Types of batteries

In Telwin's battery charger world everything rotates around the batteries used to supply power to vehicles. A short description of the main characteristics of the different production technologies of batteries used in the Automotive sector follows.

Traditional WET batteries with free acid (flooded)

"WET” batteries are traditional and have free acid (flooded); they are accumulators of the Lead-acid type that mainly have these components (see FIAMM TITANIUM photo):


  • Container (single block): made of plastic material (polypropylene - PP/PE) with 6 identical cells (12 V battery) positioned internally and insulated from each other.


  • Plate group: made of a definite number of positive and negative plates (electrodes) that vary according to the cell size and the plate data (capacity and cold-start current) of the battery. The plate groups are positioned inside each individual cell and are serial connected to each other in a way that creates the 12 V battery.


  • Polyethylene dividers (PE): normally of the sack type, the dividers are placed between a positive and a negative plate to prevent direct contact between the two oppositely-charged electrodes, which avoids short-circuits that would break the battery.


  • Electrolyte: a sulphuric acid solution (H2SO4) diluted in water, into which the plate groups are immersed. The plate groups are necessary for the electrochemical reaction to take place inside the cells. The reaction is at the basis of Pb-acid battery operation.


  • Connecting jumper: made of lead or lead alloys, they unite all the plates of the same charge together and serial connect the six cells in a 12 V battery.


  • Cover (single cover): made of plastic material (polypropylene - PP/PE) with 6 identical cells (12 V battery) positioned internally and insulated from each other. The cover is welded to the single block using a heat welding process that guarantees perfect mechanical sealing between the two components. The caps can be for centralised degassing (the gases produced during the charge phase inside the battery are expelled externally by a single breather hole on one of the two shorter sides of the battery) or of the single screw and/or pressure cap type, where the gas exits through a hole on the upper part of each individual cap.


  • Degassing caps: these caps can be single (screw) or multiple (pressure) and they close the holes on the cover; they are needed when filling the battery with the electrolyte for the first time during the first charge phase. The primary functions of the caps are to allow correct degassing of the gases produced inside the battery during the charge phase and to allow the battery to be topped-up with distilled water when necessary and possible.


  • Positive and negative terminals: made of Lead alloy and normally conical, the terminals are positioned on the top of the battery cover. They are sized according to standards, and the positive and negative terminals are different so the battery can be connected to the external device (load) correctly.


Wet batteries (flooded) are classed mainly in three macro categories according to the constructive technology of the electrodes (positive and negative plates):


  1. Ordinary maintenance battery: both grids of the positive and negative plates are of Lead-Antimony alloy (PbSb/PbSb) and the level of electrolyte inside the individual cells must be checked periodically and topped up by adding distilled water only (never use acid) through the degassing caps on the cover.


  1. Reduced maintenance batteries, also called “Hybrid technology” batteries: the positive grid is of Lead-Antimony alloy but with a low antimony content (PbSb), while the negative grid alloy is of Lead-Calcium (PbCa). When overcharged, these batteries have a lower "water consumption" than ordinary batteries that require maintenance, therefore the levels of electrolyte are only restored when necessary and in particular working conditions (extreme working temperature, extended overcharge, etc.). Even in this case only add distilled water (never acid) through the degassing caps on the cover.


  1. Maintenance free batteries (MF): the grids of the positive and negative plates of these batteries are produced using lead alloys without antimony and can be of the Lead/Calcium/Tin (PbCaSn) or Lead/Calcium/Tin/Silver (PbCaSnAg) type for the positive plate and Lead/Calcium (PbCa) for the negative plate. These batteries consume little water during overloads, therefore in normal usage conditions they do not need to be topped-up; they do not normally have visible caps and can be inspected directly. Maintenance free batteries often have an additional component called “Magic Eye” which is normally positioned on the cover near the third battery cell, and which gives an approximate indication on the battery conditions:


a) GREEN: battery CHARGED


c) WHITE: Electrolyte level very low (the battery needs to be replaced)

WET batteries with free acid type AFB (Advanced Flooded Battery)

