In short. A BESS controller decides when and how fast the battery charges or discharges, within the limits of the cells and converters. Reserve for outage cover must be protected from commercial uses, and the battery must follow the plant’s dispatch without upsetting generators.
Control layers
| Layer | Function |
|---|---|
| Cell and rack BMS | Protects cells: voltage, temperature, current limits |
| PCS (inverter) | Converts and controls power, voltage and frequency |
| BESS controller | Mode, set points, state of charge management |
| PMS and EMS | Plant coordination. See EMS |
Modes
- Backup reserve: hold a minimum state of charge for outages.
- Peak shaving: discharge when demand exceeds a threshold.
- Load shifting: charge when energy is cheap, discharge when expensive.
- Solar smoothing: absorb and release to hold output stable.
- Generator support: cover steps and peaks so sets run efficiently. See DG and BESS dispatch.
- Grid-forming: set voltage and frequency in an island. See grid-forming mode.
Limits to enforce
- State-of-charge window, with a protected reserve band.
- Maximum charge and discharge power versus temperature and state of charge.
- Ramp rate limits.
- Thermal and fire protection signals.
- Converter current and voltage limits, including fault current.
Fault behaviour
Inverters limit fault current to a small multiple of rating, so protection must be designed for it. See inverter fault behaviour.
Charging from a generator
Charging adds load. Limit the charge rate so that a generator running at moderate load does not get overloaded, and use the generator at an efficient loading point.
Illustrative example
A site uses a 500 kW, 1 MWh battery. The reserve band holds 40 percent for outage cover, and peak shaving uses only the remaining window. During a utility outage the battery picks up critical load for the first seconds, then hands over to the generator, and is recharged at a limited rate once the generator runs.
What the BESS controller manages
| Function | Detail |
|---|---|
| Charge and discharge setpoint | Within power and state-of-charge limits |
| State of charge window | For example 10 to 90 percent to protect life |
| Temperature limits | Derate or stop outside safe range |
| Grid support modes | Peak shaving, backup, frequency response |
| Protection coordination | Battery management system, converter, breaker |
Control hierarchy inside the battery system
- The battery management system (BMS) protects cells and can disconnect.
- The power conversion system (PCS) controls power flow.
- The site controller sets targets for the PCS.
The BMS has authority to limit or stop the PCS. The site controller cannot override that.
Energy check (illustrative)
A 1 MWh battery with a 10 to 90 percent window has 0.8 MWh usable. Discharging 400 kW to shave a peak lasts 0.8 ÷ 0.4 = 2 hours, less conversion losses, say 1.8 hours. Size to the duration of the peak plus a margin.
Interaction with the generator
The BESS can ride through the generator start delay, smooth load steps and let the generator run at a healthy load. Make sure charging does not overload the generator.
Safety
Battery fire safety, ventilation and gas detection depend on chemistry and local rules. Follow the manufacturer’s requirements and applicable standards.
See DG plus BESS dispatch and grid-forming coordination.
The master engineering question
If the grid fails right now, how much usable battery energy is guaranteed for the critical load, and what prevents commercial modes from eroding it?
What to send us
Send battery, PCS and tariff details. Send your application details.
Questions engineers ask
Who controls the battery, PCS or controller?
The PCS follows controller set points inside BMS limits.
Can a battery replace a UPS?
It can serve some functions, but check transfer time and design requirements.
What state of charge to reserve?
Based on required outage duration and critical load.
Does charge rate matter for ageing?
Yes. Follow manufacturer limits.
This guide gives general principles. Follow the battery manufacturer’s limits.