Sunsynk Battery Charge and Discharge Rates in South Africa

Key Takeaways

• Your Sunsynk inverter's maximum current rating often dictates the battery's charge and discharge rates, even if the battery itself can handle higher currents.
• Correct battery cable sizing is critical for safety and performance, with 25mm² copper cables having a maximum safe limit of 122.5 A.
• Cold temperatures, particularly below 10°C, will significantly reduce or completely stop your battery from charging, though discharge may still be possible.
• An F51 'Batt Temp High' fault on L/G Series batteries indicates low temperatures, not high, and requires warming the battery above 5°C.

The Inverter's Limit on Battery Charge and Discharge Rates

When setting up or troubleshooting your Sunsynk energy system, it is important to understand that the maximum rate at which your battery can charge or discharge is often limited by the inverter, not solely by the battery's specifications. Even if your battery bank is capable of delivering or accepting a high current, the inverter will cap this at its own maximum current rating.

Sunsynk states that "When selecting the charge and discharge current limits you will always be limited to the lowest current value whether that is the inverter or the batteries." This means that a 3.6 kW single-phase inverter, for example, has a maximum charge/discharge rate of 90 A, regardless of whether you have one or two batteries connected that could theoretically handle more.

To calculate the maximum charge/discharge power of your battery, you multiply the current in Amperes (A) by the Battery Management System (BMS) voltage.

The following table outlines the maximum charge/discharge rates for various single-phase Sunsynk inverters:

When configuring your inverter settings, ensure the battery type is set to "Lithium" and that "Activate Battery" is checked. The "Battery Capacity" setting should correspond to the number of Sunsynk batteries used, typically "x 100 Ah (for 5kW Batteries, bigger batteries will be more than 100A)." Similarly, the "Max A Charge" and "Max A Discharge" settings should be adjusted to "the number of Sunsynk batteries used x 50 A (for 5kW 0.5C Batteries, other batteries may vary)."

Sizing Battery Cables and Fuses for Safe Operation

Proper cable sizing and appropriate fusing are essential for the safe and efficient operation of your Sunsynk battery system. Incorrect sizing can lead to overheating, damage to equipment, and even void your warranty.

Sunsynk specifies that "The standard battery cables for Sunsynk batteries are 25mm² copper cables." These cables "can handle a maximum charge/discharge rate of 122.5 A." It is critical to note that "the current passed through them should NEVER exceed 122.5A. This can cause damage to the inverter and batteries and will void your manufacturer warranty." For larger inverters, such as the 8.8 kW model with a 190 A limit, 25mm² cables are insufficient, and heavier gauge cabling is required.

If you are making your own battery power cables, be aware that "these figures are accurate for the standard battery cable length of 1.5 metres." For longer lengths, you must "calculate your DC requirements for distance/ voltage / current capability / voltage drop." As a guideline, for a "51.2V nominal battery such as the W5.3," the voltage drop "should be no more than 2%."

Furthermore, "all interconnecting battery cable sizes are the same length as cables of differing lengths can cause battery unbalances."

The safety of your system also relies on correctly rated protective devices. "The fuse isolator/breaker between the inverter and batteries should also be appropriately rated for the passthrough of current." The recommended size for the DC breaker can be found in your inverter's manual. If you are using a busbar, "the busbar must be rated for the appropriate DC current being sent to/from the inverter."

Understanding and Preventing Unbalanced Batteries

Battery unbalance occurs when individual battery packs within a bank have different States of Charge (SOC) or voltages. This can reduce the overall performance and lifespan of your battery system. Sunsynk identifies several causes for unbalanced batteries, including "Voltage difference between batteries, insufficient charging (not reaching 100% every 7-10 days), or adding a new battery to the system."

To maintain accurate SOC readings and prevent unbalance, "it is strongly recommended that the batteries undergo a 100% charge at least once every 7-10 days." This allows the Battery Management System (BMS) to calibrate all packs.

When adding a new battery to an existing system, careful consideration is needed. Sunsynk recommends that "the additional battery's specifications closely match those of the existing battery, with a permissible deviation of no more than 5% on either side of the specified values." For older SunBatt Batteries, it is recommended that "the new Battery (or Batteries) become the new Master Battery with any additional new Batteries as Slave1 etc and the original Batteries becoming the last Slave / Slaves of the Battery Bank." However, for newer models like the L5.1, L5.3, G-Series, and W-series, Sunsynk Support can remotely update software, which may simplify the process. After connecting an additional battery, "it is essential that all batteries charge up to 100% to calibrate before being used in normal operation."

Cold Weather Effects on Battery Charging (South African Context)

South Africa experiences significant temperature variations, particularly in regions like the Highveld, Free State, and Eastern Cape highlands, where sub-zero nights are routine. These cold conditions can severely impact the charging performance of lithium batteries.

