If you’re without any recommended charging information for an AGM battery yet you need to charge it then it’s important that you use an optimum charging rate for your safety and for it to last!
What charge rate should you use?
This post provides tips and suggestions as to what charge rate and voltage you should use for an AGM battery.
Note: Always refer to the manufacturer’s battery specifications for charging information.
What Charge Rate (Amps) to Use for an AGM Battery?
The optimal charge rate (amps) to use depends largely on the capacity of the battery (Ah) and the battery type.
Generally speaking, the larger the capacity is, the higher the charge rate you can use to charge the battery safely.
Check the manufacturer charging instructions for a given battery type. They’ll usually indicate the recommended charge rate, voltage, and temperature combination to use.
Using a Car Battery Charger
With that said, a quick safe rule of thumb to determine the charge rate using an external battery charger is to divide the battery capacity by a factor of 10 and then select a standard charge as close as possible to the result you’ve obtained.
For example, with a 12V, 50 Ah AGM battery, divide 50Ah by a factor of 10 to get 5.
The optimal charge rate to use in this case is 5 amps. Therefore use a 12V charger set to charge at 5 amps.
Using a Car Alternator to Charge the Battery
When using the car alternator to charge, there’s no option to set the current limit.
Make sure though that you’re using the type of battery – AGM or other and the capacity recommended by the car manufacturer.
Why you Shouldn’t Use a Higher Charge Rate
Using a higher charge rate risks shortening the life of the battery. There’s also the danger that the battery can overheat, bulge and explode too.
How Long an AGM Battery Takes to Charge
As a rough guide, it takes about 6- 12 hours to fully charge a drained car 50Ah battery using a 10 amp charger. Ultimately though the charge time depends on
- its capacity (Ah). A larger capacity battery for example, a 100Ah takes longer to fully charge compared to a smaller 50Ah capacity battery
- the charger current setting (2 amp, 5 amp), and
- how deeply discharged it is (partially or fully discharged).
Signs an AGM Battery is Fully Charged
Manual Charger
With a manual charger, use the time specified in the battery charger manual as a guide to determine when likely the battery will be fully charged.
Make sure to correctly specify on the charger the type of battery being charged to avoid undercharging or damaging the battery.
Switch off the charger after the specified charging time (check the charger manual) to avoid overcharging.
Automatic (Smart) Charger
With a modern automatic charger, the charger detects whether the battery is fully charged or not and accordingly switches off the charger.
Related Questions
1. What Voltage Should you Charge a Deep Cycle Battery To?
Unfortunately, there are several variables to consider – for example, the battery chemistry – AGM, GEL, or other and whether the battery is regularly cycled or used in a float application.
Always consult the battery manufacturer’s guidelines.
That said, it is not uncommon for 12V AGM batteries in standby mode to be charged to about 13.5 and 13.8V.
For regular cycle use – charging and discharging, higher charge settings of 14.4 -14.5V tend to be used for AGM batteries,
2. Should you Charge a Battery at 2 or 10 Amps?
Always consult the manufacturer’s instructions for what charge rate to use.
That said, several car battery manufacturers recommend a 10 amp charge rate for a voltage-regulated charger that should be able to charge the battery for anywhere from about 6-12 hours depending on how deeply discharged it is.
The lower 2 amp charge rate is generally used to float charge the batteries after the bulk charging phase.
Selecting such a low charge rate for a standard – 45-50Ah AGM car battery leads to a longer charging time which may not work for you!
Closing Thoughts
What charge rate (amps) and voltage to use largely depends on the battery capacity, chemistry, and application – cycle or standby use.
Always refer to the battery manufacturer’s specifications for what charge rate in amps and voltage to use.
A quick safe rule of thumb to determine the charge rate using an external battery charger is to divide the battery capacity by a factor of 10 and then select a standard charge as close as possible to the result you’ve obtained.
Related Topics
