Strange battery charger operation?

Pskhaat

2005 Expedition Trophy Champion
On my last trip, I burned out some old aux. battery cable. In the meantime I've been keeping the aux. (and a few other batteries I have) charged via two older Craftsman float chargers (1.5A). They both have worked flawlessly for many years.

Now when I charge ANY battery both of these chargers audibly click on and off, cycling every 15-30 seconds or so. I know they turn "off" because I have an after-run fan that I can hear the fan speed change when they're on. However if I connect a voltmeter to them they show consistent 12v.

I'm pretty sure these charges don't modulate their charge.

What would cause pretty basic 12v chargers to cycle like this?
 
I only get this condition with my cheepo century charger when I'm attempting to charge a cooked battery....

Spence
 

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