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# Testing a battery cell

In the picture opposite, you can see a nice representation of the structure of a battery cell. Thus, you can also clearly see the anode, the separator and the cathode. In addition, the insulation material, the housing and the positive and negative terminals.

What 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

**The battery cell insulation test**
Here we determine whether no contamination has occurred in the production of the battery cell. If contamination does occur, we have to deal with leakage currents in the cell and in some cases even breakdown through the insulation material.

This is one of the causes of 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 partial discharge/flash-over detection. And this is very easy to do with, for example, a battery cell insulation tester. This test ensures a safer battery and immediately checks a number of parameters that determine whether the battery cell is properly manufactured.

**The battery cell impedance test**
Inherently to the construction of the battery cell, we have to deal with a certain resistance and parallel capacitance of the materials used. Naturally, we want to keep this resistance as low as possible because of the internal heating of the battery. And with dynamic loads, capacitance plays a role in addition to resistance. See below a drawing of how we can represent a battery cell electronically.

We can determine the internal resistance 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. Internal resistance measurement in batteries is also defined 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 that 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 capacitance is also important. This measurement is performed at a specific frequency (usually 1kHz) or is performed over a wide range of frequencies in order to even better determine the dynamic behaviour of the battery. With a single battery cell, the impedance is generally extremely low and it is very important to use specific instrumentation for this 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 battery impedance measurement, please refer to a presentation at the 2020 Energy Storage Event.

**Determining the capacity of a battery**
Determining the capacity of a battery actually depends on a large number of variables:

- Maximum charging voltage
- Maximum discharge voltage
- Battery temperature
- Desired number of charge/discharge cycles

And some of these variable are contradictory values. You want the highest possible charge voltage and the lowest possible discharge voltage to maximise battery capacity. However, this does come at the expense of service life or the number of charge/discharge cycles.

We naturally want as long a battery life as possible including maximum capacity after several years but to achieve that, we need to start with a low charge voltage and should not discharge too far. Which in turn, of course, results in less capacity available for the battery’s application.

There is also an influence of dynamic charging or discharging with a constant current. And then how high is that current relative to 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 or if that is not enough we also have the influence of temperature. If it is cold then the battery has less capacity. If it is too hot when charging the battery then it again affects its lifespan.

You understand; for all temperature conditions, there are different parameters 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 entirely depending on the application for which the battery will be used.

**The service life/charge and discharge 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 this is called the 100% capacity. Then you start discharging and recharging this battery cell from a certain percentage. And if you do that often enough, depending on your cell you get a similar graph as below:

**Charging with too high a voltage**
Finally, we would like to show you two graphs to compare charging voltage with battery life.

**What equipment can we use for testing battery cell capacity?**

For charging, we use a DC power supply with a constant or dynamic charging current. The diode is to prevent the battery cell from later discharging towards the power supply. Once the battery is charged, we can start discharging using the Electronic load. And that too can be 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, for example, is used to record the exact value. The power meter often has the added advantage that it can also integrate and thus give a Wh value. We also recommend a temperature gauge to monitor the cell temperature when charging and discharging as well.

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

We also see more and more bi-directional DC power supplies (the dc power supply and dc load in one) on the market these days which is obviously ideal for battery testing.

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

For battery cells and possibly small modules with a bit more power, we also have Itech M3400 series. This series consists of several models with power ratings 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 of connecting a temperature sensor so that this can also be included in the charge/discharge parameters.

Now, with battery cells for buses, trucks and high performance cars, we are also moving to cells with much higher currents. Now often up to around 1000A but there are already developments up to even higher currents. For this, we have solutions from MagnaPower, for example, in the TS series. In that 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. For example, it comes in a version from 0.6V to 12V with a maximum current of 1200A. With master/slave switching, this can be extended to 6000A.

From Itech, we have versions in the IT6000B series of bi-directional DC power supplies / loads from 80V to over 2000A.

But what if we want to test multiple cells at the same time? This can obviously be done by duplicating the test setups but multi-channel test setups are also possible. And these are then often test setups with also advanced software for structured tracking of the measured values.

The Chroma 17011 series allows 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 supplied with the comprehensive Battery Pro software. (For detailed information of the software see Battery test software.)

With the aforementioned Itech M3400 series, we can also achieve this by controlling multiple units from the IT5300 software. Again, please refer to another page regarding the software options in our product portfolio for testing battery cells, modules and packs.

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