how to measure battery capacity

anthony

People often ask how do you measure battery capacity and what the numbers actually mean. So here’s a clear explanation you can follow whether you’re checking a car battery, UPS battery, power bank, or any rechargeable battery.

🧐 What “capacity” actually means

Battery capacity tells you how much energy the battery can store.

Usually written as:

mAh (for small batteries, phones, power banks)
Ah (for larger batteries, UPS, solar, car)

Example:

2000 mAh = 2 Ah
100 Ah battery can theoretically deliver 1 amp for 100 hours, or 10 amps for 10 hours.
MAH to AH convert

3 main ways to measure battery capacity

1. Use a battery tester / analyzer

Some testers actually discharge the battery and measure the real usable capacity.
This is the most accurate method.

2. Measure discharge time

If you know the discharge current, you can calculate capacity:

Capacity (Ah) = Current (A) × Time (hours)

Example:

battery discharges at 5A for 10 hours →
5 × 10 = 50Ah

This is how labs and engineers do it.

3. Use smart chargers

Some smart chargers show:

charged capacity
discharged capacity
percentage left

Not perfect, but better than guessing.

Important: voltage ≠ capacity

A full battery and a weak battery can both show similar voltage.
Voltage alone does not tell you how much energy is left.

State of charge ≠ actual capacity

Sometimes a battery shows “100%” but the real capacity is lower because the battery has aged.
Example:
A 100Ah battery may only provide 60Ah after years of use.

🧪 If the battery is old…

Actual capacity goes down with:

age
heat
deep discharges
number of cycles

So measuring is the only real way to know.

For solar or UPS systems

Look for:

Ah
cycle life
discharge curves

Many companies list nominal capacity, not usable capacity, so real performance might be lower.

🧠 Quick summary

To measure capacity, you basically:

discharge the battery
track time and current
calculate capacity using Ah = A × hours

If the number is much lower than the rated capacity, the battery has aged or is damaged.

Final thoughts

Measuring battery capacity is mostly about controlled discharge. Voltage alone can’t tell you the real number. If you want an exact reading, use a proper tester or a measurement method rather than relying on what the label says.

Hope this helps someone searching the same question!

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