They are cutting edge Pb-acid batteries, and their design is based on the technical development of traditional, maintenance free flooded batteries. These batteries were developed over the last years for application to Micro Hybrid vehicles with Start&Stop systems. The main construction characteristics that make AFB batteries different from traditional flooded batteries are: better electrolyte reserve above the plates, negative plates with grids having special alloys and optimised active material for specific functions, double-layer dividers for increasing charge/discharge cycles in the Start&Stop mode, increased electrode (plate) resistance to corrosion. The main advantages when compared to traditional, maintenance free flooded batteries are: more resistant charge/discharge cycles, higher starting power in particular at low temperatures, longer life cycle (when measured in terms of energy output), no maintenance.


See photo of the FIAMM Ecoforce AFB battery.

VRLA AGM batteries

VRLA (Valve Regulated Lead-Acid battery) type batteries with AGM (Absorbent Glass Mat) technology are Pb-acid batteries where the positive grid is made of a PbCaSn alloy and the negative grid is made of PbCa; they are the only batteries that are completely maintenance free.


The main characteristic that distinguishes a VRLA AGM battery from a traditional flooded battery is the technology used to recombine the gases. These batteries operate on an “oxygen recombination” cycle.


With a traditional flooded lead battery, during the charge phase the two gases in water, namely hydrogen and oxygen, separate. These two gases exit from the caps on the lid and the level of electrolyte inside the battery consequently reduces.


With VRLA AGM batteries, the acid is contained in a special microporous divider made of glass microfiber (Absorbent Glass Mat) that is impregnated with a controlled quantity of electrolyte during the production phase. During the recharge phase, the oxygen that is released by the positive plate because of water dissociation can migrate towards the negative plate, to which it fixes. It then combines with the hydrogen, which recovers the water that had dissociated.


A closed electrochemical cycle is created in this way, which initially and during normal use does not emit gas externally and/or does not consume water.


If the battery is overcharged and a large quantity of gas develops inside it, the excess is released by a safety valve, positioned inside the cover of each cell, that opens. This valve was designed to open at a pressure of about 0.2 bar in new batteries, but in normal working conditions it is closed because it must stop air from entering the battery (the oxygen would discharge the negative plate). This is why these batteries are called VRLA (Valve Regulated Lead-Acid Batteries) with AGM (Absorbent Glass Mat) technology.


It is clear that opening the cover would damage the valves and as such should never be done.



VRLA AGM batteries are more resistant to extreme temperatures, discharge and charge cycles, vibrations and mechanical shock. They are also completely maintenance free, easier to charge (in particular the dynamic type), and they have greater starting power than traditional flooded batteries.


These batteries are very suitable for use with Micro Hybrid vehicles with Start&Stop + B.E.R. systems and this why they have become more widely used over these last years.



See photo of the FIAMM Ecoforce AGM battery.

GEL batteries

GEL batteries are made using one of the production technologies used to make VRLA-type batteries. The substantial difference between AGM and GEL batteries is that in the latter the electrolyte is not a liquid but is contained in a special silica gel in which the plate group is immersed, while with AGM batteries the electrolyte is completely absorbed by the special glass microfiber divider.


GEL batteries are not normally used for starting applications in the automotive sector because high working temperatures inside the vehicle motor area significantly increase the volume of the gel, which causes repercussions on electrical performance and battery life. On the other hand, very low temperatures cause the GEL to concentrate inside the cell, which increases the internal resistance of the battery; this has a negative effect on the cold start current (-18°C), which becomes much lower than that of AGM or flooded batteries.



GEL batteries are therefore more suitable for energy applications than for power applications, and they are used in industrial applications where high resistance to discharge and charge cycles and/or greater buffer mode life are required. They are also used for powering on-board services in the nautical and free time (Motorhomes) sectors as an alternative to AGM batteries.




Thanks to FIAMM SpA for the pictures and the support provided solely to implement the part relative to the type of batteries. Trademarks and images belong to FIAMM SpA; any transfer of the same to a third party is forbidden; the brand and the images cannot be used for any other purpose except made for the present document.