AGM Batteries

AGM Batteries: What They Are

AGM (Absorbed Glass Mat) batteries are sealed lead-acid batteries where the electrolyte is absorbed in a fiberglass mat between the plates. They are maintenance-free, spill-resistant, tolerate vibration, and have low internal resistance for higher current delivery and faster charging. A 12 V AGM contains six 2 V cells; common capacities range from small 1–12 Ah UPS modules to 70–125 Ah automotive/marine sizes and larger deep-cycle options.

When to Choose AGM vs. Alternatives

• Choose AGM if you need reliable starting power, occasional deep cycling, safe indoor or mobile mounting, and simple maintenance with a moderate budget.
• Consider Flooded Lead-Acid if lowest upfront cost is the priority and you can maintain water levels and provide ventilation.
• Consider Gel for sensitive electronics at low currents and hot environments; note slower charge acceptance and stricter charging limits.
• Consider LiFePO4 for the lightest weight, 2,000–4,000+ cycles, and deep (80–100%) usable capacity; higher initial cost and requires a compatible charging system.

How to Choose an AGM Battery

1) Pick the right type

• Starting AGM (SLI): High CCA for engines; not designed for repeated deep discharge.
• Deep-Cycle AGM: Built for house loads, trolling motors, inverters; supports frequent 50% depth of discharge (DoD).
• Dual-Purpose AGM: Balanced cranking and cycling for small boats and RVs with limited space.

2) Size for runtime and loads

• Capacity (Ah): More Ah = longer runtime. For longest life, plan around 50% DoD (use half the rated Ah).
• Quick estimate: Runtime (hours) ≈ (Ah × 12 V × 0.5 × 0.85) ÷ Load Watts.
• Example: A 100 Ah 12 V AGM running a 100 W fridge ≈ (100×12×0.5×0.85)/100 ≈ 5 hours.
• For inverters, add a 10–15% buffer for conversion losses and surge currents.

3) Match fitment and connections

• Voltage: 12 V is common; 6 V deep-cycle units can be put in series for 12 V or 24 V systems.
• Group size: Ensure the case size fits (e.g., Group 24 ~70–85 Ah, 27 ~85–100 Ah, 31 ~95–125 Ah). Check tray and hold-downs.
• Terminals: SAE posts, M6/M8 studs, or fast-on tabs (F1/F2) on small SLA. Verify polarity and cable reach.

4) Compare key specs

• CCA/MCA: For engine starting; higher is better for cold climates.
• Reserve Capacity (RC): Minutes at 25 A—useful for vehicles and boats with accessories.
• Cycle life: Typical 300–800 cycles at 50% DoD; cycles drop sharply with deeper discharge.
• Charge acceptance/internal resistance: Lower resistance allows faster charging from alternators or chargers.
• Temperature rating: Capacity falls in cold (≈80% at 32°F) and heat shortens life; look for heat-resistant designs if needed.

Charging and Maintenance

• Use an AGM-compatible charger or profile. For 12 V AGM at 77°F (25°C): Bulk/Absorption ≈ 14.2–14.7 V, Float ≈ 13.2–13.8 V.
• Charge current: 0.1C–0.3C (e.g., 10–30 A for a 100 Ah battery). Higher currents may cause heat and shorten life.
• Temperature compensation: About −3 to −4 mV/°C per cell (≈ −0.018 to −0.024 V/°C for a 12 V battery) when outside 68–86°F.
• Avoid overcharge and equalization. AGMs are sealed; venting damages capacity.
• Storage: Fully charge before storage; self-discharge is ~1–3%/month. Recharge if open-circuit voltage falls below ~12.6–12.7 V.
• Depth of discharge: For best longevity, target ≤50% DoD; occasional 80% is acceptable but reduces cycle life.

Installation and Safety

• Orientation: Can be mounted on their side; avoid mounting upside down. Provide basic ventilation.
• Connections: Clean, tight terminals; use correct torque and anti-corrosion protection. Fuse within 7–12 inches of the battery on positive leads.
• Alternators: Most vehicle alternators are compatible if the regulator holds ~14.2–14.7 V. Smart alternators may require a DC-DC charger for full charge.
• Environment: Keep away from engine/exhaust heat. Every 18°F (10°C) rise above 77°F can roughly halve service life.

Pros and Cons

• Pros: Maintenance-free; spill-resistant; high cranking power; better vibration resistance than flooded; faster charging; lower self-discharge.
• Cons: Heavier and lower energy density than lithium; higher cost than flooded; sensitive to overcharging and prolonged heat; fewer cycles than LiFePO4.

Common Use Cases

• Automotive starting: High CCA, supports stop/start systems when specified.
• Marine/RV house banks: Deep-cycle or dual-purpose for trolling motors, lights, fridges, and inverters.
• Backup power/UPS: Small AGM (1–18 Ah) modules for emergency lighting, alarms, telecom, and UPS packs.
• Solar and off-grid: Reliable storage with simple charge control; good in cold climates.
• Mobility/equipment: Scooters, wheelchairs, floor machines, and winches where sealed, rugged power is needed.

FAQs

Can I replace a flooded battery with an AGM in my vehicle?

Often yes. Match group size, polarity, and CCA. Ensure the charging system maintains ~14.2–14.7 V and consider reprogramming in vehicles that specify battery type.

Do I need a special charger for AGM?

Use a charger with an AGM mode. Incorrect voltages can dry out the mats and shorten life.

How long do AGM batteries last?

Expect 3–6 years in mixed use or 300–800 cycles at ~50% DoD, depending on temperature, charging, and depth of discharge.

Can I connect AGMs in series or parallel?

Yes. Use identical age/capacity models; keep cables equal-length. For series strings, charge with a compatible multi-bank or monitor individual voltages.

What open-circuit voltage means “full” for a 12 V AGM?

After resting, 12.8–12.9 V is typically full; around 12.2 V is roughly 50% state of charge.