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Old 01-15-2018, 07:23 PM
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Default AGM battery - NOT for classic cars.

Robin's 2010 Escape needs a battery. Her OEM battery lasted seven years!

Upon removal of the battery and its covers, the Motorcraft label revealed 'AGM' which means, Absorbed Glass Mat design. This is different from conventional lead acid (also called, 'flooded cell') batteries. The car's charging system is also designed for this battery, so don't use a standard bench or float battery charger. Unattended, it will overcharge and ruin the AGM battery. Also, don't use it in your classic car.

Expect this from AGM batteries:
•Product life lasts up to two times longer (ours lasted 7 yrs)
•Extremely heat and cold tolerant (great in Texas or Calgary)
•Ideal for vehicles equipped with lots of electronics (heated seats, navigation system, parking assist)
•Durable - Outstanding vibration resistance
•Safer - No free-flowing electrolyte or spill hazard

Wonderful, right? Maybe, if these rules are folllowed...
Charge AGM batteries at a constant current rate of no more than 10 times I20 to a voltage of 14.1V (example; For C20=100Ahr, the max charge current is 10x5A or 50A). Many 'bench chargers' peak at 15-volts which will kill this battery.

Temperature plays a big role here:
Charging voltages are based on a temperature of 77F (25C). For average operating temperatures below this range (colder than) the maximum voltage set point should be compensated with an increase at a rate of 0.018 volts / F (0.032 volts/ C.) For average operating temperatures above this range (warmer than) the maximum voltage set point should be compensated with a decrease at a rate of 0.018 volts / F (0.03 volts/ C.) Example: at 65F and 14.2 volt set point Corrected Voltage = 14.2 - ((77-65) x (0.018)) = 13.82 volts.

Modern car charging systems are designed for this compensation. Classic car charging systems are NOT. So in a nut shell, your classic car was designed to charge a standard lead-acid battery.

As a rule of thumb, AGM batteries should be use to replace OEM AGM batteries. But cars with AGM charging systems work just fine on standard lead-acid batteries so they are backward compatible. - Dave
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