The most likely and most common reason is low battery temperature t[°C].

The battery capacity measurement is performed under the following conditions:

• DOD on batteries set at 90% (10-100%)
• Battery temperature 25-30 °C
• Charging current 0,2 C
• Discharge current 0,2 C

Example of test procedure:

1. The inverter has the PV array switched off! (DC switch in OFF position), operating mode is self use

2. Fully charged (SOC=100%) battery T58 at two pieces (11,5 kWh) I set the discharge current about 9,8 A

(11 500 x 0,2=2300 : 236 (nominal battery voltage) = 9,8) and the constant battery temperature is in the range of 25-30 °C

3. I connect an appliance to the AC side that has a higher power than the discharge power of the batteries in our case a 2.5 kW heater (9.8 A - discharge current x 236 V voltage with fully charged batteries = 2313 W)

4. After the battery is discharged to SOC 10% (approx. 5 h), I will take a battery capacity reading which should be approx. 10,35kWh – 2,5% = 10,09 kWh – 500 Wh = 9,59 kWh (nominal capacity multiplied by DOD 90% – 2,5% loss on conversion from DC to AC - inverter self-consumption considering 100 Wh per inverter - Hybrid G4 (100 Wh x 5 h = 500 Wh))

For example, I can find this reading in the SolaX cloud in the "stats report" under on-grid daily yield and read the starting value when I start the test and the ending value when the battery has been discharged to 10%.

Are you planning to add a new battery to your existing system? Then you need to follow a few rules to add a new battery so that the whole system works properly.

Especially in the winter months, we can occasionally observe a sudden drop in the SOC capacity of batteries, for example from 30% to 15%, etc. This phenomenon can of course have many causes.

One and very common reason is that the battery does not perform full cycles in the winter months, i.e. it does not charge to 100% and then discharges to its minimum set SOC limit, but it works in so-called part cycles, i.e. the battery is charged by a few % and subsequently discharges to the set minimum SOC limit. These so-called part cycles have an effect on the final SOC calculation of the battery, as LiFePo4 technology is a very hard source and the voltage level between a fully charged and discharged battery is very low, see Attached chart:

It can be seen from the graph above that the SOC of the battery is calculated based on both the voltage and the temperature of the batteries, and during the use of the battery, both the temperature of the battery itself and the temperature of the environment change, and depending on this, the voltage on the battery increases or decreases, but the SOC may still remain the same when the battery is not in use. This can lead to an inaccurate actual battery SOC calculation. That's why it's good to run full cycles on the battery to correctly calculate the SOC at least several times a month.

In the winter, the energy produced from the sun is not very high, and the inverter in Self-use mode primarily covers household consumption and no longer has enough or no energy left to recharge its batteries. The batteries are still communicating with the inverter - they provide it with information about the SOC level, the voltage level, and thus the batteries gradually discharge even below the set SOC level. The inverter automatically recharges the batteries if their SOC drops to a critical level of 5% back to the set SOC level (eg 10%) and this deep cyclic battery draining can lead to a shortening of their life. In principle, we have three options for taking care of the battery so that this adverse phenomenon does not occur.

1. Increasing the minimum SOC value in self use mode from the default 10% to at least 20%, or you can also turn on battery charging from the network at night to a set level. In the set charging window for batteries, the consumption is covered from the network.
   

    

   

    

2. Switching the inverter to Back up mode, where the inverter behaves the same as in self use mode, but the batteries are only discharged to 30% SOC level. Optional setting of the charging window for batteries as in self use mode.
3. Recharge the batteries to at least 60% SOC level and turn off the batteries with their button + the main breaker on the battery. A battery SOC level of 60% and above is a safe storage value to wait out the adverse winter period, so that they can be used without problems in a more favorable period for PV. The inverter does not need to be readjusted in any way for use without a battery.

Note: Forced battery charging can be simply switched on in manual mode in the "forced charge" item, after recharging the batteries to the desired SOC level, switch back to self use mode.