i found this on line american too on charging A G M batteries your might be whats called out gassing if your CTEK is not backing off the voltage
vehicle chargers will only charge at 13.5 volts, which is suitable for running 12 volt bulbs, but this will only give you 80% of the charge when allowing for the potential difference. This shows that an American motorhome requires a good charger to look after its batteries.
The charger needs to be able to carry out a bulk charge, an absorption charge, and also a float charge. These three charge methods require performing at the correct voltage, or the process will be deemed useless.
Bulk Charge
This is the first stage of charging and shows what a fully discharged battery will go through. The battery charger fitted to your R. V. should have a maximum charge rate of 10% of the total battery, or the batteries capacity. 4 x 100 amp hour batteries will require a 40 amp charger.
If we use the above as our example the charger will charge batteries at 40 amps current, until the voltage rises in the battery to 80 – 90% fully charged. As you can see from the table, the voltage will be between 12.42 and 12.5. To allow the battery to reach this voltage, the charge voltage will be around 15 volts. There is no correct voltage for bulk charging, that is why the 10% of total battery capacity is important. It is essential not to supply the battery with too greater current and damage it.
Absorption Charge
This is the second stage of the battery charging. At this stage the voltage remains constant and the current gradually tapers off, as internal resistance increases during the charging process. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.
Float Charge
This is the final stage of the charging. After the batteries reach full charge, the charging level is reduced to a lower level (typically 12.8 to 13.2 volts). This reduces gassing and prolongs the battery life. This stage of charging if often referred to as a maintenance or trickle charge, since its main purpose is to keep an already charged battery from discharging.
If your American motorhome has a good charger fitted, you should get 4 to 6 years battery life, but often you will find that the poor battery chargers will require you to change the batteries about once a year.
As batteries age, their maintenance requirements also change. This means longer charging times and/or higher finish rate (higher ampage at the end of the charge). Usually older batteries need to be watered more often, and their capacity decreases.
if this helps Nick
