Sorry, but I'm definitely with Greg on this one guys. Neither 100% nor 10% is great for best longevity, but low end SOC is way more harmful to the cells than high end is. Small differences in cells individual capacity become much magnified at lower SOC. For long storage, somewhere around 40-60% is optimal. Plus also it’s high risk that car will suddenly stop if you go too low, so why do it if it’s avoidable? This sudden stoppage can possibly even happen before you get to zero on the GOM! Further, the available propulsion power becomes noticeably reduced by the BMS when you get below ~15-20% (Especially so if it’s cold = cells high internal resistance) It does this to try and minimise the accelerated battery degradation that is actually occurring in that lower SOC state. I’m sure sooner or later we will all inadvertently go a bit too low, just don’t do it too often, is my advice.
Also bear in mind that when Eniro displays 100%, the battery is actually only at 95% in reality. (This is why regen braking still works pretty well even at 100% displayed).
Please see this interesting excerpt taken from this useful resource.
Peter.......
State of Charge
The voltage restrictions necessary to avoid the problems outlined above can be translated into recommendations for the operating range of the State of Charge of the battery shown in the following diagram.
Operating outside of these limits will adversely affect the life of the battery. See more about
State of Charge and how to measure it.
Heat is a major battery killer, either excess of it or lack of it, and Lithium secondary cells need careful temperature control.
- Low temperature operation
- Chemical reaction rates decrease in line with temperature. (Arrhenius Law) The effect of reducing the operating temperature is to reduce rate at which the active chemicals in the cell are transformed. This translates to a reduction in the current carrying capacity of the cell both for charging and discharging. In other words its power handling capacity is reduced. Details of this process are given in the section on Charging Rates
Futhermore, at low temperatures, the reduced reaction rate (and perhaps contraction of the electrode materials) slows down, and makes makes more difficult, the insertion of the Lithium ions into the intercallation spaces. As with over-voltage operation, when the electrodes can not accomodate the current flow, the result is reduced power and Lithium plating of the anode with irreversible capacity loss.
- High temperature operation
- Operating at high temperatures brings on a different set of problems which can result in the destruction of the cell. In this case, the Arrhenius effect helps to get higher power out of the cell by increasing the reaction rate, but higher currents give rise to higher I2R heat dissipation and thus even higher temperatures. This can be the start of positive temperature feedback and unless heat is removed faster than it is generated the result will be thermal runaway.
- Thermal runaway
- Several stages are involved in the build up to thermal runaway and each one results in progressively more permanenet damage to the cell.
- The first stage is the breakdown of the thin passivating SEI layer on the anode, due to overheating or physical penetration. The initial overheating may be caused by excessive currents, overcharging or high external ambient temperature.The breakdown of the SEI layer starts at the relatively low temperature of 80ºC and once this layer is breached the electrolyte reacts with the carbon anode just as it did during the formation process but at a higher, uncontrolled, temperature. This is an exothermal reaction which drives the temperature up still further.
- (Lithium Titanate anodes do not depend on an SEI layer and hence can be used at higher rates.)
- As the temperature builds up, heat from anode reaction causes the breakdown of the organic solvents used in the electrolyte releasing flammable hydrocarbon gases (Ethane, Methane and others) but no Oxygen. This typically starts at 110 ºC but with some electrolytes it can be as as low as 70ºC. The gas generation due to the breakdown of the electrolyte causes pressure to build up inside the cell. Although the temperature increases to beyond the flashpoint of the gases released by the electrolyte, the gases do not burn because there is no free Oxygen in the cell to sustain a fire.
- The cells are normally fitted with a safety vent which allows the controlled release of the gases to relieve the internal pressure in the cell avoiding the possibility of an uncontrolled rupture of the cell - otherwise known as an explosion, or more euphemistically, "rapid disassembly" of the cell. Once the hot gases are released to the atmosphere they can of course burn in the air.
- At around 135 ºC the polymer separator melts, allowing the short circuits between the electrodes.
- Eventually heat from the electrolyte breakdown causes breakdown of the metal oxide cathode material releasing Oxygen which enables burning of both the electrolyte and the gases inside the cell.
- The breakdown of the cathode is also highly exothermic sending the temperature and pressure even higher. The cathode breakdown starts at around 200 ºC for Lithium Cobalt Oxide cells but at higher temperatures for other cathode chemistries.
By this time the pressure is also extremely high and it's time to run for the door.
- Law suits will follow.
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