I am working with PD Buddy Sink, a small USB-C PD negotiation and power breakout board. It takes USB-C PD input, performs the PD negotiation, and produces DC power. My problem is that the PD Buddy Sink does not block reverse current flow. I am attempting to use this device to charge an SLA battery. I have a SLA-05VDC-SL-A relay-based charge controller just after the sink. The problem is occurring when the relay closes and voltage from my battery bridges through the relay and into the sink's output, causing it to attempt to negotiate with the battery, resulting in a never-ending startup cycle and no resulting "forward" current flow.
I have spoken with the maker of this device and she recommended a diode. I've tried a Schottky 1N5822 and a Rectifier RL207, both of which are leaking reverse voltage on the order of about 15-20 millivolts. This is still too much for the PD Buddy Sink, resulting in the setup cycle.
Is there another solution to my problem to effectively "completely" eliminate reverse current flow from my battery/relay to the sink? I've considered a buck converter to drop the sink's 15-20 volt output to 14.x volts, but not sure if that would prevent the reverse flow I'm working against now.