For two lead gel batteries I bought the CTEK MXS 10 battery charger. This has a maximum charging current of 10A. The device is described as usable for any kind of car lead batteries, AMG lead batteries and most gel lead batteries.
The 10A version has also a temperature sensor. With this, the device adjusts the output-charging voltage depended of the ambiance/ battery temperature.
The normal maximum charging voltage @20°C is 14,3V. Wet lead batteries have a maximum charging voltage of 14,4V @ 20°C. Over this voltage, the battery begins to gas. That means, the battery loses water as h2. This gas is explosive. This wet batteries can be refiled with distilled water. else they have a capacity lose. For wet batteries it is a need, that they come to the gassing-phase, that the acid-layers are mixed.
Gel lead batteries have solid content. The lead is bound in gel. It is not recommended to charge such batteries with voltages over 13,8V @ 20°C . Over this voltage, they can go over in the gassing-phase. An because the lead is bound and the cells are airtight, it is not possible to refill water. This batteries are maintenance free.
For my first battery (gel lead-acid battery “starting bull 12V 95Ah / 595 33” http://www.batteriepool.de/Autobatterien/bis-100Ah/Banner-Starting-Bull-12V-95Ah-595-33::78.html) it worked perfect. Without attaching the external temperature sensor the charger charged with maximum 14,3V. As I could observe, the battery didn’t gas (I heard no “blub” or boiling noise) .
My second battery caused fast “blub” noise over 14V charging voltage. From the datasheet of the second battery (oerlinkon batteries 12CP150, 12V 150Ah, Datasheet 12CP150), and other source in the Internet, the most gel lead-acid batteries are known for a maximum charging voltage of 13,8V.
To change the maximum charging voltage I had the idea, to “change” the actual ambiance temperature, by heating or cooling the sensor. Then I had the better idea, just to change the value, the sensor measures electronically. If the temperature raises, the charger lowers the output voltage.
For a test of the sensor, I heated it up with my hand around it while I measured the resistance. It has lowered. This means, that it is only a NTC-resistor in the temperature-sensor. So I replaced the external-sensor by a potentiometer. With this, I can adjust the maximum output-voltage of the charger. Problem solved!