Installation - 2
♦It is good engineering practice to either twist or shield the sense and power wires.
♦Twist the power wires together and keep them as short as possible.
♦Twist the sense wires together but do not twist them together with the power wires.
♦If possible, shield the sense wires. Connect the shield to the case.
♦Keep the total cable length as short as possible.
♦Use low resistance fixture contacts.
Remote Sense Connections
The sense connections provide remote sense capability at the fixture. Sense connections on each card are through the same connectors that house the power connections.
Remote sensing allows the output voltages to be sensed at the cell, thus compensating for any losses in the wiring. On the Agilent E4374A cards, the compliance voltage (the voltage that the Agilent MCCD can provide in excess of the programmable rating) can be up to 5.5 volts to compensate for any IR voltage drop in the wiring between the channel output and the cell connections. This higher compliance voltage allows the full 5 V to be applied directly to the cell with a maximum of 0.5 volt loss in the wiring. If the charging voltage at the lithium ion cell is between 4.0 and 4.1 volts, the higher compliance voltage can compensate for a maximum of 1.4 volt to 1.5 volt loss in the wiring.
The following table gives the resistance values of various wire sizes so that you can calculate the voltage drops for various wire lengths and diameters. Larger and shorter wires result in lower voltage drops. The table also gives the maximum wire that limit the voltage drop to 1.4 volts with a maximum current of 2A. (1.4 volts is the difference between the 5.5 compliance voltage available at the power connector and the 4.1 volts required to charge a typical lithium ion cell.)
Table 2-5. Resistance of Stranded Copper Conductors
AWG No.
18
20
22
24
mm2
0.825
0.519
0.324
0.205
Resistance (at 20 deg. C)
Ω/mΩ/ft
0.022 | 0.0066 |
0.034 | 0.0105 |
0.055 | 0.0169 |
0.087 | 0.0267 |
Maximum length in meters to limit
Voltage drop to 1.4 V @ 2A (total length of + and - leads)
30
20
12
8
As an example, assume that you are using AWG #24 wire for your power connections and your charging voltage is 4.1 volts at 2 amperes. Using this diameter wire and assuming a maximum current of 2 amperes, the maximum distance from the power connector to the cell is limited to about 4 meters. This is because with a total wire length of 8 meters for both the + and − power leads, the maximum voltage drop in the wiring is 1.4 volts (2A X 0.7Ω). With a charging voltage of 4.1 volts required at the cell and a compliance voltage of 5.5 volts, this is the maximum voltage drop that the Agilent MCCD can tolerate.
NOTE: This example does not account for any additional lead path resistance that may be present such as fixture contact resistance, or fixture relays. If additional resistance is present, lead length must be reduced yet further.
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