20th Annual Technical Forum: Batteries

20th Annual Technical Forum: Batteries

Q: What’s the difference between a regular car battery and an AGM car battery?

A. A conventional 12-volt lead acid car battery contains liquid electrolyte (sulfuric acid) between the lead cell plates while an “Absorbent Glass Mat” (AGM) battery contains no liquid. The acid is suspended in boron silicate (glass) mats sandwiched between the lead cell plates inside an AGM battery. That makes AGM batteries spill-proof, and also allows closer spacing of the cell plates.

The cell plates inside an AGM battery may be flat like those in a conventional wet cell battery, or they may be spiral wound like those in Optima batteries. The glass mats help cushion the cell plates, making the battery more resistant to vibration damage. This along with the “recombinant” chemistry in an AGM battery help extend battery life significantly compared to conventional wet cell batteries.

When an AGM battery is recharging, the oxygen and hydrogen gas that is produced recombines to form water instead of evaporating out of the battery case. This reduces gassing and loss of electrolyte to almost nothing. By comparison, a conventional wet lead-acid cell battery typically uses water over time, even sealed-top maintenance-free batteries. If the water level gets low enough to expose the tops of the cell plates, they quickly sulfate and lose their ability to accept or hold a charge. That’s why typical battery life in a hot climate for a wet cell lead-acid battery is only about three years. AGM batteries typically last much longer (up to two times longer claim some AGM battery manufacturers).

Another difference is that AGM batteries will usually hold a charge much longer than a wet lead-acid cell battery when a vehicle is not being driven. This makes AGM batteries a good choice for vehicles, motorcycles or RVs that may sit for long periods between use. Some AGM batteries are designed specifically to be deep cycle batteries (like marine batteries) so they can withstand being discharged without permanent damage. By comparison, a conventional wet cell starting battery must be maintained at or near full charge for maximum service life. Fully discharging the battery more than two or three times can shorten its service life.

AGM batteries also are being installed as factory equipment in some late-model vehicles that are equipped with Start/Stop gas-saving engine management systems. Start/Stop systems put a lot more stress on the battery, so if one of these vehicles needs a new battery the recommend is to replace it with another AGM battery rather than a less expensive conventional battery.

Q. If a battery keeps running down, should it be replaced?
A. It depends on the age and condition of the battery. The average battery only lasts about four to five years, and may only last three in a really hot climate. The age of the battery should always be considered, but more importantly is the condition of the battery itself. The battery should always be tested to see if it will accept and hold a charge.

The fastest and easiest way to check the condition of a battery is with a conductance battery tester. Unlike a load tester that requires a battery to be at least 75 percent charged before it can be accurately tested, a conductance tester can be used on most batteries regardless of their state of charge.If the battery tests bad, your customer needs a new battery. If the battery tests good, the next thing to check would be the output of the charging system.

The charging voltage is checked with the engine idling and all the lights and accessories off. Most charging systems will put out 13.5 to 14.5 volts at idle depending on the temperature and the battery’s level of charge. Less than 13.5 volts would probably indicate a charging problem such as a loose or slipping alternator belt, a bad wiring connector at the alternator, loose or corroded battery cables, or even a bad alternator. An alternator can be bench tested to see if its current and voltage output is within specifications.
If the battery and charging system both test good, but the battery is running down, the fault is likely a key off electrical drain. There is always a small drain on the battery after the key is turned off to maintain the “keep alive” memories in the PCM and other modules. Many modules have internal timers that either turn off the module to put it into a “sleep mode,” or power down the module to a “standby mode” after a certain length of time when the key is turned off. Some of these modules power down in steps and time out at different rates.


Don’t Let The Dust Settle On Battery Inventory

Most everyone knows the importance of stock rotation and battery freshness.  A wise man at an auto parts store chain once said, “If a battery needs dusting, it has been here too long.”

First and foremost, Interstate Batteries has a “Freshness Guarantee” policy so the battery will be very fresh when received at the store. With regard to batteries and freshness; there are actually a number of technical reasons why it should be sold before the dust settles on it or it becomes stale.

● All batteries self-discharge due to the chemically opposing negative and positive plates internally. (5 percent – 8 percent per month at 80F (26.7C).

● The higher the temperature, the more a battery will self-discharge.

● As 12-volt batteries discharge while sitting on the shelf, the lead dioxide (PbO2) of the positive plate combines with Hydrogen (H+) and sulfate (SO4). Once the battery discharges below approximately 12.45 volts (approximately 75 percent) without recharge, the sulfate often starts to harden, creating an internal sulfate barrier characteristic on the plate.

● If a battery sits unused and uncharged for months before you receive it, change vendors.

● A partially sulfated battery will not meet its intended power characteristics.

● An alternator is not a charger and will not de-sulfate a battery under normal conditions.

Bottom line: Technically speaking, there is a verifiable reason Interstate is serious about “Guaranteed Freshness.” Don’t let the dust settle or allow the battery to become stale and sulfated. Give your customer the “Freshest Guaranteed” battery possible.

— Courtesy of Interstate Batteries

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