car emergency start battery

The USB ports have to be connected to corresponding USB ports on chargers or batteries, and they negotiate what voltage to put there and how much current to draw. It is not just dumb electronics.

I knew the data rates and currents were always being upgraded but did not know the voltage was raised in some cases. I had thought they were always just 5 volts.

Did a little more research and found out there is a lot of electronics in those battery packs to do what I thought was a simple job of starting a car or providing voltage to charge a cell phone.

Project Farm did a study of these units.

Joe Gwinn

Classic USB was 5 volts. Newer versions can negitiate for 20 or 48, up to 240 watts.

Some USB-C connectors are rated at 5A!

John

The c connector is the first USB connector that makes sense. It uses several pins for power.

I assume that someone sells the negotiation chips to let us design a box that gets lots of power from USB. There are chips and baby boards for PoE, a similar situation.

PoE has the advantage that lots of power is available from even the wimpiest compliant hubs.

Have you tried anything like these?

Other people make almost drop-in parts too.

A USB port delivering 15.5V without anything requesting that or even without anything connected to it? I would treat that whole unit as suspicious. If it was mine, I'd return it.

By negotiating up, as requested from the connected device. Never down, because if it starts high you might fry older and potentially still expensive USB hardware.

Regulation occurs via switch mode conversion. On a portable car starter with several Li-Ion batteries in series via buck.

If it's a decent enough car jump starter, yes. However, I have seen some very sorry "designs" in that field.