, but during normal driving both cars use about the same amount of power.. and not much is lost to heat since lithium-ion is very efficient under those conditions.. about 10 times more efficient than nimh chemistries. This is why the new Sonata Hybrid using LG lithium-ion is so revolutionary.

I suspect AESC engineered their cells not to need liquid cooling, and as a bonus to be capable of fast recharging. In the long run it may turn out to have been the best solution, I get very nervous with liquid cooling inside a battery pack.

I think we're still focusing on the wrong end of the problem.

Both companies use a LiMn variant, both are laminated and have similar physical sizes. There's no reason for either of them to be liquid cooled if we only look at the physical capabilities of the cells.

The most significant difference is the HYBRID aspect. By law, auto manufacturers are on the hook for emissions performance for at least 10 years. We know that battery performance will change with temperature - and that means that as internal resistance and capacity change so will the emissions profile. And the emissions profile needs to stay above the limit for GM to sell the Volt.

In addition, as already pointed out, hybrid power use in CS mode is more dramatic than for an EV (or a PHEV in CD mode).

So GM has both a regulatory and operational need for thermal management. The Leaf has neither.

Probably not, it just has a pretty wide range between bars. If you look elsewhere you'll see a graph of what each bar represents and it takes a pretty big change to move even just one bar.

TRONZ said:

Ya know, after 1000 miles of driving I have never seen my batt temp reading go up or down at all. Maybe mine is broken or something.

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guess i wont know a lot until we get to some warm weather (looks out at the snow piling up...7 inches and counting) but i have never seen batt temps get past halfway and frequently see it 2 notches below half.

we dont usually get the real hot weather, but there has been too many all time records being broken for me to be comfortable with saying what our typical weather is like. for centuries, the high temp in Olympia was 100º, hit twice, once in the 50's and again in 2006.

since then, its hit 100 five times including our 2009 stretch when it was over 90º 11 of 12 days including 8 or 9 in a row. previous record was 3 days in a row. we hit over 100 4 times including our new all time high of 104.5 but i digress.

back to topic; i see thermal management as insurance against an inefficient system. batteries are not there to generate heat. they are their to store power. so an efficient system would not need it.