How to Test Your Battery’s Actual Capacity (Not Just the Voltage)

—A Simple DIY Guide to Measuring Amp-Hours (Ah)

Most people check their battery with a multimeter, see 12.6 volts, and think, “Looks good to me!” But here’s the problem:

Voltage only tells you the battery’s state of charge—not its true health or capacity.

A battery can read full voltage and still be nearly dead inside.

If you really want to know how much usable energy your deep-cycle battery still has, you need to test its amp-hour (Ah) capacity. That’s how you find out whether it’s still a 200Ah battery or if it’s faded to 120 or 60Ah over time.

And yes, you can do this at home, with simple tools.

What You’re Testing: Actual Capacity vs. Rated Capacity

If your battery says 200Ah at 20-hour rate, that means:

• It should supply 10 amps for 20 hours (10A × 20h =2100Ah)
• If it can only do that for 10 hours before hitting 10.5 volts, it’s down to ~50% of its rated capacity

This test shows you how much usable energy is left in the battery not just whether it’s charged or not.

What You’ll Need

• A fully charged battery (start with a 100% charge)
• A known constant load (like a 12V DC light bulb, fan, or resistive load rated around 10A)
• A DC watt meter or amp-hour counter
  Recommended: MakerHawk Electronic Tester
  • Make sure to follow all instructions to avoid damaging the tester. A 12V battery at 10 amps is 120W!
  • Depending on the capacity of your battery it could take an entire 24 hours!
• A voltmeter or battery monitor (optional, for backup voltage tracking)
• A timer or just keep an eye on the clock

Step-by-Step: DIY Amp-Hour Test

1. Fully Charge the Battery

• Use a smart charger to bring the battery to 100%
• Let it rest for a couple hours to stabilize

2. Set Up Your Load

• Connect your known load to the battery (ideally 10A for a 100Ah test)
• Start your timer or watch the display if using a watt/amp-hour meter

3. Monitor Voltage

• You’re aiming to discharge the battery down to 10.5V (for 12V systems) or down to 5.25V(for 6V batteries).
• Below 10.5V is considered fully discharged

4. Stop the Test at 10.5V

• Record the total amp-hours used (from your meter or from time × amps)
• That number is your real-world battery capacity

Example:

• You drew 10.0 amps for 12.3 hours
• 10 × 12.3 = 123.0Ah
• Your “200Ah” battery now only holds 123.0Ah = ~61% capacity remaining

Things to Watch Out For

• Don’t try this on starter batteries they aren’t designed for deep discharge
• Make sure your battery is in a safe, ventilated space—especially flooded types
• Only do this a few times a year it’s a deep discharge, which wears batteries out

Why This Matters

If you’re building a solar system, running an RV, or relying on batteries off-grid, knowing your actual capacity helps you:

• Size your battery bank more accurately
• Spot batteries that are dying early
• Decide when it’s time to replace
  • If a battery produces less than 50% of rated capacity, then it is time to start thinking of replacing it.

And if you’re using multiple batteries in a bank, one weak one can drag the whole system down. A simple Ah test helps you identify the problem before it kills the rest.

Recommended Tools

• MakerHawk Electronic Tester
• Victron Bluetooth Battery Monitor
• NOCO Genius10 Smart Charger

Final Thoughts

Don’t just trust voltage—measure capacity.

Knowing how many amp-hours your battery can actually deliver is the best way to make smart decisions about replacements, upgrades, and daily usage.

It’s a little more work but it’s a whole lot smarter than guessing.