What is a battery?

A battery is like a dam — it stores electrical energy so you can use it whenever you need it.

Just as a dam holds back water and releases it on demand to generate power, a battery holds electrical charge and releases it whenever you switch on a device. It’s the silent backbone of every inverter, UPS, and backup system.

Types of batteries

For inverters and UPS systems, you’ll mostly come across three families:

3. SMF Battery (Sealed Maintenance Free)

Fully sealed, no water top-up required, and safe to install almost anywhere. Common in UPS systems, alarm panels, and small inverters.

Series & parallel connections

When you connect more than one battery, the way you wire them changes the result:

• Series → voltages add up, capacity (Ah) stays the same. Two 12V/100Ah in series = 24V/100Ah.
• Parallel → voltage stays the same, capacities add up. Two 12V/100Ah in parallel = 12V/200Ah.

Capacity of a battery — what does “Ah” mean?

Ampere-hour (Ah) is the standard unit of a battery’s storage capacity. It tells you how much current the battery can deliver for how long.

Mathematically, Ah = discharge current (Amps) × time (hours). A 100Ah battery can — in theory — deliver 10A for 10 hours, or 5A for 20 hours.

How Ah relates to plates

Inside a lead-acid battery are alternating positive and negative plates. More plates → more surface area → more storage capacity. Here’s the typical relationship:

What affects battery life?

No battery lasts forever, but the way you use it makes a huge difference. Five things drive how many years you’ll get out of it:

• Discharge rate — pulling heavy current shortens life.
• Number of discharges — every charge/discharge cycle counts.
• Deep discharges — draining the battery below ~50% repeatedly is especially harsh.
• Temperature — both the battery’s internal temperature and the room temperature matter. Hot rooms = shorter life.
• Specification — a battery built for daily deep cycling lasts much longer than one built for occasional standby.

How do you size a battery?

Sizing means picking the right Ah so the battery lasts the backup hours you need. There are two formulas you’ll use, plus a “de-rating” factor.

Battery discharge current

Battery Ah capacity

De-rating factors at 25°C (SMF lead-acid)

Batteries can’t deliver their full rated capacity over short backup windows. The shorter the backup, the bigger the de-rating penalty:

Battery backup chart

Use this as a quick reference for matching inverter capacity, battery voltage and Ah needed for different backup durations. Backup times are in minutes.

Charging current & charging time

Charging current depends on the spec of your battery, but a safe rule of thumb is:

Sulphation — the silent killer

Sulphation is the buildup of lead sulphate crystals on the positive plate of a lead-acid battery. A little is normal — but left unchecked, it permanently reduces capacity.

During discharge, active material on the plates converts to lead sulphate. The deeper the discharge, the more sulphate forms. A proper recharge converts that sulphate back into lead dioxide (positive plate) and sponge lead (negative plate).

Specific gravity — checking battery health

Specific gravity is the ratio of the density of a liquid to the density of water — measured with a hydrometer.

Water has a specific gravity of 1.000 by definition. In a lead-acid battery, the electrolyte is a diluted solution of sulphuric acid and water. In a fully charged battery, the specific gravity of the electrolyte is typically 1.250 – 1.280 at 27 °C.

The takeaway

A battery isn’t just a box — it’s a chemical system. Treat it well by avoiding deep discharges, keeping it cool, charging it correctly, and checking specific gravity from time to time, and you’ll get the most out of every Ah you paid for.

Buying a battery soon? Match its Ah to the backup hours you need, factor in the de-rating, and pick a chemistry (flooded, tubular, or SMF) that fits your usage pattern.