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AGM Batteries And Testing

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Two of the greatest changes in starting/charging systems during the past few model years have been the expanded use of the Absorbed Glass Mat (AGM) battery as standard equipment and the accelerating pricing structures of both AGM and conventional flooded-cell batteries. In the past, I knew many shop managers who simply replaced any battery that had been completely discharged or that had exceeded a typical warranty period of four years. They set this policy because recharging and testing the battery would, in most cases, consume more shop time and space than the remaining battery life was worth. But, testing methods have now become much more important since the retail price on many AGM and flooded-cell batteries is either approaching or exceeding $200.

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AGM Electrical Properties

Electrically speaking, AGM batteries have less internal resistance and, because their tightly packed cell plates and battery mats are more reactive, they can be recharged up to five times faster than flooded-cell batteries. Due to glass-mat construction, the cold-weather performance of an AGM is also less affected by subzero temperatures than a typical flooded-cell battery.

When testing open-circuit voltage, you’re probably going to see voltages in the 12.8-volt range rather than the 12.6-volt range seen in fully charged flooded-cell batteries. AGMs also have a lower self-discharge rate, and AGM battery life is longer because an AGM can endure a deep discharge cycle of up to 80% of its capacity without affecting battery life. These combined features allow AGMs to function more efficiently in vehicles equipped with multiple high-load accessories like heated seats and windshields.

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Recharging AGMs

Unlike a flooded-cell battery, an AGM can be immediately damaged by overheating the electrolyte. Once the water boils from the electrolyte through a pressure sensitive venting valve, it can’t be replaced. For this reason, the internal core temperature of an AGM battery shouldn’t exceed 120° F during the recharging process. Similarly, AGM batteries can be charged up to 14.4 volts without evaporating the water from the electrolyte. A typical “float” or maintenance charge should be about 13.8 volts.

The charging voltage of any battery is affected by the battery’s state-of-charge (SOC). If the battery is in a good state-of-health (SOH), most chargers will maintain a high-amperage, low-voltage “bulk” charging mode until it regains its charge. In contrast, the “saturation” mode is a low-amperage, high-voltage charging rate that occurs as it approaches a full charge. The “float” or maintenance mode incorporated into most modern chargers will hover between 13.2 volts and 13.4 volts, while the amperage rate will range between zero and 1.5 to 2.0 volts.

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Charging voltage and amperage can also be affected by the battery’s SOH. If, for example, the positive and negative plates in one battery cell are shorted together, the battery will never be able to achieve full voltage. Consequently, the battery will never enter the saturation charging mode. In contrast, battery plate sulfation caused by prolonged storage will cause a high-voltage, low-amperage charging rate due to the battery plates being coated with lead sulfate crystals precipitated from the electrolyte.

To remedy sulfation, many “smart” battery chargers incorporate a “de-sulfation” mode. The mode will typically hover around 15.0 volts at low amperage rates, which tends to break the lead sulfate away from the negative battery plates and restore the battery’s chemical activity. Lastly, always remember that normal discharge/recharge cycles will eventually erode the battery plates, which will reduce their storage capacity and their overall mechanical reliability.

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Battery Charging Diagnostics

If you’re recharging batteries with an “old-school,” flooded-cell battery charger, you’re likely going to ruin an AGM by overheating the battery’s electrolyte. Any modern charger incorporates an AGM charging mode, and the most sophisticated battery chargers also contain a “diagnostic charging mode,” which is based upon the above bulk, saturation and float charging modes. This feature eliminates wasted shop time by displaying a “defective battery” (or equivalent warning) if the battery doesn’t support a specific charging rate profile.

Charging System Issues

Now that we’re in the era of computer-controlled battery charging systems, charging voltage can be influenced by operating conditions as well as accessory load, battery SOC and battery SOH. For example, the power-train control modules (PCMs) in some vehicles, like BMWs, are programmed to adjust their charging rates to compensate for normal battery plate wear. The upshot of this is that a modern charging system controlled by various inputs to the engine control module (ECM) can more efficiently maintain an AGM battery than can a conventional voltage regulator, which responds only to battery voltage and ambient temperatures. With that said, my general recommendation for replacing worn batteries is to replace both flooded-cell and AGM with their original equipment (OE) equivalent.

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Since a conventional voltage-regulated charging system relies on voltage feedback from the battery to the voltage regulator, I’d also recommend that the alternator, voltage regulator and battery share clean battery positive and battery negative connections to prevent overcharging. In off-road applications, I’d recommend installing stainless steel star washers on each bolted connection and applying an aerosol anti-corrosion spray  to all electrical and battery terminal connections. In any case, remember that a voltage drop in excess of 0.5 volts can, depending upon the amperage flowing through the circuit, increase charging voltages well beyond the 14.4-volt limit for AGMs.

Article adapted from Underhood Service.

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