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AGM batteries are extremely popular in today’s market. The leading automakers use such units while assembling their vehicles due to the fact that they have a longer life cycle than regular lead-acid analogs. Some people even say that AGM batteries do not need maintenance.

This is partially true. The purchase of the best AGM battery charger and its proper exploitation are still crucial for the smooth operation of the unit. It is also important to correctly calculate the charging time, which is a stumbling block for many drivers. The article below should help you address this issue.

General charging principles

Everything is quite logical: the less energy your battery has, the longer it would take to replenish it. Depending on the degree of discharge, from several to 12 hours should be spent to achieve 100%.

Some drivers treat their cars like smartphones: they think that once the battery is full, the charging would stop. This is not the case. If you do not control the process, the unit can overheat and a short circuit can occur. Therefore, you should closely monitor the voltage, current, and temperature. If the temperature rises above 125 F, stop charging without hesitation and take a break for cooling.

Slow charging (10A or less) is the best option for AGM units. It prevents overheating and extends the battery’s life cycle. Most drivers use 3-level 12 V / 10 A chargers.

Determining the charging time

It is easy to calculate the charging time. Just take the following steps:

  1. Find data on reserve capacity (in minutes) in manufacturer’s instructions and multiply it by 0,6. For example, 200 minutes can be translated into 120 amp-hours (Ah)
  2. Measure the open-circuit voltage (the voltage maintained in a battery when it is exposed to zero load) with a voltmeter. Experts strongly recommend not to start charging before you get this indicator. For an AGM unit, 11,8 V would mean that the battery is totally discharged, 12 V – 25% charge, 12,3 V – 50%, 12,6 V – 75%, and 12,8-12,9 V – 100%
  3. With voltage numbers at hand, you will have an idea of how much energy is needed to achieve 100%. If your voltmeter shows 12,3 V, this means that you need 50% = 120 Ah / 2 = 60 Ah
  4. It is better to add 20% to avoid internal resistance: 60 Ah * 1,2 = 72 Ah
  5. If charging at 10 A, 72 Ah should be divided by 10 to calculate the charging time. So, 7,2 hours are needed to fill the energy deposits by 100%
  6. Use a voltmeter to check whether charging was successful and the necessary level was achieved

Keep in mind that amperage may change throughout charging. If it falls below 10A at certain points, an estimated time would not be enough to reach 100%. Therefore, the above calculations provide approximate data, and you should not get surprised if your unit is half-charged after 7,2 hours.

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