The basics of AGM batteries

There has been much confusion and concern regarding what some people consider “new battery technology.” More specifically, the misunderstanding is about AGM (Absorbed Glass Mat) batteries that have arrived on the truck fleet scene within the last one to two years.

AGM battery technology, which offers improvements over other sealed battery technologies, including additional life and performance, has been around for decades.

The primary questions I receive from fleet managers about AGM batteries are:

  • How do AGM truck batteries differ from conventional batteries?
  • Are all AGM batteries the same?
  • Will my regular wheel (shop) chargers work on AGMs?
  • Can I expect twice the life from an AGM?
  • Will using or changing AGMs cure my fleet’s battery problems?

The Design

AGM batteries are lead-acid batteries that hold or suspend the electrolyte in a high-absorption micro-glass mat that acts as the separator material between the positive and negative plates.

AGM batteries do not emit hydrogen and oxygen externally like flooded lead-acid batteries. Instead, AGMs have an oxygen-recombination chemistry wherein oxygen recombines between the negative and positive plates during discharge and recharge.

An AGM battery has a much lower internal resistance than flooded lead-acid batteries therefore allowing a higher recharge acceptance.

What’s more, AGM batteries are Valve Regulated Lead Acid (sealed) batteries that do not have an open vent system like flooded lead acid. With their unique internal assembly and physical design, an AGM battery’s vibration resistance is normally high (5 to 10 times that of flooded batteries.)

The Differences

Similar to flooded batteries, there are differences between AGM batteries. Some AGMs have the common characteristics mentioned previously. These as known as Single Stage AGMs.

Other AGM batteries have these same characteristics plus have other attributes. These batteries, referred to as Dual Stage Performance AGMs, feature:

  • Higher recharge acceptance characteristics than flooded or Single Stage AGMs. Dual Stage performance AGMs have an even lower internal resistance than other AGMs due to their improved lead purity levels - approximately 99.9 percent pure.
  • An improved power capability at extremely low temperatures than their flooded or Single Stage AGM batteries counterparts.
  • Three to four times the amount of deep cycles than Single Stage AGMs due to the active material density levels. Unless the AGM battery produces approximately 400 deep cycles to 80 percent depth of discharge (DoD) and/or 600 to 700 cycles at 50 percent DoD, it is not a true deep cycle.

In addition, Dual Stage Performance AGMs can produce both high levels of CCA (Cold Cranking Amps) and vehicle starts due to the number of plates per cell, and hundreds of deep cycles at the same time.

Battery Chargers

AGM batteries generally require a charge voltage regulation of 14.4 to 14.8 volts. Most wheeled (shop) chargers offer a voltage regulation of 13.8 to 14.2 volts on the “Low” setting and 14.4 to 14.8 volts on the “Medium” setting. The “High” or “Boost” settings are typically 15.2 to 17 volts.

Low to Medium settings can normally be used without problems. Do not use the High or Boost setting for AGM batteries as this may allow them to become hot to the touch and damage the battery during charging.

Battery Life

Can twice the life from an AGM battery be expected? It depends on the application.

A Dual Stage Performance AGM battery can easily add months or years if you are using the correct battery for the application. If a deep cycle application is required, or if improved recharge acceptance is necessary, an AGM single or dual stage may be necessary.

Remember, all AGMs are not created equal. Interstate’s 31-AGM7 battery, for example, is a Dual Stage Performance AGM designed with 99.99 percent pure lead and ultra-thin plates, which can result in two to three times the life of other flooded and AGM batteries in deeper cycling applications.

Battery Problems

For some short-run truck applications, AGMs may provide improved battery life, but do not think that AGM batteries will cure all of a fleet’s battery problems.

The suitability of AGMs depends on application issues, types of usage, life expectations, etc. Answer these few questions then seek out expert advice and training to assist you:

  • How long are your batteries lasting currently, or what are your expectations for battery life?
  • What are your charging voltage levels at the vehicle’s battery?
  • How many of the batteries that you replace are actually bad, or how good is your current testing and PMI program?
  • Does your application require the batteries to be cycled to deep or medium levels (e.g. more than 30 percent depth of discharge)?
  • Are your batteries suffering from lack of adequate charge acceptance due to use or connections? Are they constantly becoming discharged?
  • Do you have specific applications within your fleet where battery life is exceptionally low (e.g., liftgates, refrigeration units, electric APUs)?

Gale Kimbrough is the engineering and tech services manager for Interstate Batteries, the number one replacement brand battery in North America (www.interstatebatteries.com). Often called The Battery Man – due to his education, knowledge and experience in the battery industry, he has been with Interstate Batteries as a technical expert for more than 25 years. He serves on several national technical committees in the automotive and trucking industry and has conducted classes for more than 35,000 technicians around the world.

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