This may be off topic, but the part about the Prius battery needing voltage, to start charging reminds me of a similar situation, with my 36v 6 battery (older) Club Car golf cart... Last year, after 7 months away from FL, I asked a friend to put a charge in the cart. He wrote back, that my charger wouldn't work... Hmmm.. that's a $300 bill for a used charger... Sure enough, when I tried it myself... nada... Last ditch stand... used my 12V car charger to put a minimal charge into 2 batteries at a time, just to be able to move the cart. VOILA! Last ditch try to used the cart charger... Perfect... Seems the circuit in the charger requires some minimum charge to trigger the automatic cycle.
And one more... re: safety... five years ago, after charging the cart batteries on a hot day... decided to tighten the cable terminals with pliers. Oooops... BIG BOOM... knocked me on my tail, singed my eyebrows, and sent the plastic battery caps into the air, to land about 100 ft. away and bulged the sides of two batteries.... 36 volts and high amps, cause a big hot spark, and nitrogen gas is pretty powerful. Nothing like a personal experience to learn a lesson.
And one more... re: safety... five years ago, after charging the cart batteries on a hot day... decided to tighten the cable terminals with pliers. Oooops... BIG BOOM... knocked me on my tail, singed my eyebrows, and sent the plastic battery caps into the air, to land about 100 ft. away and bulged the sides of two batteries.... 36 volts and high amps, cause a big hot spark, and nitrogen gas is pretty powerful. Nothing like a personal experience to learn a lesson.