If you only have 10 cells then you need a controller that has an adjustable float. "12v" is a nominal value. A "12v" lead acid battery is actually 12.6v to 13.0v at rest depending on the type. Flooded lead acid is normally 12.6, some of the special sealed starved electrolyte gel cels rest close to 13 volts. A flooded lead acid battery won't gas until about 14.2, a gel shouldn't be allowed to go that high unless the maker says it's okay. Gassing in a gel can ruin it.
Your pack really is 12.0v under a light load if it only has 10 cells. A flooded lead acid battery that had 12.0v under a light load would be stone cold dead and sulfating. Appliances and controllers for 12v nominal batteries usually assume lead acid chemistry, so they would think your pack is dead or near dead.
Your 10 cell pack could go to 14.5 if all the cells are balanced, but if they aren't you will trash the pack because sealed NiMH should never go above 1.45v per cell. It doesn't sound like you have a battery management chip on this stack to prevent overcharging. An 11 cell pack isn't as likely to trash a weak cell at 14.5v. 15.95v is the max for 11 cells. You could have a completely shorted cell in an 11 cell pack it would still stay under limits for the remaining cells if you held it to 14.5v.
Maybe GHurd could join us here. He has controller kits, I think. Those might work for you.
