Simple Circuit Monitors Health of –48V Telecom Lead-Acid Battery Backup Systems Jon Munson Telecommunications infrastructure has always been powered by voltages that are negative with respect to ground to minimize corrosion in buried cable. Telcos typically use –48V power, with backup power supplied by large battery arrays to carry the system through utility outages. These power backup systems traditionally comprise four 12V lead-acid batteries in series, though newer lithium cell technology promises to make inroads as systems are updated. Every battery backup system must be continually monitored for the charge state and health of the batteries. In fact, although stacking batteries is easy, it can be difficult to build a monitoring system that can measure and digitize both the condition of individual cells, and monitor the high voltage potential of the combined cells. Enter the LTC6803 multicell battery stack monitor. The LTC6803 is designed to measure and digitize individual cell potentials in large lithium cell stacks with total potentials 100Ω Figure 1. Isolated lead-acid telecom battery-stack monitor 100nF 10µH 12V SLA 10µH 12V SLA 10µH 12V SLA 10µH 12V SLA –48V 38 | July 2011 : LT Journal of Analog Innovation beyond 60V (surviving surges to 75V). Although the LTC6803 is ostensibly designed to monitor lithium-based battery systems, it can just as well be used to support traditional –48V lead-acid battery stacks. Regardless of cell chemistry, all the measuring potentials are below ground CSBO CSBI CS2B CSB µC_CSB SDOI SDO SDO2 SDO µC_MISO SCKO SDI SDI2 SDI µC_MOSI SCK2 SCK µC_SCK VCC2 SDOEB V+ 100Ω PDZ7.5B 4.7µF 100Ω PDZ7.5B 4.7µF 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 100Ω PDZ7.5B 4.7µF 10k 10k 10k SCKI 2.2k C12 VMODE S12 GPIO2 AVCC2 C11 GPIO1 I1 ON S11 WDTB I2 VL C10 LTC6803-1 AV– LTM2883-5S DO1 DO2 V– NC S10 TOS C9 VREG S9 VREF C8 VTEMP2 S8 VTEMP1 C7 NC S7 V– C6 S1 S6 C1 C5 S2 S5 C2 C4 S3 S4 C3 VCC µC_VCC 1µF AV+ 1µF 1µF V+ GND2 GND µC_GND design ideas Although the LTC6803 is ostensibly designed to monitor lithium-based battery systems, it can also be used to support traditional –48V lead-acid battery stacks. 10µH C(n + 3) 100Ω 10k S(n + 3) 7.5V 4.7µF DISCHARGE SWITCH (LTC6803 INTERNAL) VBAT 3 C(n + 2) VBAT 12V 7.5V SLA 100Ω 4.7µF 10k S(n + 2) DISCHARGE SWITCH (LTC6803 INTERNAL) VBAT 3 C(n + 1) 100Ω 10k S(n + 1) 7.5V 10µH 4.7µF DISCHARGE SWITCH (LTC6803 INTERNAL) Figure 2. Voltage-divider structure for each 12V battery measurement or possibly floating during maintenance procedures. Ideally, these batteries should be measured by circuitry that is independent of the relative grounding between the batteries and the central-office equipment, thus Galvanic isolation is desirable. A SIMPLE SOLUTION FOR LEAD-ACID STACKS Since the ADC range for an individual LTC6803 input channel maxes out at 5.37V, divider networks are used to spread each 12V battery potential across three channels. Figure 1 shows how. Each battery potential is acquired by summing triplets of input channel readings (CN inputs). Here the cellbalancing controls (SN output discharge switches) are re-purposed to continually activate voltage dividers using external 10k resistors by setting all DCC configuration bits to 1. In this way, each channel is converting a 4V nominal potential. The 4.7µF bypass capacitors accurately hold the intermediate voltages as small ADC sampling currents flow, while 100Ω series resistors and 10µ H inductors provide hot-insert surge limiting. For best accuracy, the STCVDC conversion command (0x60) should be used so that the always-enabled discharge switches remain on throughout the conversion process. When communication has stopped and the part times-out, or it is directly commanded to standby mode, the balancing discharge switches are turned off and the dividers are effectively disconnected so that no appreciable battery drain occurs. A simplified equivalent circuit of a particular divider section is shown in Figure 2. An LTM2883 SPI data isolator is used so that the circuit accommodates any grounding differential with respect VBAT 3 Cn to the associated microprocessor circuitry. The LTM2883 also provides isolated DC power rails that can furnish several hundred mW if needed. CONCLUSION The LTC6803 provides a flexible solution for telecom battery stack measurement, including stacks using 12V lead-acid batteries. The 12V units are measured by summing the readings of three input channels that have been hardware configured to split the 12V into sub-measurements, thus achieving an effective full-scale range of 16.1V for each battery. Isolation of the data acquisition function from the processor support is important for elimination of grounding errors and safety hazards and is readily provided by the LTM2883 SPI isolator module. n July 2011 : LT Journal of Analog Innovation | 39