How to make a super capacitor jump starter that doesn't empty into battery?

Question

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?

Answer 1

You do not need to disconnect car battery. A relatively dead battery will have s high impedance and when I first apply say 12 to 15V to it the current into the battery will be low compared to the current into the system motor.
While there will be some loss of charge it will be a small portion over the time taken to start the engine.

Answer 2

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.

Answer 3

Put the cap charging circuit (resistor/diode in parallel with a dc boost circuit ( 14.5volt max)) on the cars "run" circuit. Have the cap connected across a separate contactor with the contactor's coil hooked up to "start" . run the cap hot wire straight to the starter. your cap can charge if need be with the ignition in "run" . with the engine running the cap will recharge at a safe rate if you designed the charge circuit well. with the key "off" cap is isolated. There will be a short period with the cap connected to the battery before the contact inside the starter motor engages after the starter gear slides into the crankshaft's gear. The amount of current going into the battery for this short a time will be insignificant. the 14.5 volt spike to the system will not hurt anything either.

Answer 4

It seems problem goes both ways -- preventing either component from discharging into the other due to any difference in potential.

Also, if your "super capacitor jump starter" is dead, then your system should be able to "fallback" to just using the battery instead too.

Here is a simple schematic that might help accomplish this:

^{simulate this circuit – Schematic created using CircuitLab}

Note -- that "circular voltage element" depicted is just your rectified alternator. In most standard vehicle electrical systems,

1. It's only ever "on" when the engine is successfully running, and always disconnected otherwise.
2. It has a special built-in voltage regulator ensuring a steady 13.5-14.5V no matter how the RPM or your electrical load changes during vehicle operation.