A battery energy storage system connected to the NEM is an integrated resource system: a single connection that both produces and consumes electricity through bidirectional units. Its Generator Performance Standards (GPS) must describe how it behaves while charging as well as while discharging, and in the transition between the two. Many first-draft GPS proposals treat charging as an afterthought. Reviewers do not.
This article covers the clauses and studies where charging mode changes the answer. Our article on what a battery energy storage system is covers the basics.
Two Ratings, Not One
The first thing reviewers check is that the GPS states separate ratings for discharging and charging, and the battery's energy capacity in MWh. A battery's maximum charging power is often lower than its maximum discharge power, and the two derate differently with ambient temperature because cooling demand, cell limits and auxiliary load differ by direction.
Keep the rated active power a constant value in the GPS and describe temperature derating as a separate curve, for each direction. Other clauses refer back to the rated value, so a rating that moves with temperature creates confusion throughout the document. State whether derating is linear between the temperature points given.
Reactive Power Capability (S5.2.5.1)
The Rules define maximum active power for an integrated resource system differently depending on the access standard level:
| Purpose | Pmax for an integrated resource system | Practical effect |
|---|---|---|
| Automatic access standard | The higher of active power capability and maximum demand, with all units in service | 0.395 x Pmax is based on the larger of the two directions |
| Negotiated access standard | Either active power capability or maximum demand, considered separately | Capability can be negotiated for each direction against its own base |
| Both | Less any applicable temperature derating | Derating curves must be stated for each direction |
The capability study must cover the full active power range from full charge to full discharge, not just zero to full output. A combination that often binds is charging at high ambient temperature while supplying capacitive reactive power, where the inverter's current limit and the derated charging rating meet. An import derating in that corner is a common negotiation point.
Frequency Response Through Zero (S5.2.5.11)
For an integrated resource system, the automatic access standard requires a proportional response in both directions and, for bidirectional units, a smooth change between production and consumption. A battery charging at full rate during high frequency should, as frequency falls, reduce its charging smoothly through zero and continue into discharge, with no step, pause or mode-change transient at the crossover.
That transition is a specific study case. Ramp frequency so the droop response carries the battery from full charge through zero into discharge, and show the active power trace is continuous. Check it again at commissioning.
Two other points apply:
- the droop base must be defined clearly and scale correctly with the number of units in service
- operational droop and deadband must comply with AEMO's Primary Frequency Response Requirements, not just the S5.2.5.11 capability range
Our article on S5.2.5.11 frequency control covers droop and deadband in detail.
Fault Ride-Through in Charging Mode
Every fault ride-through case run in discharge must also be run in charge. The plant's behaviour can differ: in charging mode active current flows the other way, so the current available for reactive injection, the active power recovery after clearance, and the DC-side energy balance during the fault all change. Active power recovery requirements are often stated for discharging mode, so confirm with the NSP and AEMO how recovery in charging mode will be assessed.
Very Low System Strength and Charging
At very low short-circuit ratio, a battery may be unable to absorb its rated charging power. That is not a control failure. A weak network simply cannot transfer that much power into a load at acceptable voltage. Reviewers recognise this, and NSPs have readily accepted an intertrip or runback that limits charging under extreme low-SCR contingencies.
They are much less willing to accept the same for discharging or idle operation without evidence that it is genuinely needed. If you propose an intertrip, support it separately for each mode, with simulation runs that show the limitation rather than runs that simply fail.
For the S5.2.5.15 withstand SCR demonstration, charging at very low SCR raises the same transfer limit. Artificially raising the source voltage to make rated charging possible has not been accepted. AEMO's approach for batteries in charging mode has been revised in recent years, with charging power capped at what the network can physically transfer at the tested SCR and X/R. Check the current system strength impact assessment guidelines before designing the study.
The Load Side of the Connection
Because the battery consumes electricity, reviewers may expect the performance standards to address the requirements that apply to loads, alongside Schedule 5.2. Items that have been raised include:
- load current balance between phases while charging
- harmonic and flicker emissions in charging mode, which can differ from discharge
- behaviour on loss of supply and on reconnection
- auxiliary load, which is present in every mode including idle
Confirm with the NSP early which load-side requirements it will assess. Since the AEMC's 2025 access standards rule, plant classification has moved further towards plant type rather than registration category, which makes this question more important for hybrids and co-located loads.
Modelling a Battery for Charging Cases
BESS Model and Study Checks for Charging Mode
- Separate charging and discharging ratings and derating curves in the GPS, user guide and models
- Reactive capability across the full range from full charge to full discharge
- Frequency response ramp through the charge to discharge transition
- Fault ride-through matrix repeated at full charge
- Withstand SCR demonstration in charge, discharge and idle, using the current AEMO methodology
- State of charge representation in the model stated, including whether it limits active power
- Reduced-unit modelling instructions for each direction, with worked values
- Harmonic and flicker emissions assessed in charging mode as well as discharge
- Any intertrip or runback in charging mode justified with evidence
State of charge deserves a specific mention. Standard dynamic model tests assume available energy is not limiting, but reviewers do ask how state of charge is represented and whether the model would limit power if it reached a bound. Say so in the user guide.
Get Both Directions Right
A battery's GPS is two performance standards in one document. Treating charging mode with the same care as discharge from the first draft avoids a predictable set of review comments.
ElectraGlobe prepares GPS proposals and studies for standalone and co-located batteries, including charging mode assessment and withstand SCR demonstration. See our AEMO GPS applications service, our article on FCAS revenue for batteries, or contact us.
Frequently Asked Questions
What is an integrated resource system?
It is the NER category for a connection that both produces and consumes electricity through bidirectional units, such as a battery, or a combination of generation and storage behind one connection point. Its performance standards cover both directions of power flow.
Does a battery need separate charging and discharging ratings in the GPS?
Yes. Reviewers expect separate active power ratings for charging and discharging, separate temperature derating curves, and the energy capacity in MWh.
Why can a battery not charge at full power on a weak grid?
At very low system strength, the network cannot transfer rated power into a load at acceptable voltage. This is a physical power transfer limit rather than a control issue, and NSPs have accepted charging runbacks or intertrips for those conditions when they are properly evidenced.
How should a battery respond to frequency when it is charging?
It should reduce charging as frequency falls and increase it as frequency rises, and the response must pass smoothly through zero into discharge (or the reverse) without a step or pause at the crossover.