Dangers include (but are not limited to):high current and fire risk; explosive or highly flammable battery reagents; toxic battery reagents.
The positive electrode is a lithium metal oxide (eg lithium cobalt oxide, lithium chromium oxide), and the negative electrode is carbon activated to adsorb metallic lithium. The electrolyte is normally an organic solvent containing a lithium salt.
Positive: e- + Li+ + CoOx -> LiCoOx
Negative: Li -> e- + Li+
Mass of reagents to produce 1 mole of electrons: not meaningful due to variable levels of carbon adsorbtion.
Voltage: 3.6v nominal; 2.8v discharged; 4.3v charged
Maximum discharge: ~5C
Cycle Life: 400-500
Oh boy. Dangers include but not limited to: massive energy can start fires, cause burns or cause the car to become uncontrollable; lithium and lithium compounds in the battery are poisonous, and must be disposed of in ways that are good to the environment; lithium is a very reactive metal that will burn or explode on exposure to air or water, including the water in human flesh; haloated hydrocarbon electrolyte is both poisonous and flammable, and deadly by fume inhalation; overcharging or over-discharging these batteries makes them catch fire or explode; these batteries will spill and probably catch fire if they are damaged, say by an accident; the individual cells may be heavy enough to require care in handling to prevent injury.
Now the good news. Lithium ion cells are smaller, lighter and discharge better than lead acid. They're also expensive. They require lots of care to make sure that they are charged correctly and that they do not get short circuited - probably a fuse and an overvoltage protector per battery, as a minimum.
These figures vary a lot - these ones are from PowerStream's website. They're originally from a Chinese company called Thunder Sky. These things must be worked out from the manufacturer of your cell, because they vary a lot, and more importantly if you get them wrong you'll start a fire!
Lithium Ion Cells are predictable in terms of discharge - the discharge state is reflected in the open circuit voltage. Here is a table of typical values:-
| O/C Voltage | SOC (%) | DOD (%) |
|---|---|---|
| 4.2 | 100 | 0 |
| 4.1 | 90 | 10 |
| 4.0 | 80 | 20 |
| 3.92 | 70 | 30 |
| 3.84 | 60 | 40 |
| 3.75 | 50 | 50 |
| 3.65 | 60 | 40 |
| 3.55 | 70 | 30 |
| 3.5 | 80 | 20 |
| 3.4 | 90 | 10 |
| 3.2 | 100 | 0 |
More information about the Thunder Sky battery is available from the excellent Metric Mind website. In particular, they have some safety checks (including shooting the battery with what looks suspiciously like an AK-47, judging from calibre and velocity) and also some life-cycle predictions.
The life cycle predictions can be used to estimate battery costs. Here is the same table as the Optimas, but with Thunder Sky data:-
| Discharge depth | Number of cycles | discharge storage product |
|---|---|---|
| 10% | 3000 | 300C |
| 20% | 2500 | 500C |
| 30% | 2000 | 600C |
| 50% | 1500 | 750C |
| 80% | 500 | 400C |
Lastly a series of discharge curves may be used to estimate Peukert's Number. Here is a graph, again pinched from Metric Mind:-
By taking the 0.1C curve and the 1C curve, the Peukert Number can be calculated. It is 1.016, which is pretty small.
The maximum continuous discharge rate for these cells is recommended as the C/3 rate - which is to say 67A. That doesn't provide much power.
The maximum peak power rate is recommended by the manufacturer as 160A, although the curve shows that a discharge of 300A is possible. At this current there is a considerable drop in battery voltage.
For performance designs, it might be a good idea to use either ultracapacitors, or to use a small lead-acid pack to provide acceleration current, and re-charge this from the LiIon pack or from regenerative braking.
The way to charge lithium cells is with a constant voltage, that may need to be current limited. For the Thunder Sky lithiums, they recommend limiting to a voltage of 4.2v per cell, and limiting to a current of C/3. Typically 80% of charge is achieved before voltage limiting starts, and the remaining charge takes a little over 2 hours. Charging is complete when the current becomes less than 0.01C, and the voltage may be retained on the cells while the charger is connected.
It seems unlikely that any other charging system but regenerative braking is going to be able to deliver this kind of power. So the power is going to be limited by source, not by the battery.
Regenerative charging should be carried out at the same rate as the current limit for charging - that is, at a current of 0.3C.
| Manufacturer | Part | Chemistry | Voltage | Capacity | Weight(kg) | Dimensions(mm) | Peak Power | Continuous Power | Cost | Cycles | Peukert Number | Energy Weight Wh/kg |
Wear Cost (per kWh per cycle) |
Notes |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Thunder Sky | LP9393A | Lithium Ion | 2.6-4.3 (nom. 3.6) | 160Ah @ 3.6v 576Wh |
5.5kg | 145x62x230 | 720W (900W) | 120W | $250.00 | 500@80% 1500@50% |
1.016 | 130.9 | $0.87 $0.46 |
Experimental |
| SAFT | VL_2P3S | Li-ion | 10.8 | 84Ah@3hr 907.2Wh |
8kg | ?x?x? | ??? | ??? | ~€2200 | 1500 | ??? | 113.4 | ~€1.62 | about 30p a mile! |
| Valence Technology | Saphion U-Charge | Lithium Ion | 12v nom. | 45Ah @ 12v 461Wh |
7kg | 197x132x186(U1) | 1200W | 1200W | ??? | 2000@80% |
1.036 | 66 | ??? | Lead Acid Replacement |
Although lithium ion batteries are often held up as the great white hope of electric vehicle technology, they are not as practical as might be hoped. Not only do they have a low cycle life, and so a high cost over the lifetime of the battery, but they also have a low shelf life, lasting only five or so years from factory to dustbin, regardless of use. A quick calculation shows that a 200 mile range battery that does 500 cycles will have to do 100,000 miles in five years to properly use the battery. For commercial vehicles - taxis, buses, and trucks - this might make sense, but for private vehicles it seems unlikely that the current lithium technology will ever make sense.
Nevertheless several lithium-ion based electric vehicles exist, and their performance is impressive: the T-Zero uses lithium batteries, as does the Venturi Fetish.
This page is part of an Open Source Electric Car Project, and is written and maintained by Simon. At this stage these pages are constantly under revision. Thoughts and comments are welcome.