# Super capacitors high current charging and discharging

simulate this circuit – Schematic created using CircuitLab

simulate this circuit

I've been playing around with the idea of using super capacitors as the main way to start my car (basically the car is a pain to start at times due to the way the engine is setup and being stuck without a way to start the thing at times is a real pain). I've seen Maxwell sell super capacitor bank for trucks which you hook up to the starter motor and alternator to allow the truck to be shut off and run ancillaries whilst still being able to start the truck when you need as the batteries are isolated from the super capacitor starting bank. Could buy one of these but they are extremely expensive/overkill and I'm happy to have a play to make my own.

Attached is a rough circuit diagram from what I've found so far. May be wrong but its a start/what I could find from researching various things online. Some Notes on it:

• There are two power sources; Alternator output and a low power solar panel. The alternator is meant to be the main source of power/charge with the solar panel there only to top up the capacitors due to the leakage current
• Leakage current needs to be low, so any balancing after the alternator power has been removed needs to be highly efficient else I'll go to start the car and it won't work (especially if I've left it for a few days). This rules out resistive balancing.
• Active balancing of capacitors is ok, but all the ones I've found only have very small balancing capability and from what I could gather, if you want to use large amounts of current >10A to charge it fast, they just can't keep up and you can get voltage imbalances leading to damaged capacitors/explosions.
• Capacitors used will be 3kF or 3.4kF, probably Maxwell branded as I can get a hold of these with a max rated voltage of probably 2.85v. Max used voltage is aiming to be around 2.5v.
• Max voltage out of the alternator should be around 15v
• Max voltage out of the solar panel is around 19v, but typically a lot lower than that
• Buck/Boost converter used to try and keep the capacitors as close as possible to 15v. This is only from experience when starting the car, the higher the battery voltage when starting, the better/easier it starts.
• Output current of this setup when in series unit needs to be in the order of 200A at 15-11V
• Option 2 is something that I'm not 100% sure on. Being able to charge in parallel at 2.5V in my head seemed like a good idea as I could have individual buck converters to charge each capacitor to maximum voltage with a lot more current that I thought I could whilst maintaining capacitor balancing from the off the shelf products, thereby charging faster. Then they would automatically work in series when the start motor is engaged.
• I forgot to put this on the image but I'd need a diode on the output to make sure I'm not getting any back EMF etc from the starter motor.

Any comments on suggestions on which way to go would be appreciated. Nothing here is set in stone and I'm happy to take suggestions on any part of it. This is more of a learning exercise as much as anything else. I have some level of electrical knowledge but this is generally limited. If I've done something dumb or if I ask dumb questions go easy (I do try to research things before I ask more questions).

EDIT: finally managed to get circuit schematic maker working. Both solutions now attached as schematics.

Thanks Tyler

• In the edit window you can add a schematic or two for your options that I think would be a lot easier to follow than your sketch. It's the 7th icon counting from the left. – jbarlow Aug 6 '16 at 7:05
• Thanks for the tip but I couldn't get it to load for some reason. Internet connection here is terrible is the likely cause. I'll try again later – Tyler P Aug 6 '16 at 8:39