Capacitor to help with staring?

Haven't we all been there?
Those pesky cold starts? 

Where the voltage drop is getting severe, motor turns but voltage going low, giving us a weak spark. 

Making it even harder to get going. 
​​​​​​
Maybe we even overchoke her. 

Only to have her fire right up after given a jump. 

What if. 
We install a super capacitor? 
​​​​​​Ideally close to the starter. 

Wouldt that help out a lot with that voltage drop? 
Giving is a much higher chance to get the bike going. 

And even when capacitor depletes and we go back to battery only. 
It would just be a matter of turning off and charge the capacitor a while. 
Right? 

So why have I not seen this modification done? 
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Good idea? 

A capacitor can give you only a short impulse of voltage power, he charge up quick but discharge even faster.

I agree with Scirocco.  In theory you could do it but (and I haven't done the calculations) the capacitor would probably be bigger than the bike itself.  Also, capacitors leak down over time.  Assuming it was charged when you shut your bike off....it may well be discharged when you go to start it again.  A good battery is a better alternative.

+1 good battery

hardrockminer post=846288I agree with Scirocco.  In theory you could do it but (and I haven't done the calculations) the capacitor would probably be bigger than the bike itself.  Also, capacitors leak down over time.  Assuming it was charged when you shut your bike off....it may well be discharged when you go to start it again.  A good battery is a better alternative.

And if it's not discharged, a capacitor that big could ruin your entire day VERY quickly, e.g. if it explodes (as caps sometimes do) or you're working on the bike and accidentally discharge it.

To your other point, even if the cap weren't bigger than the bike, it would probably be too big to be practical.

Hemmek post=846285
So why have I not seen this modification done? 
​
Good idea? 

I suspect you haven't seen it because there isn't really a need for such a thing.

If someone's bike is difficult to start, that person will most likely try to bring the electrical system back into spec rather than putting a potentially dangerous ancillary item on it.

Having said that, you brought up a good idea - unfortunately just not a practical one. But don't stop thinking about interesting solutions! They're great thought exercises and might even turn into something to build and use.

Assuming it would work, adding a capacitor would be a band-aid fix.  Check & set everything as per the factory service manual and especially make sure the battery is good.  If necessary, keep a good motorcycle smart charger connected.  I have a Ctek US 0.8 smart charger & plug it in whenever the motorcycle isn't being used.  My current battery is a Scorpion AGM & is now 3-1/2 years old.  The previous scorpion agm lasted exactly 2 years, but I didn't keep it on a charger >> only charged it once a month or so w an old schumacher mc charger. 

EDIT:  I made a bad simplification in my math, which made my numbers much worse.  I would have failed that physics test.  Oops.  I've recalculated with what I hope is a less bad simplification, and updated the numbers.  Super caps are actually closer to viable than I had initially guessed.

Based on your formatting I was trying to find the rhyme in you stanzas. 

The short answer is that batteries (chemical storage) hold a LOT more energy than capacitors (electrical charge storage).  And while super caps have really big numbers (Farads!) most of them still don't hold much energy (compared to a battery), and are either very large or have high internal resistance (so you can't get that energy out quickly).

Here is the very long answer:

Random numbers for discussion.  What does it take to start a motorcycle?  I'm going to say the battery needs to provide 50A for 5 seconds.  I'll also assume that (using magic), the capacitor starts at 14V and ends at 10V during the starting event.

You can turn these numbers into total energy required.  
Average Voltage: 12 V (lots of hand waving here)
Average Power: 600 W
Total Energy: 3000 W-s, or Joules  

So we need 3000 J, and it's easy (or not) to turn capacitance into Joules:   hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html

Example 1:  A coke bottle electrolytic
This is a normal capacitor (not super), but the biggest I could find in stock at digikey.

www.digikey.com/en/products/detail/kemet/ALS70A135QT025/6928375

Size: 8.74" long and 3.5" dia.
Capacity:  1.3F

Using the numbers above (5V drop, 1.3F) I get 62.4 Joules in this big boy.  If we had 48 of these on the bike, we'd be all set for one start.  

Example 2:  A regular old super cap

www.digikey.com/en/products/detail/tecat...BLS-7-5-16-2/9929761

Size: 3.1" x 2.4" x 0.5"
Capacity:  7.5F

So with this you get 360 Joules.  You would need 9 of these to get just one start.  Not too bad.

Example 3:  A hoking super cap
That's not a super cap....  This is a super cap!

www.digikey.com/en/products/detail/eaton...R-16R2507-R/10461228

Size: 16.4" x 6.9" x 2.6"
Capacity:  500F

This time I get 24,000 J, which is actually a lot.  That's enough energy for 8 starts.  Cool.  But it's bigger than the normal battery, costs $600, and will only hold that charge for 7 days due to internal self discharge.  

The real answer is lithium batteries.  For the same amount of stored energy, they are lighter, smaller, have less voltage drop under load, a higher cycle life, and a wider operating tamp range.

Of course, they also like to blow up, need complicated management electronics, and can't really be swapped directly into a 12V system.  

On the other hand, lead-acid batteries are filled with acid which tends to eat the paint off of old KZs, which I personally am really sick of.

But if you're looking to move away from lead-acid, lithium is the direction you should look.

Personally I just buy sealed AGM batteries, bring them inside in the winter, and from time to time put them on a Battery Tender trickle charger to keep them full.  They last for years this way.

 

Something similar has been developed successfully. Nology Hot Wires. Spark plug wires with a built in capacitor & earth strap.

Nology HotWires Spark Plug Wires Ignition Wires Manufacturer

Whoa, got a lot of good answers in a short time
Thank you all  

​I must clarify one thing thou, I didn't intend to replace the battery  
Just helping out with the voltage drop during the starting event. 

Also getting more energy to the starter  

My actual plan, was to go down the lithium road and mount the battery way in the rear behind the seat, capacitor closer to the starter relay.

As I see there are two advantages. 
More CCA (I have not done the math yet but I assume) 

And also I am hoping it would smooth out any voltage spikes, extending the lifetime of the lithium. 

The biggest problem I see with this solution are actually two. 

I would need a second fuse connected on the capacitors + terminal. 

And it would add complexity each and every time in working on the bike. 

For example taking out the battery over winter will prove quite hazardous. 

Since the battery leads will still have plenty potential. Only safe way I guess would be disconnecting the negative battery lead turn on the lights and wait for the capacitor to drain. 

Thoughts?
​​

 

Ha ha, Nology HotWires, only for those that still believe in the tooth fairy.

Voltage drop at coil primaries on older bikes is often caused by the serpentine route through sometimes corroded connectors and often flakey switch contacts that carry current for other loads as well.

If, as often is the case, voltage at coil primaries is less than battery voltage at rest, that can be safely solved with a simple relay, bypassing the main wiring harness & switches to supply coil primaries with direct battery voltage.  An example below:
www.wgcarbs.com/index.php/using-joomla/e...-categories/89-coils

Good Ridin'
slmjim & Z1BEBE