How to choose a LVD for solar PV?
The low voltage disconnects, or LVD as it's also known, is a device that protects your batteries from being ruined by discharging too much.
They are designed to cut off the current supply when it can no more sustain a certain level of voltage.
The LVD prevents our expensive electronics and appliances from being broken due to excessive discharge of power because they prevent electrical flow below certain voltages, typically around 10%.
This helps extend the product life span while still providing good performance at all times.
A Charge Controller that protects your battery from overcharging and a Low Voltage Disconnect will protect it against low voltage. Together they are absolutely necessary for the lifetime of your batteries to be prolonged.
Important
Lead-acid batteries are most cost-effective when you don't use more than 1/3 of their rated capacity.
This corresponds to a resting voltage of about 12.35 V, but the LVD setpoint will be lower because it bounces back after some time without loads attached.
Many people suggest a maximum DoD of 50%, which corresponds to a resting voltage of around 12.2 V. This would be much more expensive in the long run, but it is sometimes more feasible.
All inverters have some kind of LVD built in to prevent them from running on too low a voltage, but these features are often not adjustable or do not have enough range for proper protection of the batteries.
This means you will almost always need an LVD if you are concerned to protect your batteries.
What to Look for picking a LVD?
The LVD is connected between the battery and the loads to automatically disconnect your devices when a low voltage is detected. It must be rated for both the nominal voltage of your battery bank and the maximum current that your system can draw.
For small 12V systems, there are devices are available that come in a range of 10A-50A.
For higher amperages or voltages, a relay is typically used in combination with an electronic voltage display. Since the relay will still draw some current when it is turned on, this is not recommended for small systems.
The relay will have a peak and continuous rating. The maximum amperage the relay can handle is called its “peak” while the minimum it can carry without tripping is known as its “continuous.”
Your load in watts determines what you should be looking for on your specific model of relays.
The formula is (W)/(V) =(A).
Calculate the peak and continuous amperages of your system to provide a safety buffer.
Divide the expected load in watts by battery voltage, which will give you amps. Add about 20% more power for the best results.
Your inverter may have a user-configurable LVD built-in, but the voltage setting is crucial. If your system will only be running ac loads (no DC), you can use this feature if it's set to an appropriate voltage.
Choose an UL listed & CSA certified
One of the most important aspects when installing your solar panels is choosing a low voltage disconnect.
Along with being UL listed and CSA certified, this device should be rated for off-grid applications as well as those on-grid power to ensure that you can use it in either case without worrying about interrupting service or damaging equipment.
It's also recommended to choose an item from one of the top brands such as Schneider Electric so that you have peace of mind knowing they are reputable manufacturers who will offer high-quality products designed by experts in their field.
Automatic 12V/24V battery detection
When you're designing a home solar system, choosing the right low voltage disconnect is vitally important.
Choosing one that has Automatic 12V/24V battery detection can save time and money on maintenance over the long run.
Audible or visual alarm output activates before the disconnect
A low voltage disconnect has an audible or visual alarm output that activates before the device is even disconnected.
User selection for disconnect thresholds
The low voltage disconnect should offer users multiple thresholds to choose from, such as 9.3~12.1V for 12V battery and 18.6~24.2V for 24-volt.
Setting the disconnect voltage
The resting voltage of 12.35 V is the target to which our battery should settle after a few hours without any charging or load on it, but when measured at the terminals when supplying current to loads, it may be lower and difficult to predict accurately.
Because higher amperages cause voltage sag, your system won't always cut off at the same charge level if your loads aren't consistent.
All you have to do now is accept it. The resistance of wires between an LVD sensor and a battery amplifies the drop in voltages; as a result, it's best to place the LVD as close to the batteries as possible while using oversized cables to get more reliable results when these variable load conditions arise.
As a result, the only way to get it “right” is to set up and test your system. The LVD voltage should be estimated at 12.0 V as a starting point. (If your device is 24-V or 48-V, multiply the voltages by 2 or 4.)
Set the LVD for this, detach the solar panels or charging circuit, and operate the device with all regular loads attached until the battery discharges and the LVD turns off the loads.
Wait 2 hours without charging or discharging before measuring the battery voltage.
If your calculated voltage is now too high in comparison to your goal of 12.35 V, reduce the LVD by the difference.
Similarly, if necessary, go up. Connect the solar panels to the batteries and charge them. Repeat the process until you get that right.
