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Battery Monitoring

Note: The terms ‘cell’ and ‘monobloc’ are used interchangeably in this context.


What does LEM bring to standby battery monitoring?

LEM is a Swiss-based, high quality, high technology components manufacturer and the world's leading manufacturer of hall-effect and fluxgate current measuring devices. For over 25 years LEM has measured the current and voltage parameters of batteries. LEM current transducers monitor the world's largest battery‚ BESS Alaska, rated at 45 Megawatts, at 5000 volts. In 2003 LEM acquired Guardian Link, whose innovative, patented battery analysis and monitoring systems won many industry and government awards. This advanced technology is now being integrated by LEM into the most cost-effective intelligent transducers ever manufactured for standby battery systems.


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Why should I install continuous monitoring rather than carry out regular maintenance?

While regular maintenance is certainly of value, faults can develop between visits, which may prevent the battery performing as specified. In addition, unless comprehensive maintenance routines are routinely performed, incorporating thermal mapping and noise & ripple measurement for example, these inspections may not be completely effective. For example, despite diligently carrying out IEEE recommended maintenance procedures, the UPS parcel service in the US lost 40% of their $3M battery installation less than half way through its design life. After testing, the conclusion of the Facilities Engineering Manager, Joseph Parrino, was that a continuous monitoring system was essential in early detection to minimize catastrophic failures. (Proc. BATTCON 2004)


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What is the Sentinel system?

The Sentinel system is a continuous standby battery monitoring system, based on a single chip (integrated circuit). The module is integrated with a single cell or monobloc, 2.0 volts to 12 volts nominal. Sentinels are intelligent transducers, measuring cell terminal voltage, cell temperature and cell impedance. Cell voltage is measured in both static (floating) and dynamic (discharging) conditions.


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Is the Sentinel system cost-effective?

In addition to preventing catastrophic failure, continuous monitoring reduces maintenance visits and through early detection, allows the cell or cells to be proactively replaced before the entire battery is affected.

Many battery monitoring systems however can cost 40-50% of the cost of the battery itself. In addition to the cost of the equipment, 'hidden' costs like installation can add another 25-200% on top of replacement cell expense.

LEM believes that battery monitoring is important, effective monitoring, however, should not be more than 10-15% of the cost of the battery.

Also, Sentinel kits are designed to be installed in typically 25% of the time of other systems, with inexpensive pre-terminated communications links and power & sense leads. This approach can lead to ROI's of less than one year.


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What is special about the Sentinel system?

This is the first time that cell temperature, impedance and terminal volts have been combined in a single module for a single cell or monobloc. Combining accurate temperature, discharge, float, and ripple current measurements, the Sentinel system is the most comprehensive battery monitoring system available.


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Is the Sentinel system only an intelligent transducer module?

No. The system also has two extremely cost effective monitors, one for up to 250 cells and four strings (MicroGuard) plus a PC based software program for visual display, analysis and archiving (CellView).

In addition LEM are producing high quality float and discharge current transducers specifically for standby battery systems.


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The Sentinel module is powered from the cell; will this run the cell down?

Although the Sentinel module draws its power from the cell it is integrated with, the system is designed to operate in 'sleep' mode unless actually measuring the cell (less than 1/10th of a second every 5-10 minutes). In sleep mode the Sentinel draws less than one thousandth of an amp from the cell. On float charge there is no effect on the charge current.


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What if the battery is 'off line' for some time?

If the battery is left open circuit for a period, the Sentinel will sleep unless polled by the monitoring system. If left operating in the normal manner, on average the module will draw less than 1 ampere-hour per month from its cell.


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Why is it better to monitor the cell temperature, rather than the room ambient?

Although not very common, the threat of thermal runaway still exists in certain types of cells. Monitoring room ambient will not detect an incipient thermal runaway condition. The Sentinel module, monitoring the actual cell skin temperature, will detect abnormal cell temperatures and signal an alarm before runaway can occur.

Research by Telstra Laboratories & Bell Communications found that only a small increase in individual cell terminal voltage was required to initiate a thermal runaway condition. (Proc. INTELEC 1996). Periodic maintenance is unlikely to detect this condition, even if thermal mapping is carried out, since Telstra & Bell found that time periods of less than 200 hours could be significant.


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Do I have to have different Sentinels for different bloc voltages?

Yes it is necessary to order bloc specific Sentinel. In the case of a 2V bloc, a Sentinel LV and for 6V or 12V bloc, a Sentinel HV.


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Does impedance testing create any noise on the DC link?

No; testing individual cells locally is the best way to avoid creating any electrical noise at the battery terminals. In addition, it is also the optimum way to avoid including charger, inverter or system components in the impedance measurements.


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How does Sentinel ensure reliable impedance measurements?

In order to avoid false readings from a cell on float charge, it is essential to test a cell at a level of current sufficient to penetrate the 'surface' charge present in-circuit. The test pattern of the Sentinel system is programmed to optimize the test current, reducing this problem to a minimum.


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How effective is impedance in detecting faults anyway?

To date, measuring the impedance / conductance / resistance of a cell is the most powerful way of detecting deterioration or incipient failure. Impedance is known to change with most failure modes, or exponentially over service life, and the trending of cell impedance values is the most effective utilisation of these changes.


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Which is the most effective test method, impedance, conductance or resistance?

Different companies will advocate one or other of these, depending on which suits their own equipment. In truth, if the testing is carried out 'intelligently' (that is, knowing what frequencies and test current levels elicit what cell reactions) there is little practical difference between them. Conductance is, after all, only the reciprocal of cell resistance, and resistance is the same as impedance at certain test frequencies.


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Is the Sentinel safe?

Sentinel modules are designed to international safety standards and rated to 600 volts DC, and the communications link is isolated to 5000 volts in every module. The Sentinel casing is high impact tested, to prevent personal electrical shock through accidental damage, and is manufactured from flame retardant materials, with a high resistance index. Sentinel is also tested to ensure compliance with all relevant EMC standards.

Sentinel Power and Sense Leads (PSL's) are also designed to 600 volts DC, and double insulated cables in both PSL and comms leads, meet industry standards.


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Can Sentinel cause a short-circuit if there is a fault?

All Sentinels are internally fused and will open in the event of any faults. An over-temperature in the module will cause the Sentinel to immediately shut down all power components.


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Sentinel is a 'one-wire' communications system, what if it has a communications failure?

There are no active components in the line, so the integrity of the system is extremely high. Although not necessary for the average installation, for added security the last module in the line (in addition to the first) may be connected back to the monitor. Completing the ring doubles the integrity of the communications system and makes the possibility of failure virtually non-existent.


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