Hi all,
I'm embarking on a real-world educational exercise in finding out how long a Tesla Model S traction battery might last over its lifetime.
I'm building an automated charger/discharger for around 16 x 18650 cells. The aim is to constantly charge then discharge the individual cells under varying conditions, and plot battery capacity as a function of charge/discharge cycles.
Extrapolating from that test, I hope to come up with a figure of how many miles a 60kwh pack may last. While I appreciate there are warranties, and battery replacement schemes, and other market (and marketing) influences, I would like to concentrate on the physical batteries themselves.
I understand that the Model S, 60KWH pack uses 7104 individual Panasonic NCR18650A cells, wired in a 74 parallel, 96 series 74P96S configuration. Please correct me if I'm wrong, and give me a reliable source. I used wikipedia (Tesla Model S - Wikipedia, the free encyclopedia)
I'd like to start with testing 3 different scenarios. All of the voltage and current figures below are per individual cell, not per battery pack.
1. General base line test:
- Charge at 0.85A to 4.1V
- Discharge at 1A to 3V
- Repeat the above till the battery dies
2. Performance/Lead-foot test:
- Charge at 4.69A to 4V (this symbolises a 120kw supercharger)
- Rest 5 minutes
- Discharge at 8.8A for 6 seconds (this symbolises accelerating to 97km/h at max motor power of 225kw)
- Discharge at 0.564A for 3 minutes (this symbolises cruising)
- repeat the last two heavy acceleration and cruising steps till batt = 3.3V then go back to step 1- charge via supercharger
- Every 20 cycles, charge at 0.85A to 4.1V, discharge to 3V and record capacity
3. General driving test:
- your feedback here!
As a start, I would guess many people would charge via a 15A 240V outlet? Is this a fair assumption? Are there any stats on how many people use what charge current?
Then I would discharge at between 0.5A and brief periods of 5A bursts symbolising city stop/start driving.
I would discharge to say 3.3V per cell (so roughly 30% battery capacity), then charge to 4V (about 80%), then repeat the whole process.
The Panasonic datasheet for the NCR18650A battery only goes up to 500 cycles. This graph may be OK for a laptop battery pack designer, but not for an electric vehicle designer.
View attachment 66717
The graph above shows what happens in a lifetime of a cell that is charged to 4.2V and discharged to 2.5V, at 3A. This is very harsh on the battery, and not what it would see in a vehicle application. That's why my proposed tests are more focused on what a normal EV cell would experience.
As you can see, I haven't quite worked out what I want to do for the "general driving" test. What do you think would be a realistic test?
Naturally this isn't designed to be a fully scientific test, just an indicative test. Think Mythbusters logic. It will give us some idea of what we might expect, not a definitive answer. Only two batteries per charge scheme will be tested. If there is a large discrepancy between the two cells, I'll have to throw out the data for that test. The test will take some months to do continuously, day and night, and is again therefore not totally indicative of what a normal battery pack would go through.
If the test takes 6 months, technology will move on in the mean time, and better batteries will no doubt be popping up sooner than this. Even now, Panasonic has released the NCR18650B, which has a greater capacity, though it's anyone's guess as to what cell Tesla will choose for their next battery pack design. If funds permit, I'll get tests on the newer NCR18650B in anticipation of Tesla adopting this battery.
I'll try to get the ongoing results published to a live server so that anyone interested can have a look at how it's going. The test might prove nothing, but we might also find something interesting in the data.
Let me know your thoughts!
Regards, Phil
I'm embarking on a real-world educational exercise in finding out how long a Tesla Model S traction battery might last over its lifetime.
I'm building an automated charger/discharger for around 16 x 18650 cells. The aim is to constantly charge then discharge the individual cells under varying conditions, and plot battery capacity as a function of charge/discharge cycles.
Extrapolating from that test, I hope to come up with a figure of how many miles a 60kwh pack may last. While I appreciate there are warranties, and battery replacement schemes, and other market (and marketing) influences, I would like to concentrate on the physical batteries themselves.
I understand that the Model S, 60KWH pack uses 7104 individual Panasonic NCR18650A cells, wired in a 74 parallel, 96 series 74P96S configuration. Please correct me if I'm wrong, and give me a reliable source. I used wikipedia (Tesla Model S - Wikipedia, the free encyclopedia)
I'd like to start with testing 3 different scenarios. All of the voltage and current figures below are per individual cell, not per battery pack.
1. General base line test:
- Charge at 0.85A to 4.1V
- Discharge at 1A to 3V
- Repeat the above till the battery dies
2. Performance/Lead-foot test:
- Charge at 4.69A to 4V (this symbolises a 120kw supercharger)
- Rest 5 minutes
- Discharge at 8.8A for 6 seconds (this symbolises accelerating to 97km/h at max motor power of 225kw)
- Discharge at 0.564A for 3 minutes (this symbolises cruising)
- repeat the last two heavy acceleration and cruising steps till batt = 3.3V then go back to step 1- charge via supercharger
- Every 20 cycles, charge at 0.85A to 4.1V, discharge to 3V and record capacity
3. General driving test:
- your feedback here!
As a start, I would guess many people would charge via a 15A 240V outlet? Is this a fair assumption? Are there any stats on how many people use what charge current?
Then I would discharge at between 0.5A and brief periods of 5A bursts symbolising city stop/start driving.
I would discharge to say 3.3V per cell (so roughly 30% battery capacity), then charge to 4V (about 80%), then repeat the whole process.
The Panasonic datasheet for the NCR18650A battery only goes up to 500 cycles. This graph may be OK for a laptop battery pack designer, but not for an electric vehicle designer.
View attachment 66717
The graph above shows what happens in a lifetime of a cell that is charged to 4.2V and discharged to 2.5V, at 3A. This is very harsh on the battery, and not what it would see in a vehicle application. That's why my proposed tests are more focused on what a normal EV cell would experience.
As you can see, I haven't quite worked out what I want to do for the "general driving" test. What do you think would be a realistic test?
Naturally this isn't designed to be a fully scientific test, just an indicative test. Think Mythbusters logic. It will give us some idea of what we might expect, not a definitive answer. Only two batteries per charge scheme will be tested. If there is a large discrepancy between the two cells, I'll have to throw out the data for that test. The test will take some months to do continuously, day and night, and is again therefore not totally indicative of what a normal battery pack would go through.
If the test takes 6 months, technology will move on in the mean time, and better batteries will no doubt be popping up sooner than this. Even now, Panasonic has released the NCR18650B, which has a greater capacity, though it's anyone's guess as to what cell Tesla will choose for their next battery pack design. If funds permit, I'll get tests on the newer NCR18650B in anticipation of Tesla adopting this battery.
I'll try to get the ongoing results published to a live server so that anyone interested can have a look at how it's going. The test might prove nothing, but we might also find something interesting in the data.
Let me know your thoughts!
Regards, Phil