3-22 BOP HIPWR 111315The BOP output voltage is set to the desired battery voltage and current limit is set to therequired charging current value. The BOP will go into current limit, charging the battery with aconstant current. As the battery's voltage nears the BOP voltage setpoint, the BOP will switch tovoltage mode when the battery voltage reaches the “capture” range of the BOP voltage loop.The voltage loop has a resolution of 14 bits (either local or digital control) versus 12-bit resolu-tion for the voltage-limit loop. Consequently, the voltage loop accuracy (0.03% of rated output) isten times better than voltage-limit loop accuracy.The charging process continues as long as the voltage remains within the “capture” range of thevoltage mode. Charging current decreases gradually towards the battery's leakage value. Thisconfiguration eliminates the dithering (alternating between current and voltage limit mode)induced by the battery's internal resistance. The only drawback to using voltage mode is that thecharging current limit resolution/ accuracy is 12-bit/ 0.5%, versus 14-bit/ 0.1% for current mode.A similar process will take place when discharging the battery. By setting the BOP voltage to alower level than the battery's voltage value, the battery discharge current will be established bythe negative current limit setpoint. The discharging value can be different than for charging byusing independent limits to set different values for positive and negative current limit.3.3.6.1.2 BATTERY OPERATIONS USING CURRENT MODEThe following description explains the drawbacks of using current mode for battery operations.In voltage mode the voltage feedback comes from the remote sense terminals (OUT_S,COM_S). However In current mode the voltage limit feedback comes from the local output mon-itoring terminals (OUT_MON & COM_MON). This protects the load against excessive voltage ifa sensing connection is lost while in current mode.The result is that if current mode is used to charge a battery (with or without local sensing), thecharging process will stop (putting the BOP into voltage limit mode) at a voltage lower than thevoltage limit set value. This is due to a) the parasitic voltage drop on the connecting wiring andb) the internal battery's resistance. The BOP, now in voltage-limit mode, senses that battery volt-age is lower than the voltage-limit set value and switches back to current mode. From this point,the BOP will change between current mode and voltage-limit mode (dither). The dithering ratewill depend on the battery's internal impedance, the charging set current, the parasitic connec-tion's resistance and the time it takes the BOP to switch between modes. In some cases, for aparasitic voltage drop equal to, or less than, the accuracy of the voltage limit loop (0.3% of ratedoutput), the dithering will stop. While dithering between current mode and voltage limit mode,the front panel meter will show an approximate average output voltage which will be higher thanthe voltage limit set value.Discharging the battery using current mode operation must be carefully monitored. If the voltagelimit is unchanged and the current is set to a negative value, the battery will discharge at theBOP current setpoint value. If the process is not monitored and stopped, the battery will be com-pletely depleted. The BOP will even try to reverse-charge the battery to the negative voltagelimit value. Therefore, if current mode is chosen, it is recommended that independent limits beused, and the negative voltage limit be set to minimum (0.2% of rated output voltage).If the positive voltage-limit is set lower than the battery's voltage, then more than one loop willtry to control the output. Consequently, the battery will discharge at a much higher current thanthe BOP current setpoint. As previously described, if not monitored and the process stopped,the discharge will completely deplete the battery.
