Testing a battery pack

Testing a battery pack is obviously an extension of testing a battery module. Again, we can perform the following tests:

– Determining the impedance of the complete battery pack – Determination of the capacity of the battery pack – The number of charge/discharge cycles

Compared to testing battery modules, here we are dealing with higher powers (kWh), higher currents and more and more also higher voltages. And this in itself also presents new challenges. We currently see battery packs for buses and trucks with voltages up to around 800V. However, developments are already going up to 1000 and 1200V battery packs. The biggest challenge, however, is in the charging currents. Using a pantograph, we can charge at quite high currents. But the electric truck does not currently have a pantograph. Therein lies the challenge a cooled charging connectors to still have a manageable cable that can handle high charging currents. And when we hear what the future plans are especially for long-distance trucks, we are moving towards charging currents of 2000A and more at a charging voltage of up to 1500V. There are some challenges ahead.

DETERMINING THE BATTERY IMPEDANCE

We can determine the internal resistance in a battery pack quite simply by means of a step response measurement in the load of the battery. See also the explanation of this method in the page testing a battery cell. However, you need a somewhat heavier load for this. There are now also bi-directional DC power supplies that can handle these voltages and currents well.

DETERMINING THE CAPACITY OF THE BATTERY PACK

And again, basically the same conditions apply as we mentioned when testing a battery module, only now we are working with higher powers. Especially at these high powers, we see the advantages of using (regenerative) bi-directional DC power supplies.

For small packs, we have in TTMS’ product portfolio two quadrant DC power supplies in the voltage range of 0-80V through to 0-1500V at powers from 5kW to 18kW. These power supplies are completely tailored to these applications and have the option of an autonomous charge and discharge cycle with the customer’s desired parameters. Based on set values like Charge/Discharge voltage, current and time can be charged or discharged. In addition, we then have the stop values for charging and discharging in the form of stop voltage, stop current and stop capacity. With a single bi-directional DC power supply, there is also a possibility of connecting a temperature sensor so that the temperature of the Battery can also be a stop value.

With quick chargers for cars, however, we are already seeing outputs of 22kW or even 44kW. For trucks and buses, this is already running up to 600kW and more. We expect the first chargers at 900kW and 1200kW to hit the market soon. For boats, we have already seen charging stations at 6MW.

With most bi-directional DC power supplies, we can build a test system by putting a number of units in a master/slave configuration to create configurations even into the MWs.

However, we also have solutions with bi-directional DC power supplies of 60kW (max 300kW) and a modular system with 15kW to 160kW modules which can be combined to >1.2MW installations with a maximum voltage of 1500V.

THE NUMBER OF CHARGE/DISCHARGE CYCLES OF A BATTERY PACK

If these tests are performed at all on the entire pack (usually limited to the battery module), they are lengthy and costly tests. The same hardware can of course be used for the tests as for the capacity test and then often supplemented with comprehensive battery test software.

Cinergia’s B2C hardware also allows us to test up to three packs with a maximum power of 53kW per pack simultaneously.


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