SLA (Sealed Lead‑Acid) Batteries: How to Choose, Size, and Use

SLA batteries (also called VRLA, including AGM and Gel) are sealed, maintenance‑free lead‑acid batteries designed for reliable standby and moderate cyclic use. They are widely used in UPS systems, alarms, mobility scooters, emergency lighting, and small solar storage. Most packs are 6V or 12V; each cell provides 2V.

When to Choose SLA vs Alternatives

• Use SLA if you need low‑cost standby power (UPS, alarm), occasional cycling (toys, gate openers), or a drop‑in replacement matching common 6V/12V sizes and spade terminals.
• Avoid SLA if weight/space are tight, you cycle daily or deeply (e.g., off‑grid), or you need long cycle life. Consider LiFePO4 for 3–4x lower weight and 5–10x more cycles, if your charger/equipment supports it.

Types of SLA (Choose the Right Construction)

• AGM (Absorbent Glass Mat): Most common. Good all‑around choice for UPS, alarms, mobility. Charges faster than Gel.
• Gel: Better deep‑cycle tolerance and vibration resistance; requires lower charge voltage. Good for mobility and cyclic use where heat is controlled.
• High‑Rate UPS: Optimized for short, high current bursts; best for UPS runtime of minutes.
• Deep‑Cycle AGM/Gel: Thicker plates for repeated cycling; longer cycle life than general‑purpose standby models.

How to Size Capacity and Estimate Runtime

1. Find your load (W): Check the device label or UPS specs.
2. Compute battery energy (Wh): Wh ≈ Voltage × Ah (at the 20‑hour rating).
3. Derate for real‑world use: Use 60–70% of rated Wh to account for discharge rate (Peukert effect), inverter/UPS losses, and cutoff voltage.
4. Runtime (hours) ≈ (Voltage × Ah × 0.6–0.7) ÷ Load (W).

Example: A 12V 9Ah battery powering a 60W device → 12 × 9 = 108Wh. At ~60% usable = 65Wh. Runtime ≈ 65 ÷ 60 ≈ 1.1 hours. High current draws reduce runtime further.

Key Selection Criteria

• Voltage: Match system (6V, 12V, or series 24V/48V).
• Capacity (Ah) & Rate: Ah is rated at 20‑hour discharge; actual capacity drops at higher currents.
• Form factor & terminals: Match case size and terminal type (e.g., F1 0.187", F2 0.250").
• Use profile: Standby/float (UPS, alarms) vs deep‑cycle (mobility, solar).
• Discharge rate: High‑rate models for UPS; deep‑cycle for longer, lower‑rate draws.
• Design life: General‑purpose 3–5 years at 77°F (25°C); premium “10‑year” float models available.
• Temperature rating: Capacity drops in cold; heat shortens life.
• Certifications: UL, CE, UN38.3 for transport; flame‑retardant cases where required.

Charging and Maintenance (Best Practices)

• Use the right charger: Lead‑acid profile with bulk/absorption/float stages. Do not use lithium‑only chargers.
• Typical 12V setpoints (check datasheet): AGM absorb ~14.4–14.7V; Gel ~14.0–14.2V; float 13.5–13.8V.
• Temperature compensation: About −3 mV/°C per cell (≈ −18 mV/°C for a 12V battery) from 25°C; avoid charging below 32°F (0°C).
• Avoid deep discharge: For 12V, do not drop below ~10.5V under load; repeated deep discharge shortens life.
• Storage: Store fully charged in a cool, ventilated place; top up every 3–6 months. Self‑discharge ≈ 2–3%/month at 77°F (25°C).
• No equalization on sealed batteries: Overvoltage can vent and permanently damage cells.

Common Sizes and Use Cases

Typical Size	Example Uses
6V 4.5–12Ah	Exit/emergency lights, ride‑on toys
12V 7–9Ah	Small UPS, security panels, modem/ONT backup
12V 18–22Ah	Portable power, small mobility devices, fish finders
12V 35–55Ah	Mobility scooters, marine accessories
12V 75–120Ah	Wheelchairs, RV/solar buffer storage (light duty)

Replacement & Compatibility Notes

• Like‑for‑like: Match voltage, size, and terminal. Replacing 12V 7Ah with 12V 9Ah is often fine if it fits and the charger allows it.
• Series/parallel sets: Replace all batteries in a set at the same time. Do not mix ages or capacities.
• Orientation: Most AGM/Gel operate in any orientation except upside down; confirm in datasheet.
• UPS specifics: Many UPS require high‑rate SLA with specific dimensions and terminals; check the model’s replacement list.
• Switching to lithium: Possible in some systems but requires a compatible charger/BMS and may affect UPS fault detection. Verify with the equipment maker.

Safety & Environmental

• Charge in a ventilated area. SLA can vent small amounts of hydrogen if overcharged.
• Do not open or incinerate. Contains lead and acid. Always recycle at authorized centers.
• Protect terminals from short circuits; use proper fusing for external connections.
• Keep away from heat sources; every 18°F (10°C) above 77°F (25°C) roughly halves service life.

FAQ

How long do SLA batteries last?

General‑purpose SLA deliver about 200–500 cycles at 50% depth of discharge, or 3–5 years on float at 77°F (25°C). Heat and deep discharges reduce life.

What’s the difference between AGM and Gel?

AGM charges faster and suits most standby uses. Gel tolerates deeper cycles and heat less well during charging; it needs lower charge voltages.

Why does my UPS runtime seem shorter than the Ah rating suggests?

High discharge currents reduce usable capacity (Peukert effect), and the UPS inverter adds losses. Expect roughly 60–70% of rated Wh in many UPS scenarios.

Can I use a car battery charger?

Only if it has a compatible AGM/Gel setting with correct voltage limits and no equalization. A dedicated SLA smart charger is recommended.

How should I store an SLA battery?

Store fully charged in a cool place, recharge every 3–6 months, and keep on float at 13.5–13.8V for 12V units if long‑term standby is needed.