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**.

**Related Questions**

### 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**.

**Related Topics**

How long to charge a battery with Solar

Overcharging of batteries | Is it good or bad?

Can you charge your car battery using an extension cord (or ordinary electrical wire)?