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How a battery charger works

Tom Vondrasek | 6th May 2023 | 5 minutes to read

There are a wide variety of different battery chargers available that are suitable for charging different battery types. Using a battery charger incorrectly can damage your battery and also be dangerous, so it’s important to understand the various stages a battery charger goes through during the charging process and when it’s appropriate to use one on your battery.

This article explains how to set your battery charger as well as explaining each stage of the charging process the charger goes through:

  1. How Does a Battery Charger Work?
  2. Is It Bad to Fast Charge a 12V Car Battery?
  3. What Is the Correct Number of Amps To Use When Charging My Battery?
  4. Setting the Battery Charger
  5. 9 Stages of Charging
  6. Buying a Battery Charger

How Does a Battery Charger Work?

Battery chargers pass a DC current (in amps) through the battery for a length of time and the hope is that the battery will absorb or hold onto this. The voltage is also important, but it is the amps that re-energise it. If the amps are set too high for the battery, it will charge faster but may also damage the battery plates that hold the charge. This is why slower is better and using the right amount of current flow is important.

Is It Bad To Fast-Charge a 12V Car Battery?

In short, the answer is yes, it is. If charged too quickly the battery can heat up and damage the plates internally. Slow charging is better for a 12V lead acid car battery as it helps it hold the charge for longer. You can still fast charge if circumstances require it and you needed the battery operational quickly. Slow charging is best for the battery.

What Is the Correct Number of Amps To Use When Charging My Battery?

Image of a battery charger with leads

The good people at Century Batteries have provided an easy method to work out the correct amps required for your battery charger to recharge your battery.  Each battery has an amps per Hour (Ah) rating. It is common to see this on deep cycle batteries though even a start battery usually rated in CCA (Cold Cranking Amps) will have an Ah rating.

For a start battery the amps required to charge it is 10% of the Ah rating figure

Example of Start Battery Amps

If a start battery has an Ah rating of 50, then the amps needed to charge it is 10% of 50 which is 5 amps.

If a larger start battery has an Ah rating of 90, then the amps needed to charge it is 10% of 90 which is 9 amps.

Example of Deep Cycle Battery Amps

Deep cycle batteries are different. You can use a figure of 20% of the Ah rating if it is an AGM or Gel battery.

If a deep cycle battery has an Ah rating of 32, then the amps needed to charge it is 20% of 32 which is 6.4 amps.

If a larger deep cycle battery has an Ah rating of 75, then the amps needed to charge it is 20% of 75 which is 15 amps.

Setting the Battery Charger

Battery chargers vary a large amount depending on what you bought and how recently you bought it. Most modern chargers that are towards the top of the range will auto detect the battery, battery type and charge accordingly. Most low priced or mid-range chargers still require you to set the charge rate and battery type.  Where you need to set the amps use the formula in the previous paragraph.  Always set to the closest lowest setting to avoid overcharging.

Some modern battery chargers struggle to charge a dead flat battery if the voltage in it is too low. It needs to detect a certain voltage to kick start the charge process. You can trick it by putting a 9V battery across the terminals (positive to positive, negative to negative) and once it kicks off, you can remove it.

9 Stages of Charging

These are the automatic charging stages of a battery charger.

  1. Diagnosis – Determines whether the battery can be recharged or not.
  2. Desulphation – Removes sulfation of the lead plates so the battery can hold a greater charge.
  3. Pre-Charge – A low voltage and current is used to slowly increase battery condition.
  4. Soft Start – An increased voltage and the current is slowly increased over time to minimise battery stress.
  5. Bulk Charge (Constant Current) – Charging is at maximum current, then is gradually reduced.
  6. Absorption (Constant Voltage) – Applies an even higher Voltage while reducing current over time to reduce battery stress.
  7. Analysis – Battery discharge rate over time is determined. Based on performance, battery is recharged or goes to the next step.
  8. Reconditioned (High Voltage Repair) – Applies maximum Voltage to check battery condition and clean plates. Charger may not complete this step if the battery is OK.
  9. Maintain – Charger maintains the battery by monitoring Voltage. If it drops below the threshold it will recharge.

Buying a Battery Charger

The formula is also good when purchasing a battery charger as you need to make sure it will work well on your battery. If you have multiple vehicles, you need to check all the batteries and see if you can get a charger to work on all of them. It is also a good idea to get one that maintains batteries so you can move it around vehicles that are not used that often. It may be more expensive though the benefit of being able to start your vehicles when you need them is a real bonus.

For battery chargers with the latest features and for vehicles ranging from 4x4s and family cars to caravans and motorcycles, head into your local Repco store or shop online today. Make sure to keep an eye out for our specials and ask our friendly staff for information regarding your battery charger if you have further questions.

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