What are the signs of a failing lead-acid vs AGM battery in emergency vehicles, and how should they be tested?

• A. Lead-acid shows fast cranking and low self-discharge; AGM maintains voltage under load; test with a simple voltmeter.
• B. Lead-acid is silent; AGM makes a buzzing sound.
• C. Lead-acid shows slow cranking and higher self-discharge; AGM maintains higher voltage longer but declines under load; test with a load test and check internal resistance.
• D. Lead-acid and AGM behave identically in all tests.

Answer: (C)

Understanding how a failing battery behaves under load is key. Lead-acid batteries tend to show aging with slower engine cranking and higher self-discharge: the starter struggles, and the battery loses charge more quickly when not in use. AGM batteries, while they can hold a healthy open-circuit voltage, often reveal weakness when you actually draw current from them—their voltage stays relatively high without a load but sags more under load because the internal resistance has increased.

This is why the described signs are the best match: a slow crank and higher self-discharge point to a failing lead-acid cell, while an AGM that holds voltage at rest but drops under load indicates aging internal resistance rather than pure capacity loss. Testing with a load test and checking internal resistance directly assesses how the battery performs under realistic starting conditions and quantifies deterioration, which is more informative than voltage alone.

To test, fully charge the battery first. Then apply a controlled load that simulates starting (or use the vehicle’s starter draw spec) for about 10–15 seconds while monitoring the voltage. In a healthy 12V system, the voltage should not collapse far under load (typically not below roughly 9.6V during cranking, though follow manufacturer guidelines). After removing the load, observe how quickly the voltage recovers. Also measure the internal resistance with a proper battery tester that reports resistance or conductance; compare the reading to the manufacturer’s spec. Higher internal resistance confirms aging and reduced ability to deliver peak current, which helps distinguish a failing battery from a healthy one.

The other statements don’t fit because they assign unusual or incorrect behaviors to the chemistries (for example, one type being inherently louder or silent, or both behaving identically), which contradicts how lead-acid and AGM designs respond to load and aging.