Sunsynk states that "The reduction in the battery charge rate initiates at approximately 10°C cell temperature." As temperatures drop further, "Even in temperatures of 0°C or below, the battery retains the capability to discharge to the load but is restricted from charging." This is a protective measure for the battery's cell chemistry.

To restore normal charging rates after exposure to cold, "the cell temperature must rise above 12°C." It is important to note that "ambient temperature may slightly differ from the cell temperature, and achieving a normal charge rate may require an ambient temperature of around 15°C." This means batteries installed in unheated garages, outdoor enclosures, or outbuildings in colder regions may struggle to charge during winter months. Conversely, in coastal areas like Durban or Cape Town, these low-temperature charging restrictions are far less likely to be encountered.

A particularly counter-intuitive fault code related to temperature is the F51 'Batt Temp High' fault on L/G Series batteries. Sunsynk confirms that "this error code is triggered when the batteries experience lower temperatures, primarily in negative temperature conditions. Although the fault is labelled as a TEMP HIGH fault, we can confirm that it is indeed related to colder temperatures." If you encounter an F51 fault on a freezing morning, the remedy is to "Allow batteries to return to operational state by ensuring temperature rises above 5 degrees Celsius, possibly using a heat mat."

V-Batt vs. BMS Voltage: Why They Might Differ

In your Sunsynk system, you might observe two different battery voltage readings: V-Batt (the voltage measured across the inverter's battery terminals) and BMS Voltage (the operating voltage reported by the battery's internal Battery Management System). Ideally, these should be very close.

However, "if there is a difference of 1.5V or larger between V-Batt and BMS Voltage," the inverter will act as a safety mechanism. In this scenario, "the inverter will stop the flow of current between the inverter and battery to protect against a possible system short or loose connection." This protective shutdown is crucial to prevent damage to your system. If you notice a consistent discrepancy of this magnitude, it warrants investigation by a qualified installer to check for loose connections or other system issues.

Recovering a Low Voltage Battery

If your Sunsynk battery has very low voltage, indicated by no lights on the front or an F56 fault, there are steps you can take to attempt recovery, provided the battery voltage is above 40V. If the voltage is below 40V, you should contact Sunsynk Technical Support.

Procedure for Low Voltage Battery Recovery (Voltage above 40V):

1. Switch Battery Settings: Access the inverter's battery settings and "switch to AGM V."
2. Adjust System Settings: Go to the system settings and change the mode to "Zero Export + Limit to Load Only" if it is not already selected.
3. Configure Grid Charge (if applicable): If you intend to use grid charging, ensure "Grid Signal" and "Grid charge" are checked. Set "Grid Charge in Grid start to 10%."
4. Set Low Voltage Thresholds: In the battery settings, change the "Shutdown and Low Battery to 38V" and the "Batt Empty V to 41V."
5. Allow Trickle Charge: "If you observe a trickle charge going to the battery, allow it to remain for an hour as the battery will be in protection mode until it surpasses approximately 5%, at which point it will begin to increase the charge."
6. Monitor and Revert: While in AGM V mode, "there will be no BMS so please ignore any readings on the BMS as these will not be accurate until put back into lithium battery mode." Once the battery has recovered sufficiently, switch the battery settings back to "Lithium."

What you can change yourself, and what you cannot

As a system owner, you can typically adjust settings related to battery capacity, maximum charge/discharge current (within the inverter's limits), and operational modes like "Zero Export + Limit to Load Only." You can also monitor for fault codes and perform basic troubleshooting like checking cable connections.

However, any work involving opening the inverter or battery enclosure, modifying DC wiring, or altering grid-protection settings must be performed by a registered person. In South Africa, such work requires a Certificate of Compliance (CoC) to ensure safety and adherence to NRS 097-2-1 standards. For instance, if the low voltage recovery procedure does not work, or if you suspect a loose connection causing V-Batt and BMS voltage discrepancies, it is time to call your installer.

What the published sources do not tell you

While Sunsynk provides comprehensive guidance, some details are not explicitly covered or may vary. The documents do not specify the exact "bigger batteries" that are "more than 100A" for capacity settings, nor do they detail the "other batteries" that "may vary" from the 0.5C charge/discharge rate. This means you may need to consult your specific battery's datasheet if it is not a standard Sunsynk 5kW unit.

The guidance on adding new batteries mentions that Sunsynk Support can remotely update software for L5.1, L5.3, G-Series, and W-series batteries, but it does not detail the process or requirements for this remote update. The documents also do not provide specific wiring diagrams for all possible multi-battery configurations, only stating that a busbar and individual battery breakers are needed for more than two batteries.

Finally, while the cold weather charging information is globally applicable, the original context of the "Winter Charging" article assumes an overnight cheap-rate tariff for grid charging, which is not a standard or widely available practice in South Africa. Therefore, relying on grid charging to warm batteries in cold South African regions may not be a viable strategy for most users.