VRLA Basics (Quick Refresher)

• What “VRLA” means: A sealed lead-acid battery with a pressure-relief valve that limits gas escape and prevents outside air from entering.
• Electrolyte handling: Oxygen recombination inside the cell minimizes water loss, so routine topping-up is unnecessary.
• Still needs ventilation: Overcharge can vent hydrogen—never locate batteries in an airtight enclosure.

Constructions Compared

AGM (Absorbent Glass Mat)

• Separator: Fine glass-fiber mat pressed tightly between plates; it absorbs electrolyte.
• Electrolyte fill: Not fully saturated—kept slightly under-filled (≈5–10%) to leave “oxygen channels” for efficient recombination.
• Implications: Low internal resistance → strong high-rate performance and lower voltage drop during discharge.

Gel (Gelled Electrolyte)

• Separator scheme: Plates are enveloped (often with a thin “leaf” separator); the electrolyte is immobilized as a gel.
• Gel pathways: Micro-voids in the gel still allow oxygen to reach the negative plate for recombination.
• Implications: Higher internal resistance than AGM but better thermal coupling from plates to case.

Performance: Capacity & High-Rate Discharge

• Capacity (same size & plate set): AGM typically delivers about 7–10% higher capacity than Gel due to higher electrolyte SG and quantity.
• High-rate output (e.g., 10–20-minute runtime): AGM often provides notably longer runtime (≈40%) thanks to lower internal resistance.

When high-rate power density matters (e.g., UPS ride-through), AGM is usually preferred. Gel can still serve but may be chosen for other environmental advantages (see temperature & life).

Temperature, Float Current, and Thermal Behavior

• Float current vs. heat: AGM’s more efficient recombination and lower resistance tend to draw higher float current than Gel at a given float voltage, which means more internal heat.
• Thermal dissipation: Gel has tighter thermal contact between plates, gel, and case, improving heat conduction by roughly ~15% compared with AGM.

Practical setup tips

• Provide air gaps (~0.5 in / 12–13 mm) between units for cooling.
• Lower float voltage in warm environments to reduce float current and internal heating.
• Watch for conditions that can trigger thermal runaway (high ambient temp + high float voltage + restricted cooling).

Life Expectancy: Float vs. Cycle

Float service (standby)

• At 25 °C (77 °F) and proper charger settings, both AGM and Gel can achieve rated float life. Life depends on:
  • Plate design (thickness, alloy), separator design, electrolyte SG, and overall thermal environment.
  • Good installation and initial commissioning.

Cycle service (repeated charge/discharge)

• Deep-cycle use: A specialized Gel “Type B” design with reinforced separators (e.g., glass-mat retainers) and phosphoric acid additive:
  • Offers up to 2× the cycle life of AGM or standard Gel (“Type A”) in deep discharges.
  • May show ~10% lower initial capacity, typically recovering after ~20 cycles.

Rules of thumb

• For frequent deep cycling (mobility devices, PV storage), Gel Type B is the durability pick.
• For lighter cycling or standby with occasional high-rate discharge, AGM is often optimal.

Selection Guide (Cheat Sheet)

Always confirm charger profiles and float/equalize setpoints with the battery’s datasheet.

Installation & Operation Best Practices

1. Float voltage per spec, temperature-compensated. Over-voltage accelerates corrosion, increases gas, and heats the battery.
2. Thermal management. Space units, avoid hot spots, and ensure room ventilation. Consider derating float voltage at higher ambient temps.
3. Commission correctly. Proper initial charging and verification improve early-life capacity and longevity.
4. Maintenance checks. Even “maintenance-free” VRLA benefits from periodic inspections for swelling, temperature anomalies, terminal torque, and charger logs.
5. Safety. Never enclose batteries in an airtight box; design for hydrogen dissipation during fault/overcharge events.

Conclusion

AGM and Gel VRLA batteries share core safety and maintenance advantages, but they excel in different niches. Choose AGM when high-rate output is paramount and thermal conditions are controlled. Choose Gel (especially Type B) when deep cycling or heat handling is the priority. Match the chemistry to the job, manage float voltage and temperature carefully, and you’ll get reliable service life.