Hooked up a 1.5 year old battery (12N7-3B Parts Unlimited) to a Deltran Battery Tender Plus unit (similar to this Battery Tender Plus Charger 12Volt Maintainer 1.25A # 0128 | eBay ).  After a few minutes, the battery acid was bubbling and sizzling.  Is this normal?   I hooked it up to the same unit last winter and didn't notice this.  

I'm far from an expert but if it's pumping 1.25 amps in constant then I'm not surprised it's bubbling. 0.25 or less I'd think would keep it up. Maybe the charger just means it has capacity to crank up to 1.25 but should back off to next to nothing when fully charged. I'm feeling less of an expert after writing that so look forward to what the 'real' experts say. Just looked at the ebay link & think it's just a standard charger with fancy advertising but my take is it pumps hard to get a flat battery up to speed then will back off to almost zero to maintain it. My little charger does that. Are all the cells bubbling or do you think there's a faulty cell?  Mind you it might not charge at all if one is faulty & can't complete the circuit.

If it was low, the charger is likely hitting it with higher voltage (I've seen as high as 17 volts!) to maximize initial charging.  If you have a voltmeter, you can verify voltage during charging. 

As battery charges, the charger should reduce the voltage (and the charge rate).  You said "similar" which could mean a lot of things...  most importantly is the amp rating.  If your actual charger is 1 amp or even 1.25 max amps, you should be fine. 

If you are using a "similar" charger with a higher rating like a 10 amp rated auto battery charger.  STOP!  Get the correct charger.

Make sure water level (use distilled only) is good and it's not just one or two cells "boiling"...  that might be a bad battery.

Keep an eye on it...  a "similar" charger will reduce charging voltage (and therefore current) as battery charges so "boiling" should subside fairly quickly as charging voltage and current drops....  unless the problem is a bad cell or two which you should be able to see as they'll be the ones doing all the bubbling.



 

Am I missing sumptin’, a 12 volt charger on a 6 volt battery?

Yup. You’re missing sumptin.
125B is an electric start using 12v battery.

12N7-3B 

Now I know, thank you.

Tenders(smart ones) charge bat. similar to this:
Initial
charge 1amp until 13.8volt
discharge 10mA until 12.8volts
calculate state of charge(SOC)

charging
charge maximum until 14.8 volts and SOC >20%
discharge 10mA until 12.8volts
calculate state of charge(SOC)

repeat until SOC > 80%

maintain
charge to 13.5volts
discharge 10mA until 12.8volts
repeat