Looks like I killed one of my small AGMs last night, but the behavior it exhibits now is quite strange.
I was playing around with a new fish-finder / chartplotter on my desk and connected it to a 5AH AGM. When I finished last night I forgot to disconnect the FF and it drained the battery overnight (the device was off, but obviously draws something even then).
This morning I found the battery at 9.4V - ouch! When I connected a Battery Tender it showed to be in "bulk charge" mode but only pulsed every so often. The meter I use with it (Watts Up) let me see the voltage pulses but strangely didn't register *any* current. Worse, the average battery voltage started *falling*!
So I took it back to my electronics bench and connected it to a variable power supply. Started out at battery voltage and slowly turned up the wick. Strange... Got all the way to 12.4V before I saw any current reading! Even at nearly 14V (measured at the battery terminals with a meter as well as the Watts Up) the current was only 100mA or so. Left at 12.4V the current finally rose to just 40mA after a half hour or so.
Take the battery off and let it rest - right back down to 9.4V!
So what happened inside this thing? The failures I've seen (generally old batteries with obvious swelling) will exhibit low terminal voltage but will still consume current if you try to charge them. Not taking any charge suggests an open cell, but why am I still seeing voltage across the terminals? The dead cell acts like a capacitor?
This also shows I probably should build a low-voltage cutoff for my kayak... I'm sure I'll forget to disconnect the darn thing from time to time, don't need to kill more batteries!
