THE TESTING OF A BATTERY CELL

In the picture you see a nice representation of the construction of a battery cell. You can clearly see the anode, the separator and the cathode. Also the insulation material, the housing and the positive and negative terminals.

Which tests can we perform on such a cell?

  • The battery cell insulation test
  • Determining the impedance of the cell
  • Obviously determining the capacity of the cell
  • The number of charge/discharge cycles
  • The overload test

 

Let’s start with the cell insulation test.
Here we determine whether or not contamination has occurred in the production of the battery cell. If this does happen, we are faced with leakage currents in the cell, and in some cases even through the insulation material.

This is one of the causes of a possible self-ignition of a Li-ion cell. To prevent this, it is wise to test each battery cell for insulation, leakage current, charge current and detection of partial discharge/flashover. This is very easy to do with for example the Chroma 11210 battery cell insulation tester.

This test ensures a safer battery and at the same time checks a number of parameters that determine whether the battery cell has been manufactured properly.

THE BATTERY CELL IMPEDANCE TEST

Inherently to the construction of the battery cell we have to deal with a certain resistance and parallel capacity of the used materials. Of course we want to keep this resistance as low as possible because of the internal heating of the battery. And in the case of dynamic loads, capacitance plays a role alongside resistance. See below for a drawing of how we can represent a battery cell electronically.

The internal resistance can be determined quite simply by means of a step response measurement in the load of the battery. This load variation gives an initial voltage variation and if we divide this voltage variation by the current we have the internal resistance. The internal resistance measurement for batteries is also laid down in various standards such as DIN EN 61951 and DIN EN 61960. This method uses two different load currents and calculates the Ri from there. There are a number of manufacturers of electronic DC loads who have integrated this as a standard measurement in the DC load, including for example Hoëcherl & Hackl with the H&H PLI series.

With dynamic loads, the internal capacity is also important. This measurement is performed at a specific frequency (usually 1kHz) or is performed over a wide range of frequencies to better determine the dynamic behaviour of the battery. With a single battery cell, the impedance is generally very low and it is very important to use specific instrumentation with measurement results in the micro and milli ohms. For possible solutions see for example the Itech IT-5100 series, the PSM3750 from Newtons 4th with BATT470m option or the Hioki BT4560.

For a detailed presentation of the battery impedance measurement we would like to refer to a presentation at the Energy Storage Event 2020.

DETERMINING THE CAPACITY OF THE BATTERY CELL

Determining the capacity of a battery is actually dependent on a large number of variables:

  • Maximum charge voltage
  • Maximum discharge voltage
  • Temperature of the battery
  • Desired number of charge/discharge cycles

 

And some of these variables are contradictory.
You want the highest possible charge voltage and the lowest possible discharge voltage to get the maximum capacity from the battery. However, this does come at the expense of lifespan, i.e. the number of charge/discharge cycles.

Of course, we want the battery to have the longest possible life, including maximum capacity after several years, but to achieve that we need to start with a low charge voltage and not discharge too far. This of course results in less capacity available for the battery’s application.

There is also an influence of the dynamic loading or discharging with a constant current. And how high is that current compared to the capacity? Do we discharge with 10Ah for one hour or do we discharge with 1Ah for 10 hours? Unfortunately, this is not a linear curve either.

And if that is not enough, there is also the influence of temperature. If it is cold, the battery has less capacity. If it is too warm when charging the battery, this again has an influence on its lifespan.

As you can see, for all temperature conditions, different parameters are possible for the ideal battery cell behaviour. Many of these parameters are also included in a good Battery Management System (BMS) for the optimal capacity / lifetime mix, depending on the application for which the battery will be used.

THE LIFE CYCLE TEST

In this test we assume a certain capacity of the battery at a certain temperature, at a certain charge voltage, at a certain maximum discharge voltage and at a certain discharge current (not dynamic) and that is called the 100% capacity. Then you discharge and recharge this battery cell from a certain percentage. And if you do that often enough, depending on your cell, you will get a similar graph as below:

CHARGING WITH TOO HIGH A VOLTAGE

Finally, we would like to show you two graphs for comparing the charging voltage with the battery life.

