Today we are discussing the battery component of an off-grid solar electric system.
Batteries store the energy that is produced during the day by solar photovoltaic (PV) panels. The energy collected in the batteries is used during cloudy days and/or at night. Grid-tie systems do not necessarily require batteries, but can be used in emergency outage situations. (Day-use PV systems, such as water pumping are designed to be used during daylight and do not require batteries.)
*An important characteristic of all batteries is that they are potentially dangerous. Batteries are made up of dangerous chemicals and carry high voltage and currents. When handling batteries, practice safe methods and follow all safety precautions.
PV System Battery Types:
- Lead-Acid (most commonly used in home solar)
- Alkaline (typically not used in home solar – mostly used for small “solar gadgets”)
- Lithium Ion (most likely the future of battery storage for solar applications)
Here in New Mexico, the most commonly used battery type for home solar electric systems is the lead-acid battery.
Lead-acid is the oldest rechargeable battery in existence. These deep cycle batteries can be discharged down as much as 80% of its state (unlike car batteries). Because of its ability to recharge repeatedly, and its long life and relative lower cost, they are the most commonly used for Off-Grid PV applications.
Traditional lead-acid batteries are called “liquid vented” or “vented” or “open” because the battery volume is directly connected with the surrounding air and releases (or vents) produced gas to flow outside. Water is needed to be refilled periodically because it is lost as battery vents waste gasses. Deep cycle batteries will last longer if the battery does not completely discharge.
Sealed lead-acid batteries are also available, and have no caps to vent, but a valve (which excess pressure can escape to prevent overcharging). This is called Valve Regulated Lead Acid (or VRLA for short). They are spill-proof and don’t require as much maintenance. They can be safely transported and handled – therefore, a great choice for remote, RV, or boat PV systems. They must be charged to lower voltages and at a lower amp rate, however.
Lead-acid batteries use charge controllers to prevent discharging and overcharging. Charge controllers monitor battery voltage. Overcharging a battery causes liquid electrolyte loss, thereby increasing maintenance needed and shortening the battery lifespan. Deep discharging of a battery also contributes to a shorter battery lifespan. An off-grid charge controller may have a low-voltage disconnect which can help prevent that from happening.
How does Low Voltage Disconnect Work?
Low Voltage Disconnect (LVD) for a charge controller works as an automatic disconnect of non-critical loads from the battery when the voltage falls below a predefined threshold. It will automatically reconnect the loads to the battery when it is being charged. It will extend the battery life by preventing over-discharge and protects equipment from operating at potentially low voltages.
Some controllers have a “load” or “LVD” which can be used for smaller loads, such as small appliance and lights. These would be considered “non-critical” loads, so the terminal will shut off to keep the battery from discharging too far. Most systems do not need the LVD function, it can only drive smaller loads, and is most often used in RV and remote systems.
It is important to remember that even though off-grid homeowners may have a charge controller with an LVD, there still is a required management for battery loads. LVD protects overcharging from DC loads only, not AC loads.