The hive will know - Battery Tenders

Is there an easy way with a multimeter or such to test a battery tender to make sure it is operating properly?

I have about 6 in the shop, but discovered today one battery died even with tender attached.

You can get a general state of health with a multimeter but you probably can't determine if the unit is operating 100% as designed.  With the unit attached to a slightly discharged battery you should see a higher voltage than one that's fully charged. As a general rule the deader the battery the higher the voltage but you should never see anything higher than 14 and change volts.

As an asside, batteries (particualurly, lead acid) can die even when on a tender.  If the acid evaporates then no battery tender can keep it working.

If it is a sealed cell or dry cell style they can usually be brought back to life by charging them in series off another battery.

I just had a battery die that was on a tender. Its hot here in Fl, you probably need a new battery. Not much to go wrong with a tender. 

Measure the battery voltage off the tender. Measure the battery voltage on the tender. If higher on the tender, the tender is charging.

A lot of sealed lead acid batteries can still be opened with no trouble or damage.

I had a tender go bad and discharge a battery. Killed the battery wouldn't hold a charge anymore. I did an ohm check across the leads unplugged and got continuity. C Checked my other tenders and no continuity unplugged. DO NOT DO A CONTINUITY CHECK PLUGGED IN.