I'm really sick of buying batteries. Especially "lightweight racing" batteries that die every 1.5 years (unless you buy lithium). On the other end, my noname group 24 battery has stayed strong for 6 years, but weighs a ton. Instead of settling for a happy medium in an optima redtop or similar, I decided to try something different.

I got this idea from lasersaber on youtube, used a 58 farad capacitor replacement on his 1.8l scion.

I've been using a 150 farad supercap on my 12a for a few weeks now. 150 farads give me a full 3.5 seconds of cranking power (wow)! I keep a small lithium jumper in case, but I haven't used it yet (and yes you can still push start if things are really bad).

I don't know the specifics of the FB's electrical system, but once the car has started, the oem alternator has enough to power everything (lights, radio, air, etc) at once, so you won't drain your cap at idle.

There are a few caveats if you're thinking of running a system like this:
If you have an alarm system/parasitic draw this definitely isn't for you.
If your engine can't hot start, takes too long to start, stalls, or runs like ****, fix that first.
If you're only using your rx7 on the street, don't really care about weight, or don't want to pray everytime you attempt to start your car, just get a normal battery.

Other than that, I feel like this is a nice solution for me. I usually only drive to events, at the events, and back. I don't have any crazy sound system or excessive power draw. I'm not an expert, but I'll throw out there that it's safer, lighter, and cheaper than Li or LA batteries. Since these caps aren't in any extreme conditions, I expect them to last longer than any battery too.

Here's how I have it in my car. Mind the extreme fire hazard wiring, still sorting everything out.

 

At 3.5 seconds, I'd rather just buy a $700 lithium battery.

 

Yeah, that crank time is a major (and the only) downside.
One solution is to run a small 4lb 10Ah lead acid battery alongside the cap to recharge it, which would bump up the time to a more reasonable ~15 seconds. I haven't had the need to test how long my small lithium jumper lasts, but in all honesty if it's not starting in 15 seconds, it's probably not starting.
If I had the money for lithium I would, just wanted to test this out.

 

When you engine gets older (like mine) hot restarts can sometimes take a few cranking sessions to rebuild compression. I'm seeing this now on my SA with the original engine which has ~150K on it. Still runs great and cold starts fine but sometimes a hot start after sitting for 20 or 30 minutes can be a challenge. Its not clear to me why this is but I chalk it up to hard driving and the mileage on the engine.

This cap wouldn't meet the bill for me. Its a cool idea tho.

 

how much does the super cap weigh?