Sizing, testing and monitoring

Yes, and this is where most monitoring vendors overclaim. Impedance and conductance are trend indicators, not capacity measurements. They catch open cells, dry-out and connection faults, and rising impedance correlates with capacity fade, but the only direct measure of capacity is a timed discharge test. AusNet’s own asset strategy states that the impracticality of discharge testing on single-bank zone substations “prevent[s] accurate measurement of the DC system available capacity”, which is why they pursue continuous cell monitoring and a capacity test method. /is-battery-monitoring-worth-the-cost/

A BMS is inside a lithium battery and protects it. A battery monitoring system is external, watches a bank (typically lead-acid) and reports condition for maintenance planning. Lithium has the first inherently; lead-acid needs the second added.

The opposite of what people expect: the absence of monitoring can void a warranty. AusNet’s published asset management strategy states that lack of monitoring “may void the DC cell warranty as the manufacturer requires proof that the DC cell was operated in a controlled regime”. Monitoring data is not just diagnostics. It is the evidence file for a future claim. Confirm what evidence your specific warranty requires at purchase, not at claim.

Options in increasing order of capability: charger or rectifier alarm contacts into an existing RTU; a monitoring unit reporting over the site’s existing SCADA link via Modbus or DNP3; a cellular-connected monitor where no SCADA link exists; or a full energy management system where the site has generation and storage as well as batteries. The choice is usually determined by what communications the site already has, not by what the monitoring can do.

Four inputs: the load in watts or amps, the required autonomy in hours or days, the maximum depth of discharge you will design to, and the derating factors for temperature and end-of-life capacity. Australian utility practice, expressed cleanly in Sydney Water’s HV specification, is the IEEE 485 method with a 10% design margin and a 25% ageing factor. For stand-alone power systems, AS/NZS 4509.2 gives the storage sizing method and autonomy guidance.

It is set by criticality and restoration time, not by convention, but Australian specifications give real benchmarks worth quoting:

Nobody else has these numbers assembled in one place, and each maps to a Valen product line.

Available capacity falls as temperature falls. AS/NZS 4509.2 provides derating curves for lead-acid at C₁₀₀ and C₂₀ rates from −10 °C to +40 °C, with correction factors running roughly 60% to 105%. Size to the minimum expected electrolyte temperature, not the annual average. Separately, and in the other direction, sustained high temperature does not reduce available capacity much but shortens life sharply, see B1.

Three tiers. Impedance or conductance testing (instruments such as impedance meters) is fast, done on a live circuit, and reliably detects open cells, dry-out and connection degradation, but requires a baseline from when the bank was new. Capacity analysers perform a cyclic load test and require the battery fully charged first. Timed discharge testing is the only direct measurement of capacity. AS 2676 covers installation, maintenance, testing and replacement obligations in Australia.