Energy management of the battery

The energy management of the battery is performed with the “SOC for backup” mainly. In the advanced setting the “SOC for grid feeding” is also available.

The default values are 100% for SOC for grid feeding, 20% for SOC for backup with lithium and 50% soc for backup with lead acid batteries. 

Comments about the State of Charge (SOC) for grid feeding

The principle of the SOC for grid feeding is that if the SOC is higher than this threshold, the battery is discharged in the grid (if grid available and grid feeding allowed). After some time, the SOC will be at the setting value and there will be no grid feeding from the battery anymore. 

The SOC for grid feeding can be used for 

• Buffering peak solar production when grid feeding power is limited. 
• Discharging the battery voluntarily for tests by a manual change of the parameter.
• Keep the battery at a lower SOC than 100% without losing the energy production

If the SOC for grid feeding is 100%, the battery voltage is maintained at the target voltage of the cycle (for example absorption voltage). 

When discharging the battery, the low boundary for voltage will be limited to undervoltage level +2% higher. That means the battery will go down to the SOC you adjusted but keeping that minimum voltage to reduce the discharging current. 

The SOC for grid feeding must be set higher than the SOC for backup.

Battery cycle for lead acid battery

The next3 is a fully automatic solar and grid charger designed to guarantee an optimum charge for most type of batteries: lead/liquid acid, lead/gel, AGM batteries or Lithium. The battery charger enters automatically into operation as soon as the irradiation is sufficient, and the photovoltaic panel voltage is sufficient. 

The charging from the grid/genset is performed according to the AC energy management settings. When charging from the grid/genset, the next3 follows the same charging cycle as the solar.

The batteries can be fully charged by the successive phases 1 to 4 described hereunder:

Bulk phase 

The bulk phase is the stage where the next3 applies the maximum charging current (if there is enough energy available on solar and/or AC source) to charge the battery. This will lead to an increase of the battery voltage up to the next phase voltage limit; absorption, equalisation or floating, depending on the charging profile adjusted.

The bulk phase will allow a quick charge thanks to the high current. For lead batteries, this phase will charge them up to 90% SOC.

It is important that the maximum battery charge current is set according to the battery specifications to prevent damaging them. This current can be limited with the setting "Charging current limit". The maximum charging current might not be reached due to diverse conditions like the solar irradiation is not enough in an off-grid system, or the available power from AC source is too low, or the ambient temperature is creating a derating on the power electronic, etc...

Absorption phase

This constant voltage phase, mainly used in lead batteries, allow to charge the last percents of the batteries. Because of keeping the voltage stable and the battery accepting less and less energy, the charging current will diminish progressively. 

It can be ended by time (if there is enough energy to keep the phase for longer periods) or by current (if the battery ends his charge before the adjusted time)

Be aware that due to the current reduction during the phase, the power required to charge the battery will also be reduced. This can cause a reduction of the PV production if the excess energy is not used for other purposes than for charging the battery.

Floating phase

When the battery is charged, a constant voltage is applied to the battery to keep it full and compensate his self-discharge.

Equalization phase

Some types of battery need equalization in order to avoid the stratification of the water and acid they contain.

This phase is allowed only for flooded/wet batteries with liquid electrolyte. During this phase, the charging voltage target is temporarily higher. It allows, on one hand, to equalize the electrolyte density (stratification control) and, on the other hand, to equalize the voltage among the cells in series/parallel of the battery bank. During this process, the charging current can be limited by parameter “equalization current”.

By default, the equalization phase is forbidden because it is incompatible with gel and AGM batteries and these are the most used batteries in the field. It can be activated/deactivated by the dedicated parameter in the battery cycle settings.

In a general manner, lead batteries charging profile consist of 3 to 4 phases while the lithium only need 2; bulk and floating.

When connected to a communicating lithium battery BMS, the charging profile is given by the BMS and cannot be adjusted in the next settings.

For more information, contact your battery supplier who will inform you on the values to be applied for his products.

Temperature compensation

For non-communicating battery (no BMS) with a nx-tempSensor, the voltage adjustment levels for charging the battery (absorption, equalization, floating…) are automatically corrected in real time according to the battery temperature.

The value of this compensation is given in V/°C for a reference temperature of 25°C by a parameter. Default value corresponds to -3mV/°C/cell which is -0.072V/°C for a 48V battery. For example at a temperature of 30°C, the voltage compensation is: (30-25)*(-0.072) = -0.36V. For a floating voltage value set to 54.4V, the effective floating voltage (compensated) will be 54.04V at 30°C.

Another example with 5°C, the compensation will be (5-25)*(-0.072) = +1.44V, so a floating voltage that goes from 54.4V to 55.84V.

SOC for end of discharge

To prevent a stop/disconnection of the battery by the BMS that would require a manuel reset or that would definitely block the system, a SOC for end of discharged can be chosen. That way, the next3 stops to discharge the battery before the signal of the BMS and before the opening of the BMS contactors that would completely unpower the whole system. The next day, or when the grid/genset or the sun are back, it is possible to recharge the battery and recover. 

An error is set if the SOC is lower than this value. The discharge of the battery is prohibited when the error is set but the charge is still allowed. The error is reset if the SOC is greater than or equal to the SOC for backup or if the bit "SOC for end of discharge" in the property: “Conditions for energy management” is not set.

By default, the function is deactivated for non-communicating batteries and activated with an initial value of 15% for communicating batteries.

Adaptative SOC for backup

The goal of this function is to prevent the battery to stay at a low state of charge during a long period of time and to avoid that the inverters are disabled due to an unwanted undervoltage. The lithium batteries are managed by the SOC given by the BMS of the battery manufacturer. One point recurrently observed in practice is that the SOC is not always accurate. It can drift and recalibrations are often done at 100% SOC when the BMS is sure that the battery is full. In practice, there are undervoltage problems when batteries are cycled at low SOC without reaching 100% regularly. That may be the case per example in self-consumption systems during the winter when the solar production is low.

To cope with this problematic situation, an advanced adaptative algorithm has been developed.

The adaptive SOC function is enabled/disable in the advanced battery menu with « Adaptive SOC for backup » (Y/N). If the function is enabled, the adaptive SOC for backup is:

• increased every day if the SOC has been <  «SOC to increase adaptive SOC for backup » during the day. The increase step is set via the value: « Adaptive SOC for backup slope». The slope is given in %/day; per example 5% per day is the default value.
• reset to its initial value: « SOC for backup» if the SOC is reaching more than « SOC to reset adaptive SOC for backup » for more than « Time before resetting adaptive SOC for backup ». This value is used to set a minimum waiting time with a fully charged battery before resetting the adaptive SOC for backup. Typically, 5minutes (300 seconds) at 99%.
• The adaptive SOC for backup pushes the « SOC for gridfeeding» and the « SOC for end of charge» upward for proper operation when it gets to the same level.
• The adaptive SOC is increased by a value « Adaptive SOC for backup undervoltage increment» if a warning or an undervoltage error has been detected. This prevents to turn off the inverters due to a low battery voltage only because SOC calculation drifted.

The following graph illustrates the behaviour: