Rick's Free Auto Repair Advice

Stop Start Battery Diagnostics

Stop Start Battery Diagnostics

Vehicles equipped with stop start systems use an AGM (absorbed glass mat) or enhanced flooded (EFB) lead-acid battery. To prevent the possibility of a no-start on a stop start system, the vehicle’s computer monitors the battery’s condition at all times. If it sees a discharge situation, it will prevent the engine shutdown and set a check engine light.

What is an AGM battery?

An AGM battery has some similarities to a standard acid (SLI) battery with; positive and negative plates, plate separators and sulfuric acid as the electrolyte. However, instead of the liquid acid sloshing around between the plates, and AGM contains an absorbent glass mat to absorb and hold the acid in place. In addition to the glass mat, the plates also have a slightly different chemical composition that provides much lower internal resistance. Because of these differences, an AGM battery accepts a charge five times faster than an SLI battery and can be discharged to up to 80% of its capacity, versus 50% for an SLI battery. AGM batteries are also less prone to sulfation, the #1 killer of lead acid batteries.

Because the battery acid is fully absorbed, AGM batteries can be mounted at odd angles.

Stop start AGM battery monitoring systems

Since certain vehicle electric system remain on during a routine engine shutdown, the power management system (PMS) must monitor current draw for entertainment, HVAC, lighting and other power accessories while the engine is off. So you’ll find current draw sensors at the battery terminals. battery cable temperature sensorThose current sensors often measure internal battery resistance because internal resistance is an indication of battery condition.

In addition to monitoring current draw and internal battery resistance, the power management system monitors voltage drop during cranking. If the voltage drops below set minimums, the PMS will set a trouble code and malfunction indicator light (MIL) on the instrument cluster. The PMS system also monitors battery temperature, an important factor to keep in mind during battery recharging. The temperature sensor can be located in the positive battery terminal or in the ECU if it’s located next to the battery.

Battery temperature affects starting and recharging

Since a car battery produces power through a chemical reaction and chemical reactions slow when cold, a car battery will produce less power when it’s cold. On the flip side, the recharging process generates heat. If the battery is already at its upper heat range, giving it a full charge and maximum voltage can damaged the plates.

Charging an AGM battery

AGM and EFB batteries require a different charging routine than standard SLI batteries even when they’re new. As they age, the stop start vehicle ajusts the charging routine based on battery degradation. That’s why re-programming the vehicle when replacing a stop start battery is critical. If you don’t have the reprogramming done, you will reduce the life of the new battery.

If you recharge an AGM or EFB battery on your own, you must use a battery charger with a dedicated AGM charging routine. Charging an AGM battery with an older battery charger designed for SLI batteries will damage the battery.

Testing a stop start AGM battery

Since AGM batteries have much lower internal resistance, you cannot test them with older capacitance style battery testers. They provide a false reading. In addition, an AGM battery will test at 13-volts when fully charged versus 12.6 for an SLI battery. If you don’t know the difference, you’ll think the AGM battery is ok if it reads 12.6

In addition to the voltage reading, you must also measure the battery’s temperture and use a temperature/state of charge charge to determine what the voltage should be at that temperature.

Replacing a stop start AGM battery

Replacing the battery itself is the same as with an SLI battery. However, you also need a compatible scan tool to tell the ECU that you’ve replaced the battery. Skip this step and you’ll be replacing that expensive stop start AGM battery within six months. That’s because the ECU keeps track of the old battery’s performance and adapts the recharging routine to that condition. In many cases, the adapted routine charges more slowly and with lower voltage to compensate for higher internal resistance caused by aging. If you install an new AGM battery in the vehicle without reprogramming the ECU, it will continue to recharge the new battery using the old routine, which will result in a constantly undercharged battery that will sulfate and die quickly.

Can you replace a stop start AGM battery with an SLI battery?

No. An SLI battery won’t perform as well and will die a rapid death. You may think you’ve saved money, but you’ll be replacing the SLI battery within the year.

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat


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