- Battery manufacturers express battery lifetime in terms of the number of cycles in a specific ambient temperature and battery operation regime (i.e., charge and discharge profile). For instance, LG Chem RESU is sold by this lifecycle tag: 60% SOH after >6,000 cycles @ 90% DOD. It means that the battery will reach to 60% of its original capacity after 6,000 cycles (one complete charge from 10% to 100% and then discharge from 100% to 10%). Well, the battery does not experience such a gentle operation in real-world. Basically, it undergoes interweaved small charge and discharge in different orders with very different high/low power. Although it significantly affects battery lifetime, no information can be found from battery documents.
- Battery constantly loses its initial capacity. If you buy a diesel generator of 10kW/10 kWh power and energy capacity, it will deliver the same amount (or very close) until the end of its lifetime. Energy and power capabilities of a battery, however, will degrade through the battery lifetime. For instance, a 10kW/10kWh battery after a year might only be able to deliver 9kW/9.5kWh power and energy although it has not yet reached its end of life. This fact is not emphasised in the contracts nor battery datasheets. Additionally, battery manufacturers won’t let you know how the battery will degrade through its lifetime. Is it going to degrade exponentially (it means more degradation when it reaches its end of life) or linearly (it means equal amount every year or month)? Well, you do not get a clue from battery datasheet or other documents which come with the battery.
- Tesla Powerwall and LG Chem RESU comes with a warranty term where battery capacity degradation rate is guaranteed to a certain kWh or MWh throughput. The kWh or MWh throughput refers to accumulated energy which either has been stored/absorbed in/from the battery. This measure is slightly better than the number of cycles because it is not limited to a specific operational condition (such as those in item 1). It gets very interesting and very real here. If you do the math with the kWh values to calculate the number of cycles, you get only 3,200 cycles in standard condition for the two popular models. Remember that the LG Chem RESU is sold with >6,000 cycles in nominal condition. Yeah, they are not stupid. They know their products very well. So, don’t pay attention to the number of cycles as it does not deliver any useful information. Check the kWh or MWh throughput guaranteed.
- We see battery energy degradation information, but you barely see anything about battery power degradation. Yes, the battery degradation is not only limited to its energy capacity (specified in kWh). If you can discharge your battery at 10kW today, you would not be able to do the same in a year or two from now. It negatively affects the amount of energy stored in the battery, which consequently reduces battery cost-effectiveness in long-run. So, it would be smart if consumers ask about power capability and its degradation when purchasing a battery solution.
- Battery degradation is not only limited to the way it is operated. Actually, the battery loses its energy and power capabilities just by sitting somewhere. This is called calendar or storage ageing. Ambient temperature and battery state of charge (i.e., the amount of energy stored in the battery) can significantly impact the calendar degradation rate. Although battery storage ageing is very slow comparatively, it is not negligible. This ageing process is more prominent when the battery is under operation in a real environment. Remember that your battery will end up sitting and doing nothing most of the times. So, it is important to know how that affects your battery. Unfortunately, the battery manufacturers don’t tell you anything about it. Be cautious about where you install your battery.
- Various Li-ion battery technologies show a round-trip efficiency of about 90% or more. This means that if you feed 10 kWh energy to the battery, you can get 9 kWh at the maximum when you needed. This is because of self-discharge, internal power depletion because of resistances and so on. The round-trip efficiency is typically given by the battery manufacturers. However, this value changes over the battery lifetime. Also, battery efficiency depends on the operational and environmental conditions. Again, there is no information given by the manufacturers.
- Batteries are said to be operational from -10 to 50 degree Celcius. While this is true, there is a hidden fact in this message. Although it is safe to operate the battery in this range of temperature, its performance is significantly altered by the storage and operating temperature. Basically, Li-Ion batteries have the highest performance while operating within 25-30 degree Celcius. Its efficiency, as well as energy/power degradation rates, are significantly degraded by the excessive high or low temperatures. No information is given to realise these impacts.
This article does not intend to scare people from purchasing and using battery storage technologies. Quite differently, it tries to educate people to make a smart decision when it comes to such an expensive investment. Although it is true that battery looks like a black-box, tremendous research efforts in the past two decades shed lights to many unknown aspects of different battery technologies. That’s why many battery management systems are developed to utilise the driven knowledge to operate battery in the best possible (i.e., economic) way. A good battery management technology can make a huge difference in battery operation and its life cycle.