Equipment Self Consumption

An important consideration regarding LVD/ LSOCD settings is the self-consumption of the solar charge controller(s), load controller(s) and any other devices in the system that are not disconnected when there is an LVD. After there is an LVD the self-consumption will continue to drain the battery. This is not much of a concern for higher LVD/ LSOCD settings or larger battery banks where there is a relatively low self-consumption. 

If LVD is set too low, it can reduce battery life and the additional self-consumption after LVD will only make it worse. This can occur when the LVD setting is too low and/or the self-consumption is relatively high in comparison to the battery bank. It is especially important that the LVD setting isn’t so low that the battery is over discharged overnight and negatively impacts the health of the battery. There are also circumstances that might lead to loss of solar production for longer periods. Conditions like snow or solar controller failure would be examples of such circumstances. If the battery drains below the minimum operating voltage of the solar controller, it will not be able to recover without voltage being applied onsite and thus likely cause permanent degradation or damage to the battery bank. 

Self-consumption can be a bigger problem with lithium batteries that are set with LVD settings that are too low. Often lithium battery manufacturers provide a minimum LVD setting that can leave the battery at ~ < 2% SOC. These settings can only be used when the self consumption is negligible. When a lithium battery reaches a very low voltage, the BMS will disconnect the battery internally. Depending on the amount of self-consumption, 12.8V nominal LiFePO4 lithium batteries with LVD settings below 12.6V may be at risk of a low voltage BMS disconnect. Therefore, LVD settings of 12.7V (12.8V LiFePO4 batteries) or higher are recommended to prevent this–especially for remote sites or sites that are not being monitored. Not only is this detrimental to the health of the battery, but if it lasts more than ~ 30 days, it may not be able to recover from a BMS low voltage cut-off at all. 

N-Load Control for Prioritized Loads and Multiple Load Circuits 

Some off-grid PV systems include loads that are more critical than other loads. One example is remote communications devices that the user would not like to disconnect during an LVD event so they can continue monitoring the system. This is where prioritized load control can help. By disconnecting the non-critical loads first, the critical loads can continue to operate longer without interruption. This is also referred to as “N-Load Control” or “load shedding”. The critical load will need lower LVD/LSOCD settings and/or a longer Warning/LVD delay setting than the other less critical loads to achieve this.

Besides prioritized load control there are other reasons to provide separate load control for more than one load circuit in the system.

  • To provide additional load control in case the sum of all loads exceeds the load control current rating of the controller
  • To provide load control for a very large load or a load with a high inrush current independently from other loads.
  • To provide scheduled, manual or automated load control functionality for specific load(s)
  • To provide High Voltage Disconnect (HVD) protection for voltage sensitive loads but not for other non-sensitive loads.

Morningstar provides several options for implementing load control for these applications. The best solution depends on the application and system configuration.

The Relay Driver (RD-1) includes 4 relay driver channels so it can control two or more separate load control circuits with external relays. This is a great option when the solar controller(s) in the system do not include built-in load control (often with TriStar and TriStar MPPT solar controllers). The RD-1 uses Voltage Threshold settings to implement LVD/LVR. The remaining RD-1 channels can also be used for  other applications for the system, such as automatic generator startup control, cooling, Fault-Alarm notifications etc…. 

An alternative to the Relay Driver would be to use two separate load control devices. On the one hand, the TriStar controller provides data monitoring of the current and Ah for the load circuit that you do not get with the Relay Driver. However, using two or more TriStar controllers for load control will cost more, take up much more space and have higher self consumption than the RD-1 with relays. Also, the high current ratings of the TriStar can be overkill for smaller critical load circuits. Using smaller, lower cost solar controllers with built-in load control without using the solar controller function is another option for 12V and 24V systems. 

If there is more than one controller in the system that has built-in load control, they can be set with different load control settings without having to add additional load control devices. If there is only one solar controller with built-in load control in the system a relay driver, TriStar PWM or a second controller with built-in load control can be set up for prioritized load control. 

The GenStar MPPT is an Integrated Series product. Therefore, is it compatible with the ReadyRelay Integrated Series accessory, making it possible to add load control circuits in addition to the GenStar 30A built-in load control. The ReadyRelay includes two relays each rated for 6A @30Vdc, 500mA @60Vdc or 6A @250Vac. These relays can be used as contact relays for remote switches, or with small loads (12V, 24V or AC loads) or larger loads with external relays. Like the relay driver, the second relay can also be used for other applications in the system at the same time. In addition to Prioritized Load Control settings, the GenStar can also be set up with weekly schedules for ON/OFF load control. For more information see the GenStar MPPT and ReadyRelay manuals on the Morningstar website.