Too little transparency in the storage market

Wed, 18/01/2017 - 16:28

The company Solarwatt, from Dresden, Germany, developed its storage system “MyReserve” completely on its own and makes the battery modules itself. In an S&WE interview at the technology centre “Solarwatt Innovation”, the centre’s head Andreas Gutsch explained his take on battery technology and its future perspectives. Here we have put together his arguments, background information and forecasts.

The current state of battery quality

What is quality? Among many relevant criteria, the cycling stability is one which is often considered. With lithium-ion technology a battery only emerges from the integration of cells and software, and the cycling stability is important especially at a cellular level. The fundamental rule says: you can’t make a good battery from a bad cell. The cycle stability is measured at KIT in the 1C/1C test. This means a repeated full discharge and recharging up to the rated capacity in periods of one hour each. When a cell only reaches 80 % of remaining capacity it is considered to be at the “end-of-life” stage, as the ageing after this progresses not linearly but at an accelerated speed. Normed tests at KIT have shown that there are massive differences in the cycle stability between different manufacturers and cell types. Many end consumers are completely unaware that the range here is from a few hundred cycles up to 6,000.

Dr. Andreas Gutsch studied chemical engineering at Karlsruhe University and after his doctorate he joined Degussa AG. Later he took on responsibility at Evonik for setting up new business fields. His entry in the field of lithium-ion technology goes back to the development of a ceramic water filtration membrane 14 years ago. Developed for drinking water preparation, this material proved to be a very good separator membrane for lithium-ion cells, and their series manufacture was the first step in the foundation of the company LiTec, which was later sold to Daimler. His next post was at the Karlsruhe Institute of Technology (KIT), where Gutsch worked on stationary storage systems. Since May 2016 he has lead the technology centre “Solarwatt Innovation” in Frechen, near Cologne, and is responsible for the complete technical development of “MyReserve”.

What is a good cell?

There are cells which can survive 6,000 full cycles in the test described above. This is good news initially, but the question is when such cells make sense. For its stationary storage “MyReserve”, Solarwatt has gone for a cell with approx. 4,000 cycles. The base calculation is a full cycle value of 250 per year for a solar electricity storage, which then gives an expected lifetime of approx. 15 years. This is the optimum from a cost-benefit perspective, as cells with even higher cycle stabilities are disproportionately expensive for a start. Additionally, if you look at periods even longer than this, then time-based “calendar ageing” plays a role, which occurs irrespective of the cycling undertaken. There are two decisive factors here; the cell temperature – the more extreme the temperature in either direction, the worse things are – and the state of charge. It is not good to leave a fully charged lithium-ion battery unused, as the voltage built up in the cells stresses the molecules.

The truth behind the specification sheet

The problem in the market for home storage is that there is too little transparency for customers. There are batteries where the specification sheet claims 4,000 full cycles, but the cells in the KIT test survive a lot fewer cycles. How can this be? Well, by using other testing methods which draw out the charging and discharging, for example, or give the cells a break between the two directions. This is indeed more in tune with real-life conditions, but cannot be documented because it takes too long to get valid results. The market is not yet transparent in this respect and we are unfortunately still lacking standard test conditions such as those for testing module power. The cycle value on specification sheets is determined according to very widely differing simulations and is not totally meaningful, at least when applied to home storage. Solarwatt uses a cell which can prove its 4,000 cycles in the KIT test. Under “drawn out” conditions such that the customer will experience, perhaps up to 6,000 cycles will be possible, but then you once again have the problem of the calendar ageing to consider.

From the car into the cellar?

A car cell designed for 500 full cycles would be fine in that application, as a suitable dimensioning could result in a battery lasting for 200,000 kilometres. Such a technology would be completely unsuitable for home storage, however. Car manufacturers which are “dressing warmly” are using cells which last 3,000 cycles and more in the KIT test. The very basic question of using a retired EV battery as home storage is a tough one. The dilemma is that the cells will degrade either through the cycling or through time itself. Batteries with a lower cycle value won’t face this question at all, and if batteries with a high cycling stability were built into cars, then due to/after 10+ years of use the calendar ageing would be too far advanced to allow for reliable use as a home storage anyway.

The role of internal resistance

The internal resistance of the cell is the second quality criterion, which initially determines the efficiency of the battery when charging and discharging, and thus also the total efficiency. The differences between the common cells on the market are very large and are related to the cycle stability. For example, the internal resistance of a smart phone cell is about 15 times as high as in cells suitable for home storage. A smart phone heats up while charging, while a good home storage must not do this because it would accelerate the calendar ageing, further increasing the internal resistance. This would be a self-strengthening process which would quickly kill the batteries.

Manufacturing the "MyReserve" battery module in Frechen (Photo: Solarwatt)

What makes things safe?

The safety risk for lithium-ion cells is an internal short-circuiting that leads to heat generation, which causes a failure of the separator and a cell fire which cannot be put out. This risk demands a conservative cell and battery concept. Unlike most other cells, the Solarwatt cell has a ceramic coating on the separator. As an internal contaminant filter it reduces ageing, but is primarily stable at high temperatures should the separator collapse – an internal short-circuiting is prevented. Providing a final barrier is an aluminium casing with a wall thickness of at least 13 mm. In the unlikely event of an internal short-circuit it ensures no flames get out – as also demanded by the safety guidelines for home storage. It is important to note in connection with this that the critical temperature which results in a “thermal runaway” differs according to the cell chemistry. No lithium-ion cell cell is “completely fireproof”, however. A control of the operating conditions is also naturally a part of the safety concept; overcharging must be impossible. The Solarwatt storage is switched off at a temperature of 50 °C, whereby each individual cell has its temperature monitored.

In the case of a fault a Solarwatt module switches itself off with an internal relay. This is a feature which should really become a standard for an important reason. With such a coded relay you can guarantee that the battery can only be operated in connection with the power electronics of the manufacturer, and exclusively under conditions which the manufacturer has defined. If there is no relay, then there is no way to guarantee a safe operation in all circumstances.

The development potential

A higher energy density at lower cost: this wish comes primarily from the automobile sector. Conventional ways of upping the energy density generally have a direct negative effect on safety. Even if a cell itself can provide more watt hours, this is eaten up by the battery containment required to make the system safe. With the current cells we are at the energy density limit and will not see any safe batteries which significantly beat what we have today. If things are pushed too hard then there is a danger of a series failure. Should that happen then it is all too late for the manufacturer. In connection with electric vehicles this means that the range is increased by designing the car around an optimally placed battery, and not trying to integrate the battery into a finished vehicle.