A 12v battery is made of six 2 volt cells. When we discuss charging we have to consider the application in which they are used and how long you want them to last. This ties in with Depth of Discharge (D.O.D.). Many variables exist but the gist is the faster you discharge and the higher the voltage on recharge or higher the current is on recharge the quicker the active material deteriorates which causes end of battery life.

Batteries typically have voltage ranges to charge for bulk / cyclic applications and float life applications. There is also a stipulation on the max initial charge current, the larger the battery the higher the current. If comparing brands an initial inrush current that is higher than another brand may be an indication of a better quality product with a lower internal resistance if made with better raw material which will also be reflected in its cost. Alternatively, it will indicate a thicker plate which is ideal if you require a deep cycle battery.

AGM batteries should bulk charging voltage between 2.4 and 2.45 volts per cell, and float voltage charge between 2.25 volt and 2.31 volts per cell. That is 14.4-14.7 volts bulk and 13.5-13.9 volts float. These figures are not temperature compensated and this needs to be factored in.

See temp. compensation charging FAQ but in short, allow for the addition of 0.03v per degree C below 25⁰C and the reduction of 0.03v per degree C for every degree above 25⁰C. Temperature compensation can not be calculated if you do not have a temperature sensor which is typically an optional extra on many chargers. It does come standard on all CTEK chargers above the 7A rating.