Would appreciate advice about a generator over-charging an 12v volt AGM battery to 18 volts.

Hello,

I was wondering if anyone might have any advice on a possible charging problem with an AGM (Absorbed Glass Mat)  Battery?

My Father-in-law has a Harbor Freight " predator brand"  generator which he recently replaced the starter battery on.  The new battery is a 12 volt AGM battery. I think that the old battery was just a lead-acid deep cycle.  
The problem is that when the generator runs it is charging the battery to 18 volts.  
I've read that AGM batteries have  a low internal resistance and I'm wondering if the charging circuit might need to be modified to handle the AGM battery?  I'm sure that whatever the charging circuit is on the generator it's pretty low tech.

Surely 18 volts is too much charging voltage for a 12 volt  AGM battery?  I was expecting more like 14 volts maximum.

Everything that I've managed to read states that AGM batteries need a special charger to charge independently.  Does anyone know why it seems to be ok to put an AGM in a machine with a normal charging system like a car or tractor, etc.?

Thanks for any advice or insight.  I really appreciate it.

Regards,
Tim

Sounds more like a generator problem than a battery problem.
Does it act the same way with other kinds of 12 volt batteries?
It's a 12 volt generator?
Or a 110 generator hooked up to an old style 12 volt battery charger?

You can try reducing the RPM to lower the voltage and  add a charge controller to the battery, which would prevent over charging.