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Everything You Need to Know About Batteries
A simple, visual walkthrough of inverter & UPS batteries — how they store energy, how long they last, and how to size them correctly.
By Kunwer Sachdev · The Inverter Man of India9 short chapters · ~7 min read 12V · 150Ah
What’s inside
- What is a battery?
- Types of batteries
- Series & parallel wiring
- Capacity (Ampere-Hour)
- What affects battery life
- Battery sizing
- Backup chart
- Charging current & time
- Sulphation
- Specific gravity
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.
Dam stores water analogy +−12VBattery stores electricity The simplest way to think about it: water in, water out — energy in, energy out.Types of batteries
For inverters and UPS systems, you’ll mostly come across three families:
1. Normal Lead-Acid Battery
The classic flooded battery. Affordable and reliable, but needs periodic top-up of distilled water and proper ventilation.
2. Tubular Battery
Built for long backup hours and deep discharges. The “positive plate” is housed in a tube — making them tougher and longer-living.
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.
Lead-Acid Flooded Tubular Deep cycle SMFSEALED · MAINTENANCE FREESMFSealedSeries & 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:
| Battery (Ah) | No. of plates |
|---|---|
| 100 Ah | 15 |
| 135 Ah | 19 |
| 150 Ah | 21 |
| 180 Ah | 23 |
| 190 Ah | 25 |
| 225 Ah | 27 |
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
KVA Rating × Rated PF × 1000Battery Nominal DC Voltage × Inverter Efficiency = Discharge Current (A)Battery Ah capacity
DC Current (A) × Backup HoursBattery De-Rating Factor = Battery Ah CapacityDe-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:
| Backup time | De-rating | Backup time | De-rating |
|---|---|---|---|
| 15 min | 0.45 | 6 hr | 0.86 |
| 30 min | 0.52 | 7 hr | 0.87 |
| 1 hr | 0.62 | 8 hr | 0.88 |
| 2 hr | 0.74 | 9 hr | 0.89 |
| 3 hr | 0.80 | 10 hr | 0.90 |
| 4 hr | 0.83 | 15 hr | 0.95 |
| 5 hr | 0.85 | 20 hr | 1.00 |
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.
| Product | Battery V | No. of batt. | Discharge (A) | 30 | 60 | 90 | 120 | 150 | 180 | 210 | 240 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1Φ → 1Φ (Single-phase input · Single-phase output) | |||||||||||
| 1 KVA | 168 V | 14 | 6 A | 7.2 | 12 | 12 | 17 | 18 | 26 | 26 | 28 |
| 2 KVA | 180 V | 15 | 12 A | 7.2 | 18 | 26 | 26 | 38 | 42 | 45 | 65 |
| 3 KVA | 180 V | 15 | 16 A | 17 | 28 | 38 | 45 | 65 | 65 | 65 | 80 |
| 5 KVA | 180 V | 15 | 25 A | 26 | 45 | 65 | 80 | 88 | 100 | 120 | 125 |
| 7.5 KVA | 192 V | 16 | 38 A | 38 | 65 | 88 | 100 | 125 | 150 | 160 | 180 |
| 10 KVA | 192 V | 16 | 47.5 A | 42 | 80 | 100 | 125 | 150 | 180 | 200 | 225 |
| 3Φ → 1Φ (Three-phase input · Single-phase output) | |||||||||||
| 3 KVA | 360 V | 30 | — | — | |||||||
| 5 KVA | 360 V | 30 | 15 A | 17 | 28 | 38 | 45 | 65 | 65 | 65 | 80 |
| 7.5 KVA | 360 V | 30 | 19 A | 18 | 38 | 45 | 65 | 65 | 65 | 100 | 120 |
| 10 KVA | 360 V | 30 | 25 A | 26 | 45 | 65 | 80 | 88 | 100 | 120 | 125 |
| 15 KVA | 360 V | 30 | 38 A | 38 | 65 | 88 | 100 | 120 | 150 | 160 | 180 |
| 20 KVA | 360 V | 30 | 50 A | 45 | 88 | 100 | 125 | 160 | 180 | 200 | 240 |
| 3Φ → 3Φ (Three-phase input · Three-phase output) | |||||||||||
| 5 KVA | 360 V | 30 | 15 A | 17 | 28 | 38 | 45 | 65 | 65 | 65 | 80 |
| 7.5 KVA | 360 V | 30 | 19 A | 18 | 38 | 45 | 65 | 65 | 65 | 100 | 120 |
| 10 KVA | 360 V | 30 | 25 A | 26 | 45 | 65 | 80 | 88 | 100 | 120 | 125 |
| 15 KVA | 360 V | 30 | 38 A | 38 | 65 | 88 | 100 | 120 | 150 | 160 | 180 |
| 20 KVA | 360 V | 30 | 46 A | 45 | 88 | 100 | 125 | 160 | 180 | 200 | 240 |
| 30 KVA | 384 V | 32 | — | 65 | 135 | 180 | 200 | 240 | 280 | 320 | 360 |
Charging current & charging time
Charging current depends on the spec of your battery, but a safe rule of thumb is:
Charging current (A) = Ah rating of battery10 Charging time (hr) = Ah rating of batteryCharging current + 2 hours The “+2 hours” accounts for the natural drop in charging efficiency near the end of a charge cycle — a real battery doesn’t accept current as eagerly when it’s nearly full. 0Time →% Charge~100%Fast charge zoneSlow taper (+ 2 hrs)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).
Healthy plateClean lead-dioxide surfaceSulphated plateWhite crystals blocking active material Prevent it: Avoid leaving batteries discharged for long stretches. Recharge promptly after deep discharges. Top up flooded batteries with distilled water as required.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.
Battery cell Electrolyte = sulphuric acid + water 1.300 1.280 1.250 1.200 1.150 1.100 1.280→ fully charged ✓| Specific gravity reading | What it means |
|---|---|
| 1.250 – 1.280 | Fully charged · healthy |
| 1.200 – 1.250 | Partially discharged |
| Below 1.150 | Discharged · needs recharge |
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.
Kunwer Sachdev
The Inverter Man of IndiaA pioneer of the Indian power-backup industry and the founder of Su-Kam Power Systems, Kunwer Sachdev has spent decades demystifying inverters, batteries and home power solutions for everyday households and businesses across India.
Please note: Mr. Sachdev is no longer associated with Su-Kam Power Systems Ltd. in any capacity.
