Guide: Battery Chargers and EFB, Calcium, AGM and Gel batteries; What is Recon, Sulphation and Stratification?

I have posted a number of questions to CTEK customer support over the last couple of years in regards to using my charger on different battery types (the user guide is light on info). Info on the web is also lacking, or sometimes incorrect (many car forums carry inaccurate info). So I compiled this guide as a personal reference and to help others. Whilst it is written primarily for the CTEK MXS 5.0 charger, much of the information will still be relevant for other charger types.

In this document:

In car charging

You can connect your CTEK MXS charger to the battery while it is installed in the car. Even the RECOND programme mode, where the max voltage peak is 15.7v (+0.3v higher than non-RECOND programmes), is perfectly safe for most cars. Typically anything under 16 volts shouldn’t cause any problems. (Though it is still worth double checking with your car manual as getting it wrong could be costly).

RECOND Programme mode

First off, only use RECOND mode with a standard Lead Acid Battery or Enhanced Flooded Battery (EFB) (this includes calcium EFB batteries). RECOND will not help an Advanced Glass Matt (AGM) battery. It could even harm a Gel battery. Find out more about what RECOND mode does in the Stratification section below. RECOND + AGM programme can be safely used on Ca-Ca (Calcium) type batteries, see more below.

Desulphation (Charging stage one)

Desulphation happens automatically, if required – not just in RECOND mode. Every CTEK charger mode starts with a desulphation step at the beginning. If the charger diagnoses a sulphated battery, it will attempt desulphation. If not, it will move on to step two.

Sulphation is where sulphuric acid crystals build up inside the battery on the electrodes. During desulphation the charger will pulse voltage peaks to help break down and remove these crystals.

CTEK chargers will spend as long as is required, or until a prefixed period of time has lapsed, on desulphation. If the charger is unable to complete this step within the time period it will show an error and the charger will not move on to the next step. You may need to attempt this process several times if sulphation is extreme. I have had success with small 7Ah AGM UPS batteries in the past, but never with a dead car battery. If after several attempts you still receive an error then you may need to replace the battery.


What the RECOND programme includes is a process called destratification, which creates a controlled gassing that remixes the acid and equalises the acid balance. This process helps to reclaim battery capacity and restore battery efficiency. The success of the reconditioning process depends on how severe the problem is.

What Stratification physically is is a situation where the electrolyte, which is denser, sinks towards the bottom of the battery cells and thereby ‘starves’ the top of each cell, thereby reducing the potential of the battery to drive current.

Stratification can happen if the alternator charging voltage is too low or the charging time is insufficient (due to short drives, or heavy use of accessories that reduce charging current), or where a battery is deeply discharged and unused for a period. Different batteries suffer more or less from stratification, depending of the battery type and design.

Because AGM or Gel batteries do not suffer with stratification, RECOND mode won’t help revive these types of battery. It may even damage the health of Gel batteries.

AGM batteries and the AGM mode

The AGM programme uses a slightly higher voltage (+0.3v) at it’s peak. That’s all. It will still attempt a desulphation stage at the start of the programme if deemed necessary by the charger. The higher voltage is just better suited to these types of batteries.

Calcium batteries

CTEK charger AGM+RECOND mode?

So if destratification doesn’t benefit an AGM battery, why does a CTEK charger provide an AGM + RECOND mode? The answer is that calcium batteries can benefit from this programme. Calcium (Ca-Ca) batteries can suffer from stratification, so using RECOND to create a gentle gassing in the battery can help restore power (See: Destratification, above). Calcium battery characteristics are also better suited to the higher voltage (+0.3v) of the AGM programme. Hence it being possible to use the AGM and RECOND programmes together.

Thanks to CTEK customer support for answering my many questions!

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Questions or comments? Let me know!