I have an HD battery tender for my Road King. Is it powerful enough to use on my car battery in a pinch?

 

Yes it will work, but it will take a long time to charge the battery. The HD tenders only charge at a 1 or 2 amp rate.

 

A tender is used to maintain a battery, not to charge.
Yes, it can be used to maintain a 12 volt battery.
If you are trying to charge a weak battery (not dead) it could take a few days.

 

A rapid charger that you can get at Pep Boys, Walmart, etc...is a good investment for less than $50. Get one less than 7amps, and you can plug it into the Harley trickle cable. Helps for those times you leave it on accessory all night long with the radio on.