I was surprised when I tried this out finding it "pumps" power from the "source" car into the "dead" car.  I assumed that this would let the power thru from the source (which is at a higher voltage) to the dead one & slow/stop the flow of power if the voltages equalized, but it doesn't.  It continues to draw on the source to feed power into the dead one (even if it makes the source drop to under 12V while the dead one gets raised to 13+V which I saw on meters hooked up on both ends).  I also tried hooking this up to a small battery to charge it and it had it at 17V pretty quick, so it doesn't regulate the voltage (over 15V is considered overcharging most batteries, but this is intended for brief use).  Also the first time I tried it, it didn't begin charging (although I had it plugged in) until I started the good car. I'm not sure what held it back initially & triggered the change, but it's instantly started charging when I plugged it in ever since.