Pipcecil
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Usable Capacity and EPA efficiency

Wed Mar 19, 2014 6:58 am

I know EPA's range test are a little wonky (using information from the wall unit, assuming a "fill up" at 90% usage, and even averaging 80% and 100% on the 2013's).

But I have been asked a few times and couldn't answer this, and I searched on the leaf boards and still couldn't find a definete answer:

How much actual usable capacity is in the Leaf battery? Is it 22.32 kWh (based on the assumtion that Nissan reserves 7% - 5% top and 2% bottom - from the pack?)? I remember reading many times people stating ~21 kWh as well. Finally some people using Leaf Spy pulled 23+ kWh on new vehicles. I know technically capacity varies from battery to battery and there is some tolerance (i.e. you could be lucky and get a slighty higher capacity battery). But what is the number that everyone has settled on as the actual usable capacity?

The final part is to take that technical usable capacity and extrapolate a miles/kWh per vehicle by year. Using 2.9 us unfair since that accounts for a full 24 kWh usage and not actually capacity usage. It should be easy to figure out once I nail down the actual usable capacity:

2011/2012: 73/X
2013: 75/X
2014: 84/X

I know the calculation results don't necessarily give a true number - but its a metric I have been asked by other leaf owners before, and I was always at a loss since I don't feel comfortable quoting 2.9 kWh/mile, I wanted something more realistic.
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TonyWilliams
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Re: Usable Capacity and EPA efficiency

Wed Mar 19, 2014 1:24 pm

This stuff has been hashed over in such minutia so many times, I'm reluctant to respond.

The 22.48kWh (plus or minus some amount) that is read on the LEAF app is strictly what's "stored", not what is usable.

Assuming a 281 GID full charge multiplied by 80 wattHours per GID = 22.48kWh.

Approximately 480 wattHours, or 6 GID, is not usable at the "bottom" of the battery, which leaves 275 GID to burn.

So, of the 22kWh stored, only about 21.3kWh can be actually used. That means 281 - 6 GID = 275 GID stored for use, at about 97% efficient to withdraw that energy for use. With each GID worth 80 wattHours stored, multiplied by 97% efficient equals 77.5 wattHours per GID, therefore:

275 * 77.5 = 21.312 kWh usable. This happens to be almost exactly what a government agency measured!!!

All this assumes a new condition warm battery (70F - 80F). More usable energy can be available if the battery is hotter, at about 1% increase per 8F increase in battery temperature above 70F. Conversely, the battery loses 1% per 4F below 70F.

As the battery degrades, obviously you won't measure 281 (actually up to about 284 GID on 2013 and newer).

Just to be clear, this data is not what you would measure "off the wall". That number will depend on the efficiency of your charger (typically 73% efficient at 120 volts and 84% at 240 volts). That means that to put 22kWh stored in the battery takes 26kWh - 30kWh from "the wall".

As to range, quite simply 4 miles per kWh driving down the highway at about 65mph will generate 4 * 21.3 = 85.2 miles range, plus or minus a few. Some really aggressive city driving can yield 3 miles per kWh, or 64 miles of range. With the heater on, it can drop to 2.5 miles per kWh, or 53 miles of range. As the battery degrades, your mileage with a 30% degraded battery would simply be that much less to the above mileages.

On the LEAF Spy app, there is a setting to adjust the GID value between 75 and 80 wattHour units, at 0.5 wattHour increments. I recommend setting that on 77.5. Set the reserve at 6 GIDs (I think it work with GIDs to consider "reserve").

Hope this helps.

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TonyWilliams
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Re: Usable Capacity and EPA efficiency

Thu Mar 20, 2014 4:00 pm

Energy from the wall from dead to 100%: 25.414 kWh
Energy from the onboard charger to battery: 22.031 kWh (86.6% charger efficiency)
Energy from the battery during discharge: 21.381 kWh (our 21kWh useable at 70F)

http://www1.eere.energy.gov/vehiclesand ... 2012_o.pdf

http://www1.eere.energy.gov/vehiclesand ... 2012_p.pdf

**********

New 70F/20C fully charged battery:

-------------------KWH-------Gid------SOC%
Rated capacity: --24 ------- 300 ---- 100
Max possible. : ---24 ------ 300 ----- 100
Stored energy: --22.5 ----- 281 ----- 95 (4.1v per cell, 393.5 volts)
Usable energy: --21.3 ----- 281 ----- 95 (4.1v per cell, 393.5 volts)
Depleted cutoff: --0.5 ------ 5 -------- 2 (zero fuel bars remaining, 300v)


************


The official battery degradation capacity value, per the Nissan service manual for LEAF is as follows. It's important to note that Nissan removed all reference to this data in the April 2011 update of the service manual.

12 of 12 bars - 100% to 85%
11 of 12 bars - 84.99% to 78.75%
10 of 12 bars - 78.74% to 72.50%
9 of 12 bars - 72.49% to 66.25%
8 of 12 bars - 60% to 66.24%


*******************


300 Gid * 80 = 24kWh

Of that 24kWh capacity, 22.48kWh is pumped into the battery, and about 0.48kWh of the 22.48kWh is held in the battery at the bottom of the SOC, leaving 22kWh pumped in the battery that is "available" to use. Unfortunately, there are losses not accounted for with the 80 kWh calculation when you pull the energy out.

The usable value is closer to 75 per Gid, not 80. Actually, for that brand new battery with 21.3kWh usable at room temperature, it is about 77.45 per Gid.

A fully charged battery at 281 Gid, minus the 6-ish Gid remaining at Turtle leaves 275 Gid used multiplied by 77.45 per Gid = 21.3kWh (actually measured in a government test at room temperature). So, I just want to be clear that there are temperature related adjustments to capacity. We see Nissan's programming for these adjustments in the winter as the Gid count actually goes UP, even though the battery capacity goes down with cold.


*********************

The 80 value checks out perfectly against the government battery measurements. At 22.031kWh from the charger to the battery, plus the absolute likely minimum of 0.300kWh to 0.400kWh still in the battery, the total divided by 281 Gid equals 79.47 to 79.83 wattHours each.

Of course, we don't know that the government test actually showed 281 Gid. Just 280 Gid would put the value at 79.75 to 80.11. So, I'm glad we have non-Nissan lab results to back up our data.

The 21.381kWh actually recovered means there is a direct battery energy recovery loss of 21.381 / 22.031, or 97.05% efficient.

If we apply that 97.05% in this example to 80 wattHours, it becomes 77.64 per Gid for "energy from the battery during discharge".

Since they did this all in a lab at room temperature, it's perfect data to apply temperature correction factors to. But, the above example is why I chose 77.5 per Gid as the default. GIDs continue to be the gold standard for determining the only thing the end user cares about; usable capacity for range.

That usable battery capacity, with temperature correction and compensated for UNusable energy will generally align with the dash miles per kWh economy meter to get ultimate range.

The displayed kWh on the LEAF Spy app should reflect this data. Conversely, we get exactly the same ultimate result with continuing to call a Gid = 80 with a 97.05% correction factor.

Now, the app would show a 281 Gid battery at 70F temperature to have:


(((281 - 6 UNusable) * 80) * .9705 energy recovery factor) * 0% temp correction = 21.351kWh usable


Isn't is great how all that works out?

When the car is being driven at 4.1 miles per kWh, the DTE value in the app will show:


4.1 * 21.351 = 87.5 miles, or just about EXACTLY what it will actually do.

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