Understanding the solar charge profile — absorb, float, and equalise stages — is essential for correctly configuring solar charge controllers to maximise battery life. In Indian solar installations, misconfigured charge profiles are the leading cause of premature battery failure. This guide covers each charging stage, correct voltage settings for lead-acid and lithium batteries, and controller configuration for Indian conditions.
Solar Charge Stages Explained
A solar charge controller manages three distinct charging stages based on battery voltage and current:
Stage 1 - BULK:
Panel delivers maximum available current
Battery voltage rises from discharged to absorption setpoint
Duration: Depends on battery state and panel size
Stage 2 - ABSORPTION:
Voltage held constant at absorption setpoint
Current gradually decreases as battery fills
Duration: 2-4 hours (adjustable on quality controllers)
Stage 3 - FLOAT:
Voltage reduced to float setpoint
Very low current maintains 100% charge
Duration: Indefinite while sun is availableAbsorption Stage: Topping Up
The absorption stage fills the last 10-20% of capacity. The controller maintains a constant voltage while the battery draws decreasing current. Correct absorption voltage is critical:
| Battery Type | Absorb Voltage (12V) | Indian Summer (40degC) |
|---|---|---|
| Flooded lead-acid | 14.4-14.8V | 14.1-14.5V |
| AGM | 14.2-14.4V | 13.9-14.1V |
| Gel | 14.0-14.2V | 13.7-13.9V |
| LiFePO4 (4S) | 14.6V | 14.6V (no temp comp) |
Temperature compensation: -18mV/degC for lead-acid (above 25degC baseline). LiFePO4 does NOT need temperature compensation.
Float Stage: Maintenance Charging
After absorption completes, voltage drops to float level to maintain 100% charge without overcharging:
- Flooded: 13.2-13.6V at 25degC
- AGM: 13.2-13.4V
- Gel: 13.0-13.2V
- LiFePO4: 13.6V (or disable float entirely — LiFePO4 prefers not to be held at voltage)
Equalise Stage: Cell Recovery
Equalisation applies a controlled overvoltage (15.0-15.5V for 12V lead-acid) for 1-3 hours to equalise cell voltages in flooded lead-acid batteries. This causes controlled gassing that mixes the electrolyte.
CRITICAL: Never equalise sealed batteries (AGM, Gel) — the gas cannot escape and the battery will bulge or explode. Never equalise LiFePO4 — the BMS will disconnect at 3.65V/cell and equalisation serves no purpose.
Settings for Different Battery Types
Typical MPPT controller settings for Indian installations:
Flooded lead-acid (Exide/Amaron inverter battery):
Absorb: 14.4V | Float: 13.4V | Equalize: 15.0V monthly
Temp comp: -18mV/degC | Absorb time: 3 hours
LiFePO4 (DIY 4S pack):
Absorb: 14.6V | Float: 13.6V or disabled | Equalize: DISABLED
Temp comp: DISABLED | Absorb time: 1-2 hours
Low disconnect: 11.2V | Reconnect: 12.4VFAQ
My solar controller only has lead-acid presets. Can I use it for LiFePO4?
Only if it has a “User” or “Custom” profile where you can set exact voltages. Set absorption to 14.6V, float to 13.6V, disable equalisation, and disable temperature compensation. If the controller does not allow disabling temp comp, the voltage will drift low in summer, undercharging the LiFePO4 pack.
How often should I equalise my inverter battery?
For flooded lead-acid: once every 30 days, or when cell voltages differ by more than 0.1V. In Indian monsoon season (high humidity, more power cuts = more cycling), equalise every 2 weeks.
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