Incompatible or faulty? B&D drill battery fully charged. Doesn't work. Other's do... Why?

[Majdnem]

Two B&D 18v NiCad batteries. They both fit my old 18v B&D EPC18 drill, but only the 2-pin/prong A18E battery works. The 3-pin ML:HPB18 is completely dead in the drill, but, after testing, seems to be fully charged (16.6V = OK for a not-new battery?).
So, is this because the batteries are importantly different in some way, or because one battery is faulty? What kind of 'faulty' would fully-charged-but-not-working be? Or are they significantly different - is the three-prong one incompatible in some way, even though it's the correct voltage and fits the drill - snaps into place fine?

If the battery is not faulty, but just incompatible, could anyone help me understand the difference between these two batteries - at least in terms of what tool the useless-to-me 3-prong one is intended for, so that i can offer it to someone who could maybe use it...? Thanks!

 

[RandomGrinch]

From what I can gather, the two batteries are compatible. The 3rd contact apparently connects to a thermistor that monitors the battery temperature when charging.
The only thing I can suggest (if there isn't a problem with the battery), is to leave it charging for longer.
If you are using the charger for the first battery, it might take longer to charge the extra capacity of the second.
Other than that, I can only think it's a battery fault.

 

[Majdnem]

Thanks, RandomGrinch - I'm thinking now that it must be a fault rather than an incompatibility, but it's the nature of the fault that i'm baffled by. The battery *is* charged up. Smart charger goes to solid green as soon as soon as connected and multimeter indicates 16.6V, which I assume is OK for a not-new battery, so why would it seem completely dead when slotted into a drill that works fine with other batteries...?

 

[RandomGrinch]

If it was a smarter device than a drill, the undervoltage detection could have kicked in (although not likely!).
Otherwise, I have seen batteries that will measure at the rated voltage by multimeter, but under load are unable to supply any current. This is why battery checkers are more suitable than multimeters at times.
Can you connect a car bulb across the output and see how long it remains lit for?
Other than that, there might be a slight difference in the tolerances between the branded and unbranded batteries that means a poor connection?