BT2200 Charge-Discharge Platform

Verifying the Number of Physical Channels Required for Charge-Discharge Applications

Cells are becoming larger to provide EVs with additional range or to provide electronics with longer run times. Defining a higher power user channel simplifies testing a large cell. A user channel is simply a group of parallel physical channels to provide the higher charge-discharge current. The user channel controls all the parallel physical channels divvying up the programmed current. 

Two examples:

1.       Combining eight physical channels on one BT2204B Charge-Discharge Module creates a user channel with a nominal current rating of 8 x (± 6.25 A) = ± 50 A.

2.       Combining the two physical channels on one BT2205A Charge-Discharge Module creates a user channel with a nominal current rating of 2 x (± 100 A) = ± 200 A.

However, when using the BT2200 to charge and discharge cells, you need to consider additional factors in determining how many physical channels to parallel into a user channel that will meet your application's requirements. The other factors include: 

• The power available on each physical channel. There is a limit on the maximum available power for each physical channel, and that limit is less than the maximum current multiplied by the maximum voltage. 

• The minimum module voltage supported at any specific value of discharge current.

• The difference in the voltage at the cell vs. the voltage at the output terminals of the chargedischarge module. A current flowing through the resistance of the wire, connectors, and fixtures causes a drop in voltage from the module to the cell. The resistance of the wire, determined by wire length and wire gauge (diameter), can be critical to a successful test. Likewise, the resistance of the connectors and contacts in the current path can also be essential to a successful operation. These factors become increasingly important at higher current levels.

A proper test set up necessitates verifying a couple of operating points.

1.       The combination of maximum charging current, maximum charging voltage, and maximum charging power shown in red, Figure 1.

2.       The combination of maximum discharge current, minimum discharge voltage and discharge power shown in blue, Figure 1.

Example Configuration 1 – BT2204B Using Parallel Channels for Higher Power

Suppose you need to charge and discharge a group of cells to the following requirements:

• Charging  Maximum current = 50 A at a maximum cell voltage = 4.2 V

• Discharging  Maximum current = -50 A at a minimum cell voltage = 2.5 V

A BT2204B module has multiple physical charge-discharge channels connected in parallel creating a user channel that will provide the ± 50 A current to each cell. For this example, assume:

• 16-gauge copper wire connects cells to the charge-discharge module; this wire diameter matches the type of connectors that mate to the connectors on the BT2204B module. Two of these wires (supply and return) connect each physical channel.

• A wire pair runs from each physical channel on the module connector to the cell. Wire pairs connect each physical channel directly to the cell to create the paralleled group.

o Cells are located 10 feet from the BT2204B module. 16-gauge wire has a resistance of about 4Ω per 1000 feet of wire, 10 feet of wire has 40 mΩ of resistance in each of the two wires (supply and return) from a physical channel.

o There is an additional 10 mΩ of resistance in each of the two current paths (supply and return) due to connectors and contacts. Adding the wire and contacts resistance makes for a total of 50 mΩ in each current path.

We can examine the number of physical channels required for both charging and discharging.