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

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?

• You dont need to disconnect the battery. The capacitor has a circuit that detects you're trying to crank the vehicle, which activates discharge instead of charging off the battery. Commented Feb 5, 2020 at 4:55
• @NatsuKage I assumed they had a circuit of some sort, hence the question. What is in the circuit is my question. Commented Feb 5, 2020 at 20:41
• As I recall, disconnecting the battery cable while the engine is running is usually a very bad idea. Commented Feb 5, 2020 at 21:16
• @ElliotAlderson it is disconnected before the engine is running. The point is to disconnected it so the capacitor Bank is not drained into the depleted battery. This would be like a dead short across the cap bank and might damage the battery if it is very depleted. After the car starts the battery is reconnected. I'm trying to figure out how to do it without disconnecting the battery. Commented Feb 6, 2020 at 18:03
• Note -- either with respect to older cars or electrical systems, or even manufacturers "cutting corners" by using cheap components or buggy designs -- the reason you sometimes hear to "never disconnect" the battery when the car is running is simply just as a "general rule of thumb" to prevent potentially damaging "transients" to potentially vulnerable or poorly protected delicate electronic components if your voltage regulator might be too "slow" to correct the output voltage as needed. But -- this is really just more of a "safety measure", not necessary a design requirement. Commented Nov 24, 2020 at 17:46

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.

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.

• It should be rather obvious that such modifications are both not allowed, and also physically impractical. Do you realize the magnitude of the currents involved, and why are you disregarding the issues previously explained in responses to your existing answer about operating a vehicle without its storage battery connected? Your proposal amounts to the same as the manual disconnection of the battery, which was well explained to be inadvisable. Commented Nov 25, 2020 at 18:11
• Well, can't you just find a "high current" spdt then? Also, with regards to the load dump you're referring to, it seems there are also special protection devices out there (Wikipedia brings up TVS diodes, varistors, etc...) to protect whatever component(s) might be vulnerable. And also in my answer above the battery is never disconnected from anything except the car starter (I think the car starter can withstand a battery disconnect. Otherwise, every car would turn into an expensive brick the second one disconnects the battery!). Commented Nov 25, 2020 at 18:22
• I take it you've never seen the "relay switch" which is used to connect a car starter? Think the scale of plumbing, not "wiring". That was a nice greasy-hands afternoon in my apartment building's parking lot. A little bending got it to work long enough to drive to the auto parts store. Commented Nov 25, 2020 at 18:24
• I've seen them, they're not that large. I haven't had to replace one yet, although you mention that can take a bit of elbow grease to do (as probably some other things involving the starter mechanism too!) Commented Nov 25, 2020 at 18:46
• You have a funny idea of "not that large" ! Anyway, the overall point you're missing is that a practical solution doesn't try to avoid the battery as a load, it just times things so that it's only exposed to the battery during the brief actual attempt at starting. No modification of the vehicle electrical system needed! Commented Nov 25, 2020 at 18:47

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.

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.
• Why the downvote? Commented Nov 24, 2020 at 16:13
• I think the battery is supposed to charge the jump starter Commented Nov 24, 2020 at 18:47
• Not really -- a standard car battery is used only for starting the engine and small-power tasks like keeping the system clock alive. When the car is finally running, however, that's when the engine alternator is supposed to charge everything. Having a car battery charge anything while the engine is off is very wasteful, especially a high-energy supercapacitor --- since, remember, even in the world of ideal components, you cannot even charge an ideal capacitor with an ideal voltage source without half of the energy already just going to waste! Commented Nov 24, 2020 at 18:57
• Furthermore, by discharging the battery into a depleted supercapacitor while the car is turned off, you're also reducing the available voltage for the starter for the next time you need to start the car. Which could also make the starting even more troublesome! Commented Nov 24, 2020 at 19:07
• how do you think the supercaps get depleted? Commented Nov 24, 2020 at 19:11