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2015 Sensata Technologies
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Introduction
1.4 Battery Regulation Methods
In an AC-coupled system, there are several methods that are used to regulate the battery voltage,
as described below:
1.
AC disconnect driven by DC controlled relays:
When the battery voltage rises above a
maximum setpoint, a battery voltage controlled relay is activated to open the AC connection to
the grid-tie inverter. This causes the critical load sub-panel to now be powered from the batteries
through the battery-based inverter. When the battery voltage falls to the low setpoint, the relay
closes and allows the grid-tie inverter to reconnect and begin generating power from the renewable
energy. If the battery voltage rises again, this cycle repeats.
Disadvantages:
• Batteries are cycled, not regulated—does not allow the batteries to be properly charged.
• Generated power from the renewable energy is wasted while the relay is opened.
• The DC relay setpoints must be set much higher than required to ensure the DC relay
doesn’t connect or interfere with normal charging (from the battery-based inverter) and
any sell back voltage settings once the utility power returns.
• No temperature-compensated regulation while charging.
2.
DC diversion driven by DC controlled relays:
When the battery voltage rises above a
maximum setpoint, a battery voltage controlled relay is used to switch on a dedicated DC diversion
load to consume any excess power. When the battery voltage falls to the low setpoint, the dedicated
diversion load turns off. If the battery voltage rises again, this cycle repeats.
Disadvantages:
• Batteries are cycled, not regulated—does not allow the batteries to be properly charged.
• Dif
fi
cult to source and size DC diversion loads to absorb the full output of the renewable
energy source.
• The regulation setpoint must be set much higher than required to ensure the diversion load
is not always in “regulation”, and that it doesn’t interfere with normal charging (from the
battery-based inverter) or any sell back voltage settings once the utility power returns.
• Since excess power is regulated on the DC side, the battery-based inverter is required to be
always on, re-converting the renewable energy from AC back to DC where it is diverted—an
extra conversion step creates energy loss and there is an unnecessary use of the inverter.
• No temperature-compensated regulation while charging.
3.
DC Diversion Controller off the battery:
When the battery voltage rises above a voltage
regulation setpoint, the DC Diversion Controller sends excess current to a dedicated DC diversion
load to maintain the battery voltage. When the battery voltage falls below the regulation setpoint,
current is no longer sent to the dedicated diversion load.
Disadvantages:
• Dif
fi
cult to source and size DC diversion loads correctly. If the load is too small, it cannot divert
enough power from the source (wind, hydro, etc.), and the battery could be overcharged. If
the diversion load is too large, it will draw more current than the rating of the controller—
causing damage or causing the controller’s protection circuits to open the load.
• Multiple controllers are usually needed even for medium sized renewable energy systems
(i.e., a 4kW/48VDC system requires at least a 70-amp controller).
• The regulation setpoint must be set much higher than required to ensure the diversion load
is not always in “regulation”, and that it doesn’t interfere with normal charging (from the
battery-based inverter) or any sell back voltage settings once the utility power returns.
• Since excess power is regulated on the DC side, the battery-based inverter is required to be
always on, re-converting the renewable energy from AC back to DC where it is diverted—this
extra conversion step creates energy loss and there is an unnecessary use of the inverter.