I have been watching videos on super capacitor jump starters for cars.
I figured out about 90% of how they work, but there is one last thing I don't understand.
The jump starters use a boost converter to charge up the capacitors from my lower voltage source, possibly even the partially depleted car battery. However in the videos, when they jump start the car, they do not disconnect the battery. Since the super capacitors are in parallel with the battery, shouldn't they push all of their current trying to balance the battery with their voltage?
I have been building my own version, and in order to jump start the vehicle I disconnect the battery from the car at the positive terminal and hook it up to the super capacitor bank. Is there a circuit that I can add that will prevent it from draining into the battery but will still allowed to discharge into the car's electrical system?
To see what I am talking about go to 12:00 in this video…
https://youtu.be/l_xojkGW2hY
My guess is maybe they have a high current MOSFET that can push up to 500A and some sort of circuit that senses when the voltage across the terminals drops suddenly from connection to the starter and triggers the MOSFET. How would you detect this if the battery was completely discharged?
Best Answer
My previous answer spawned quite a discussion afterwards, so I decided to add another answer here after a number of clarifications.
If modifications to existing vehicle circuitry are allowed, it seems the goal can be accomplished by putting a high-current SPDT "selector" before the car starter, like
simulate this circuit – Schematic created using CircuitLab
where R-(misc) is just the usual load when the car is off, often comprised of things like the clock, or radio, or even the "ECU" in some cars (which also get "reset" when the battery is disconnected). Since R-(misc) is always connected in the above circuit, resetting said components should never be required.
So, in the above circuit, once the capacitor jump-starter is charged, it's connected to the "free" (or unused) throw of the selector and the system ground, and now the vehicle may be started once the ignition is pressed -- without having the jump starter discharge anything into the depleted battery.