An AGM battery should be charged at the equivalent in amps of 0.1C or 10% of its capacity in Ah for it to last as long as possible.
To work out the optimum charging current of an AGM battery, simply multiply the battery’s capacity by 10%.
For example, the charging current of a 100Ah battery using an external charger, be set to 10A (10% x 100Ah).
Whereas you can charge the batteries using higher currents, using extremely high currents repeatedly can damage them.
(Always check the technical specifications of your battery to confirm what current rates are safe)
How to tell when the AGM battery is fully charged
Use a voltmeter (digital multi-meter set to DC volts) to measure the open-circuit voltage of the AGM battery. The voltage of a fully charged 12V battery is 12.7-12.9V (estimate).
When you use a modern automatic charger (with the AGM setting) to charge the battery, once the battery is fully charged, the LED status show and the charger stops the charging process.
With a manual charger, you need to rely on the elapsed charging time of the battery under charge depending on the charger rate and the open-circuit voltage.
For example, a 200Ah battery charges fully in about 12 hours when using a constant voltage charger set to 20 amps (see table below). What to learn how many watts of solar you need to charge a 200Ah battery? Find out here.
The table below shows estimated charging times for selected AGM battery sizes for different charger settings.
*Note: This assumes the batteries were deeply discharged to 11.2V.
|Battery capacity (Ah)||Charger set to 2 amps||Charger set to 5 amps||Charger set to 10 amps||Charger set to 20 amps|
|50Ah||30 hours||12 hours||6 hours|
|100Ah||60 hours||24 hours||12 hours||6 hours|
|200Ah||120 hours||48 hours||24 hours||12 hours|
How long it takes to charge a battery (external charger)
The time a battery takes to charge fully depends on:
- its capacity (Ah). A larger capacity battery say 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).
A fully discharged battery takes a long time to charge compared to a partially discharged battery assuming the same charge rate is used.
The table above shows estimated charge times for selected battery capacities using different charging rates (2 amp, 5 amp, 10 amp, or 20 amp).
You can also use the calculator below to estimate the charging time for an AGM battery.
Should I charge my battery at 2 or 10 amps?
The current setting you select to charge your battery depends on the capacity of the battery. In general, the recommended charge rate is 10% of the battery capacity.
If it is a 20Ah, battery, set to 2 amps (10% x 20Ah). For a 50Ah battery, set the charge rate to 5 amps (10% x 50Ah).
Setting it to a lower charge rate (amps) extends the charging time unnecessarily. On the other hand, using high charge rates repeatedly can damage the battery.
What voltage should you charge a deep cycle battery to?
The voltage settings for your charger mainly depend on how your 12V AGM battery is used. If the battery is used in standby mode, your charger’s settings should be between 13.5 and 13.8V.
If on the other hand, it is regularly discharged and then recharged (cycle use), set the charger voltage settings to between 14.4 -14.5V.