Understanding AGM Battery Ah Rating: Which is better—high capacity or consistency?

By Adam Pratt, Applications Manager, Lead Acid (Motive)

Choosing a battery for your project or equipment based on ampere-hour (Ah) capacity seems like a reasonable decision, but is it? Ah capacity may not be the most important specification to consider when selecting a battery.

What does Ah tell us about AGM battery capacity and performance?

Ampere-hour (also called amp-hour or Ah) measures the total charge a battery can hold. It represents the amount of electrical current (measured in amperes) that a battery can supply for a period of time. For example, a battery has a 207Ahr capacity @ 20Hr rating. This battery can provide (207Ahr / 20hr) or 10.35A for 20 hours.

A higher amp-hour rating signifies a larger capacity and the ability to power a device for a longer duration—the battery’s “runtime.”

What’s hidden behind a battery’s Ah rating?

Though Ah seems like the most straightforward spec to look for in a battery, it can hide important performance realities. The battery with the highest Ah capacity may not necessarily be the best battery for an application in the long run.

Some batteries show a higher initial Ah capacity but fade dramatically within the first hundred cycles or so. Others have an initial lower Ah rating but have a capacity that remains consistent over hundreds of cycles.

Life Cycle Testing shows the complete picture

Life Cycle Testing of batteries (typically done in a lab environment over time) can provide a much deeper understanding of battery capacity. Testing shows how the capacity of each battery can fluctuate after the initial start and can even fade dramatically in a short period of time.

The results of these tests can provide information to help you make an informed decision and save time and money.

Figure 1: AGM Battery Cycle Life Comparison

Comparing AES AGM to standard AGM battery cycle life

The Life Cycle Testing results in the image above show that the Trojan T875 AES AGM battery achieved 1200 cycles in testing. This can also be looked at as 1200 days of operation.

How do 1200 cycles translate to real-life usage? Typically, operators run their equipment 20 days out of 30-day month. By dividing 1200 cycles (1200 days of operations) by 20 days per month, we get 60 months of usage out of the T875-AES model.

Standard AGM batteries typically provide life cycle values that are much lower than the Trojan AES AGM batteries shown in this comparison. Even though both battery types have similar rated capacities at the beginning, the standard AGM batteries achieve less than 300 cycles compared to AES AGM’s 1200, showing a precipitous drop in discharge throughput.

Conclusion: Ah rating doesn’t tell the whole story

Life Cycle Testing demonstrated how Ah specs can hide the real story. In this case, the consistent capacity response across a longer timeframe reveals the superior performance of Trojan AES AGM over standard AGM.

About the author

Adam Pratt is the Application Manager at Trojan Battery Company, bringing 24 years of experience in Power Management, Power Quality (DataCenter UPS Systems and Surge Protection Systems) and Battery Storage.

As a Subject Matter Expert in Power Quality and Battery Storage, he conducts power audits, listens to customers and identifies their pain points, and develops complete solutions that reduce equipment downtime and increase equipment reliability.

Adam’s recent roles include Lithium Energy and Emerging Technology Specialist at C&D Technologies, as well as Application Manager for Motive Product Management Team at Trojan Battery, supporting the growth of lead acid batteries.