Recommendations on category G4

Test procedure for category G4:

IEC61427 plus Qualibat (Combined test procedure, ageing ratio 1:1.5)
See Testprocedures and ref. [3])
As long as no results are available for this combined test procedure, select the battery that has the best results (longest life time) for the two separate test procedures.

Selection of battery and charge controller for category G4:

• It is essential to use a charge controller that protects the battery from overcharging. The recommended end-of-charge-settings of the controller depend on the type of battery and the category of battery usage. The recommended batteries are given below, in order of preference, together with the recommended voltage settings for the charge controllers at charging.

  Recommended battery in order of preference	Controller settings(in V/cell at 25°C)	maint. months
  Flooded, tubular plate with electrolyte mixing	Charge: 2.4 V( 2h / day) Float 2.35 V equalisation: 2.55 V (8h / 2 weeks)	6
  VRLA, tubular plate with electrolyte mixing	Charge: 2.45 V (2h / day) Float 2.35 V equalisation: 2.5 V (10h / month)	6

• It is essential to use a charge controller that protects the battery from deep discharging. The control algorithm of the charge controller can be based on a SoC calculation or on voltage settings. The recommended end-of-discharge-settings of the controller are the following:
  • Low voltage Disconnect, for discharge current I10: 1.80 to 1.85 V/cell
  • Low voltage Disconnect, for discharge current 10% of I10: 1.95 to 2.0 V/cell
• If the battery will be operated outside the temperature range of 20-30 degrees it is preferable to use a charge controller that compensates for the effect of the battery temperature. It will typically adjust settings to ensure that charge and discharge cycles are modified based on temperature and voltage or state of charge. A separate battery temperature sensor should be used.
• It is preferable to use a charge controller with separate voltage sense terminals.
• In a system with a high charge factor and a long average time between full charge, one can consider to replace part of the storage capacity with a back up generator.

More specific recommendations for category G4:

The main battery issues of category G4 are operation at low state of charge, long time between full charge and high partial cycling intensity despite a high charge factor. Such conditions can only exist, if there are distinct differences in operation, e.g. a very high charge factor when the battery is full anyway and very poor operating conditions with cycling at low state of charge and infrequent full charge. To improve battery performance, both conditions have to be addressed.

To improve the low state of charge situation:

1. Apply the recommended values for the setpoints of the charge controller in the table above.
2. Use a charge controller with a low self-consumption and with a high efficiency.
3. Reduce the load (e.g. by replacing appliances by more efficient ones).

For flooded batteries with a charge factor of intensity level 5 and VRLA batteries with a charge factor of 4 or 5 measures should be taken to reduce the charge factor such as:

1. Optimise the orientation (tilt and azimuth) of the PV-module for the period of low irradiance (e.g. winter).
2. If a backup-generator is installed, consider reducing its operating time and making sure that the generator is not operated in the morning to increase the state of charge of the battery. PV production during the day will then likely lead to this considerable overcharging.

Particularly in a high temperature environment there should be concern about the battery thermal protection. The battery thermal control management particularly of the auxiliary cooling system may be necessary due to the high charge factor possibly leading to an overcharge requiring an extensive heat dissipation. Attention should be paid also to low environmental conditions that would require thermal protection against the electrolyte freeze.