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The charging voltages for Absorbed Glass Matt (AGM) Batteries are generally the same as for any standard battery** - BUT CHECK with your SPECIFIC Manufacturer’s Charging Instructions. Since their internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents.
The AGM battery has an extremely low internal electrical resistance. This, combined with faster acid migration, allows the AGM batteries to deliver and absorb higher rates of amperage than any other sealed batteries during discharging and charging.
For flooded wet batteries, internal resistance can be as high as 26%, which is the charging current lost to gassing, or breaking up of water. Gel acid batteries are better at approximately 16% internal resistance and require only roughly 116% of rated capacity to be fully charged. Advanced AGM has the lowest internal resistance of any battery manufactured - only 2 percent.
Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period. "C/8" is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps.
Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity.
The Concorde AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle. For a 220 AH battery, this would (theoretically) equal 880 Amps - much higher than your wiring could accommodate!
**Charging at 15.5 volts will give you a 100% charge on Lead-Acid batteries. Many authorities suggest 14.0 volts (2.25 - 2.33 volts per cell) as the maximum charge voltage for AGM’s. Refer to specific your AGM Battery manufacturer’s Charging Instructions, and compare to your Alternator & Charger output characteristics (With engine running AND lights or other loads on, voltage reading on Alternator/cCharger output should read between 13.0 and 15.0 volts).
“Concorde” AGM VRLA-AGM Batteries:
Initial charge or recharge - 14.22 to 14.4 volts at 250C (770F)
Float charge - 13.38 volts at 250C (770F)
Equalize charge - 14.4 volts at 250C (770F)
“MK” Initial charge charge 14.6 - 14.6 v
“Optima” Initial charge below 15 v - float charge 13.8 v (1 Amp)
AGM batteries will not gas under normal operating conditions, however, under severe overcharge conditions gassing is probable (to some degree).
AGM batteries can be equalized, but only under controlled constant current conditions. Always follow manufacturer’s recommendations for the correct charging voltage profile.
Three-Step “Smart” Charging”
The first step is bulk charging where up to 80% of the battery energy capacity is replaced by the charger at the maximum voltage and current amp rating of the charger. When the battery voltage reaches 14.4 volts this begins the absorption charge step. This is where the voltage is held at a constant 14.4 volts and the current (amps) declines until the battery is 98% charged. Next comes the Float Step. This is a regulated voltage of not more than 13.4 volts and usually less than 1 amp of current. This in time will bring the battery to 100% charged or close to it. The float charge will not boil or heat batteries but will maintain the batteries at 100% readiness and prevent cycling during long term inactivity. Some gel cell and AGM batteries may require special settings or chargers.
Temperature Compensation is important for all battery types, but AGM and gel cell (valve-regulated type batteries) are more sensitive to temperature. For maximum life, temperature compensation is essential when the operating temperature is outside the range of 68 to 77̊F (20 to 25̊C).
AGM''s may cost 2 to 3 times as much as flooded batteries of the same capacity. AGM batteries main advantages are no maintenance, lower self-discharge, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes. Not everyone needs (or can afford) these features. Brand names, such as Concord, Lifeline, Odyssey & Optima, are all excellent batteries, but beware of AGM batteries coming out of China.