LFP Charging Guidelines

Rolls R-Series, G-Series and S-Series drop-in LFP batteries may be charged in a 1, 2, or 3-Stage charge profile, shown below. The specifics and recommended setpoints for these charge regimes are explained in this article, with the recommended charge profile being a 2-Stage charge.

Representation of recommended 2-Stage Constant Current/Constant Voltage (CC/CV) charging. The dotted line represents the transition from CC to CV when the voltage limit is reached. Once the battery reaches the constant voltage limit, the battery is held by the charger at this voltage until the current decreases to 0.05C. At this point the battery is at 100% state-of-charge (SOC). Any current continuing to flow in the float stage is attributable to balancing activity of the BMS.

CHARGING SOURCE: LEAD-ACID BATTERY CHARGER

Customers may choose to replace lead-acid batteries with lithium models. Most lead-acid battery chargers may be used to charge Rolls R-Series, G-Series, and S-Series drop-in LFP models if the charger is properly configured to operate within recommended charge current and voltage limits.

The pre-programmed voltage settings for AGM or OPzV GEL models may be in line with the recommended drop-in LFP charge voltage settings and can sometimes be used if direct voltage control is not possible for your charger. However, flooded batteries often require higher charge voltage settings. If left configured for charging flooded batteries, the higher charge voltage can trigger the BMS to restrict charging to protect the battery, effectively resulting in a 1-stage charge. If this occurs repeatedly, or the charger cannot be configured at a lower charge voltage, it may be necessary to replace the charger for optimal balancing.

1-STAGE CHARGING – CC (CONSTANT CURRENT)

When charging with a single-stage constant current charger, charge at the recommended charge current, by operating temperature, until the battery reaches its termination voltage.

NOTE: 1-Stage CC Charging may be required if charging from a source which is not efficient to run at lower power, like a generator. However, it may only charge the battery to 90-95% SOC and provide very little time to balance cells. For these reasons, 2-Stage CC/CV charging is recommended to ensure the battery reaches full SOC.

2-STAGE CHARGING – CC/CV (CONSTANT CURRENT/CONSTANT VOLTAGE)

When charging with a two-stage constant current/constant voltage (CC/CV) charger, charge at the recommended charge current, by operating temperature, until the battery reaches the “absorption” voltage or constant voltage (CV) limit. The charger then holds the battery at CV until the charge current decreases to ≤0.05C (termination current).

The recommended absorption (constant voltage) voltage is shown below. If the charger has a pre-set voltage setting or cannot be programmed, an absorption voltage in the range below is also acceptable. Note: lower voltage will lead to longer charge times.

NOTE: If charge time is not a concern within your system architecture, reducing the absorption voltage will increase charge time, but allows the BMS more time to ensure all cells remain balanced. As batteries age, small changes in manufacturing or due to uneven wear may present themselves, requiring more time to maintain balance.

3-STAGE CHARGING - LEGACY LEAD-ACID SYSTEMS, INVERTER/CHARGER HARDWARE

When programming an inverter/charger or charge controller equipment using a 3-stage charge sequence (2-stage with an additional “float voltage” after the charge is terminated), the following charging parameters should be programmed to properly charge Rolls R-Series, G-Series and S-Series LFP batteries:

Temperature Compensation: If the inverter/charger or charge controller uses temperature compensation this should be turned off when charging Rolls R-Series, G-Series and S-Series models. Turn off the temperature compensation settings and disconnect the sensor to ensure the correct voltage regulation from the charging device.

Equalization: Equalization should never be used; elevated charge voltages are unacceptable for LFP batteries and will simply lead to the BMS disconnecting the charging path. It should be turned off, or the equalization voltage setpoint should be reduced to the appropriate system float voltage, above.

Some charger models may require additional firmware, programming, or parameters. Please contact your inverter/charger or charge controller manufacturer for assistance with these settings, if required.

CHARGING TEMPERATURE

Due to the chemistry of LFP cells, these batteries cannot accept high charge current at low operating temperatures without cell damage and permanent capacity loss.

Rolls R-Series & G-Series drop-in LFP batteries may be safely charged between 0°C to 55°C (32°F to 131°F). However, because cycle wear is accelerated below 10°C (50 °F) the charge should be limited to 0.2C (20% of battery capacity) for optimal longevity. Similarly, at high temperatures, charge current should be limited to ≤0.2C when operating at temperatures from 35°C to 55°C (95°F to 131°F) as noted below.

Rolls S-Series drop-in LFP batteries may be charged at lower temperatures due to their internal heating which allows them to compensate for external temperatures down to as low as -20°C. If the batteries are typically charged at low temperatures [-20-0°C (-4-32°F)], proper insulation is required to keep the batteries at their warmed temperature.

To maintain optimum performance and durability of Rolls R-Series LFP batteries, the following charge current limits should be followed:

NOTE: Due to the internal chemistry, LFP batteries can be discharged at lower temperatures than they may be charged. So, at low temperatures between -20°C and 0°C, R-Series and G-Series drop-in LFP batteries will still be effective in discharging, but no energy may be put into the batteries.

The recommended and maximum continuous charge current is specified for each Rolls R-Series and S-Series drop-in LFP model as a function of capacity, cell and internal BMS.