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Energy storage control (EMS): how it works and what the BST Control Box does

A battery storage system is only as good as the controller that tells it when to charge, when to discharge and when to stay out of the way. This page explains the control layers of a BESS – BMS, PCS, EMS – the use cases a controller has to handle in Hungary in 2026, and how BST Solar implements them on ComAp controllers and the BST Control Box, for new systems and for storage built by others.

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BMS, PCS, EMS – who controls what in a battery storage system

The three abbreviations are often mixed up, but they are three different layers with three different jobs. A storage system that misbehaves is almost always a problem in the connection between the layers, not in the battery itself.

BMS – battery managementLives inside the battery rack. Measures every cell's voltage and temperature, balances the cells, calculates state of charge (SoC) and state of health (SoH), and opens the contactor if a limit is exceeded. It protects the battery – it does not decide what the system should do.

PCS – power conversionThe bidirectional inverter between the DC battery and the AC grid. Executes the power setpoint it receives (±kW, kvar), handles grid protection (voltage, frequency, anti-islanding) and the grid code requirements of the DSO. Fast, but blind to prices, tariffs or the solar forecast.

EMS / controller – the brainSits above both. Reads the meters, the PV inverters, the genset, the HUPX prices, the aggregator's signal and the site's demand limit, and every second decides the setpoint the PCS has to follow. This is the layer BST Solar designs and operates – on ComAp controllers and the BST Control Box.

< 1 scontrol cycle from measurement to PCS setpoint

16sites where BST controls energy storage and diesel gensets today

+15further sites in progress

0–24remote supervision with an on-call engineer

What an energy storage controller has to do in Hungary in 2026

A controller is specified by its use cases. These are the ones we implement most often – usually several at once on the same system, with a priority order so they never fight each other.

Peak shaving – lower system usage chargeThe controller watches the 15-minute average at the main meter and discharges before the site would cross the contracted or historical peak. Every kW shaved saves the monthly demand charge of the system usage fee (RHD) – for an industrial site typically 1.5–2 million Ft per avoided 100 kW per year.

Zero export / feed-in limitWhere the DSO allows no or limited feed-in, the controller charges the battery with the PV surplus and throttles the inverters only when the battery is full. The PV park keeps producing, the connection point stays within its limit.

Price-based charging on HUPXEvery afternoon the controller receives the next day's HUPX day-ahead curve and plans charging in the cheapest and discharging in the dearest hours – in 2026 that spread is 150–250 €/MWh on a typical day. Our live price page shows what the controller sees.

aFRR through an aggregatorThe controller exposes the storage to the aggregator (Modbus TCP or IEC 60870-5-104), follows the activation setpoint within seconds and keeps the SoC in the band the prequalification requires, while reserving capacity for the site's own peak shaving.

Hybrid with PV and diesel gensetOn sites with a genset the controller runs the battery as the first reserve, starts the genset only when SoC and load require it, and shares load between PV, battery and genset without frequency excursions. Black start and island operation included.

Retrofit control of an existing storageMany systems installed in 2023–2025 run on the inverter's built-in logic with no market access. We add a ComAp controller or a BST Control Box in front of the existing PCS and BMS, after a protocol audit – the battery stays, the revenue changes.

Priority order – how several functions share one battery

The controller runs the functions in a fixed order; a higher-priority function can always take the capacity a lower one planned to use. This is the single most important setting in the whole system, and it is agreed with the owner before commissioning.

Priority	Function	Why it comes here
1	Protection and grid code	Non-negotiable: BMS limits, PCS protection, DSO setpoints.
2	Feed-in limit / zero export	A breach can cost the connection permit.
3	aFRR activation (if contracted)	Penalties for non-delivery exceed any arbitrage gain.
4	Peak shaving	One missed 15-minute peak sets the demand charge for the month.
5	Price arbitrage and self-consumption	Uses whatever capacity the functions above leave free.

How BST Solar implements it: ComAp controllers and the BST Control Box

We build the control layer on ComAp industrial controllers – InteliNeo 530 BESS for a storage unit, InteliNeo 5500/6000 as the site or microgrid EMS, InteliGen and InteliMains for gensets and grid interfaces. The BST Control Box Optimum packages the controller, the communication gateways, the metering and the remote-access router in one cabinet, pre-programmed with the use cases above and tested in our workshop before it goes to site.

Integrated and field-tested with the batteries and PCS of our catalogue: CATL, EVE, Sunnic, Delta batteries; Sinexcel, Kehua, Delta PCS
Protocols: Modbus TCP/RTU, IEC 61850, IEC 60870-5-104, SunSpec for PV inverters, CAN for BMS; DSO ripple-control and setpoint inputs
Remote supervision through ComAp WebSupervisor and the BST monitoring platform – alarms to our on-call engineer, data to the owner's dashboard
Cyber security: VPN-only access, role-based users, no inbound ports on site, firmware updates in maintenance windows
Every charging interval logged with its source – the data the grant programme's 75 % renewable-share proof is built from

BST Control Box Optimum

ComAp InteliNeo 530 BESS

ComAp InteliNeo 6000

All controllers

From concept to a running controller

1. Use-case workshopHalf a day with the owner and the electrician: tariff, demand limit, DSO conditions, PV and genset data, aggregator plans. Output: the priority list and the control concept.

2. Engineering and factory testController program, communication mapping to every device, protection settings agreed with the DSO; the complete Control Box runs against simulated meters and PCS in our workshop.

3. Commissioning and handoverOn-site tests of every use case, DSO acceptance measurement, aFRR prequalification if contracted, training of the site staff and a 3-month monitored start-up – then optional O&M.

Frequently asked questions

What is the difference between BMS, EMS and PCS?

The BMS protects the battery cells, the PCS converts DC to AC and follows a power setpoint, the EMS decides that setpoint from prices, tariffs, PV and the grid's signals. Only the EMS knows why the battery should charge now; the other two only know whether it can.

Can an existing storage system be controlled afterwards?

In most cases yes. If the PCS and the BMS expose a documented interface (Modbus, CAN, IEC 104), a ComAp controller or a BST Control Box can be placed in front of them. We start with a protocol audit and a written list of what the manufacturer allows under warranty.

Can one battery do peak shaving and aFRR at the same time?

Yes, with a priority order and a reserved SoC band for each. Typically 20–30 % of the capacity is kept for the site's peak and the rest is offered to the aggregator; the controller enforces the split every second.

Which protocols does the controller need to talk to the DSO and the aggregator?

Hungarian DSOs use ripple-control contacts or IEC 60870-5-104 setpoints for feed-in limits; aggregators use Modbus TCP or IEC 104 for activation signals. ComAp controllers support all of these natively, so no extra gateway is needed.

Does the controller use HUPX prices automatically?

Yes. The next-day day-ahead curve is loaded every afternoon and the charge/discharge plan is rebuilt; if the aggregator or the site's peak needs the capacity, the plan yields to them according to the priority order.

What does energy storage control cost?

For a C&I cabinet a pre-programmed BST Control Box with commissioning is a small fraction of the storage investment; for a containerised system the EMS, metering and DSO interface are typically 3–5 % of the total. The revenue difference between a well and a poorly controlled system is 30–40 % a year.

Let us design the control of your storage

Send the site's tariff, demand limit and the storage data – we come back within a week with a control concept and a priority list.

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