             

WHAT EQUIPMENT CAN WE USE TO TEST THE CAPACITY OF THE BATTERY CELL?

Let us start with a graphical representation of the test setup.

For charging we use a DC power supply with a constant or dynamic charge current. The diode is to prevent the battery cell from discharging towards the power supply. When the battery is charged, we can start discharging it using the Electronic Load. This can also be done with a constant discharge current or dynamic.

The voltage and current values can often be read from the power supply or load, or a power meter can be used to register the exact value. The power meter often has the additional advantage that it can also integrate and thus give a Wh value. We also recommend a temperature gauge to monitor the temperature of the cell during charging and discharging.

See also our web pages on DC power supplies and DC loads.

Nowadays we also see more and more bi-directional DC power supplies (the dc power supply and dc load in one) on the market which of course is ideal for testing batteries.

For smaller cells, these include the Itech IT6400 series up to 150W and 60V or the Rohde&Schwarz NGL200 series precision power supplies up to 60W, up to 20V and 6A.

For the battery cells (and possibly small modules) with some more power we also have the Itech M3400 series. This series consists of several models with powers from 200 to 800W, 0-60V or 0-150V and currents up to 30A.

The advantage of this unit is that special functions such as charge and discharge curves can be programmed in the unit itself. There is also the possibility to connect a temperature sensor so that this can also be included in the charge/discharge parameters.

Nowadays, the battery cells for buses, trucks and high performance cars are also moving towards cells with a much higher current. Nowadays, these cells often have currents up to about 1000A, but there are already developments to even higher currents. We have solutions for this from MagnaPower, for example, in the TS series. In this line we have DC power supplies from 5V up to 2700A and 10V up to 8000A. For single cell batteries with such high currents, H&H has developed the SCL series loads. This is available, for example, in a version from 0.6V to 12V with a maximum current of 1200A. With master/slave switching this can be expanded to 6000A. From Itech we have the IT6000B series of bi-directional DC power supplies/loads from 80V to over 2000A.

But what if we want to test multiple cells simultaneously?
This can of course be done by duplicating the test setups, but there are also multi-channel test setups possible. And these are often test stands with advanced software for structured recording of the measured values.

With the Chroma 17011 series this can be done up to 16 channels per module and 32 to 64 channels per system depending on the maximum current. The systems come fully pre-assembled and delivered with the extensive Battery Pro software. (For detailed information of the software see Battery Test Software).

With the earlier mentioned Itech M3400 series we can also achieve this by controlling multiple units from the IT5300 software. Again, I would like to refer to another page regarding the software possibilities in our product portfolio for testing battery cell, module and packs.

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WHAT OUR CUSTOMERS SAY

Arnold Memelink

Arnold Memelink

ABB

"The ABB Power Grids Jumet organisation is an innovative development and production facility of ABB that manufactures active filters and battery energy storage systems to support a stable grid. To test these systems we have acquired two Cinergia GE/EL +120kVA eplus units capable of doing grid emulation (AC + DC), operating as an electronic load (AC +DC) and being a battery emulator. We have experienced very good support from TTMS in obtaining these two Cinergia units. This included organising an on-site demonstration and supplying loan systems prior to the delivery of our final systems. This way we could start testing as early as possible. TTMS also organised training for the operators from the original manufacturer. This support allowed us to introduce the first battery energy storage systems even prior to the full delivery of the Cinergia systems."

University of Luxembourg

Sasan Rafii-Tabrizi

University of Luxembourg

“At the Energy and Environment department of the Faculty of Science, Technology and Medicine (FSTM) of the University of Luxembourg we do have different research projects related to green energy contributing to the development of eco-cities. As a long-standing partner of our research unit, TT&MS has proven that they are able to answer even the most complex questions in a professional manner. They have shown that they are always up to date with the latest technology and do not spare any effort to look at the application and even the situation on site. The exchange of ideas was always very pleasant and took place quickly. We look forward to further years of fruitful cooperation.”

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