The SK48v100 is compatible with select inverters in a closed-loop configuration where the battery programs the correct operating parameters to the inverter via CANBUS / RS485 communication as covered in Chapter 4. While this mode of operation is the most simple and precise control method, it is not a solution for everyone.
For older inverters not featuring CANBUS / RS485 communication, or loads powered directly by DC, the batteries can still be used by programming some simple setpoints. The exact terminology and setpoints of each inverter will vary, but can easily be adapted to match the parameters listed below.
Standalone Operation Charge Setpoints
- Charge Current for normal use: Up to 63A (Per Battery)
- Charge Current for fast charge: Up to 95A (Per Battery)
- Absorption Voltage: 57.6v recommended; up to 58.4v max
- Absorption Time: 15 Minutes
- Float Voltage: 55.2v
- Charging Target: 100% SOC
- Low Voltage Shutdown: 48v
There are two setpoints here for certain charge applications.
For example, if you are running a generator and have ample charging capacity, you are better off running a higher charge current to get the batteries topped off as quickly as possible. Fast charging will reduce the runtime of the generator, leading to decreased wear & tear as well as increased fuel efficiency. The increase of wear on the cells by using a higher current when generator charging is miniscule compared to the increased wear & maintenance on a generator from additional runtime, that your money is better spent fast charging.
If you are providing normal everyday charging with solar, that’s when it would be better to reduce the charge current closer to the target of 63A or less. Areas having more than 4 hours of sun exposure can charge as low as a 25A (C/4) rate and still reach a full charge while experiencing minimal wear.
Not every charger allows you to dial in the exact voltage you need. Some only allow for coarse adjustment via DIP switches, as an example.
57.6v is the most optimal absorption voltage, however if the charger doesn’t let you program this voltage, anywhere up to 58.4v is still safe for the pack.
If any cell within the pack reaches a full charge too quickly (due to being out of balance), the BMS will restrict charging to the entire pack to allow that cell to be discharged by the BMS (balanced), and when the cell is back within a normal range the BMS will stop restricting charging. The thing is, if absorption stops too soon, then the other cells may not be fully charged because charging has ended by the time the BMS has corrected the cell balance issue. By setting a 15 minute absorption duration, we allow some time for the BMS to provide balancing, but still have charge current available on demand the moment the BMS re-enables the charging circuit.
The float voltage is not actually a charging voltage; it is a voltage that can be provided by a charger that matches the resting full-charge voltage of the battery. Some chargers allow float to be disabled, however we do not recommend doing so. By leaving float enabled and set at the recommended voltage, the charger (whether solar or grid) will provide the power to loads FIRST, and the battery will make up for any additional current demanded by the loads. Floating in this manner will prevent discharging the battery while ample charge is available – whether coming from Solar, Grid or Generator. If floating was disabled or set to a lower voltage, then the battery could be significantly discharged when the user thinks that it’s been connected to a charger and should be full and ready to go.
This is contrary to the float-charge of a lead acid battery, as a lead acid battery will self-discharge when sitting and must be continually be fed charge current to keep it topped off.
A common misconception about LiFePO4 (LFP) is that it is best to operate in the 30%~90% SOC region, however this applies to every other lithium based chemistry except for LFP. If you only charged these batteries to 90%, then the BMS would never have a chance to offer any cell balancing due to the stable nature of LFP’s charge curve. If cells get out of balance, then the capacity of the entire pack is reduced to the capacity between the fully-discharged voltage of the lowest cell and the fully-charged voltage of the highest cell. If packs are not regularly balanced by the BMS, then the difference in balance can easily overcome what the BMS is able to correct, leading to quite poor performance of the system in general.
Common cells such as the NiCoAl (NCA) cells used in common EV’s are subjected to increased stresses when above 90% SOC that due to their construction, LFP cells are not.
Low Voltage Shutdown
We suggest stopping discharge at 48v. The battery won’t cut off until 46v, however if you set your low voltage shutdown at 46v, then there is a strong chance that the battery will shut down before the inverter/load, which can cause a situation where the battery is difficult to get restarted. Some chargers require presence of battery voltage on the terminals before they will turn on and begin charging.