Modern Buddy

Has anyone ever tried replacing their battery with an ultracapacitor bank? It looks like it might actually be cheaper than buying a new battery, plus capacitors perform best in the cold.

It looks like you need about 35A to cold-start a 50cc engine, so assuming a generous 3 seconds of cranking time, it looks like six(6) 2.7V 50F caps would be sufficient. The capacitors have a very low internal resistance, so they can handle well over 100 amps.

If you convert everything to stored energy, you get the equation System voltage × Crank amps x Time = ½ × Total capacitance × Capacitor voltage². Moving stuff around and plugging in the right totals, this is what I got.



You can get 50F caps from an electronics supplier for $5-6 each plus shipping, so the whole thing works out to about $40.

Does this look right to you all? Any suggestions?

When I was rebuilding the CB125, I found that it was common practice to replace the battery with a large cap, especially if you were going mimimalist cafe. Since the CB was kick-start-only, the cap merely served as a sort of "ballast" for the electrical system; it didn't have to do "starter" duty.

Since you have the kick-start option on the RH, I'm sorta wondering why you don't just toss the starter altogether and use the caps as "ballast". Just curious.

The kickstart is meant as more of a backup option. You have to extend the center stand and kick while you're standing to the left of the engine (often while bent over the seat so you can reach the throttle). The convenience of electric starting is worth it to me.

I feel compelled to list a few caveats, though the answers are likely to be "it depends".

Have you considered the leakage rate of the caps in an open circuit? Capacitors don't hold a charge forever.

How do you intend to connect 2.7 volt caps to a 13.8 volt system? I assume you'll run the six caps in series.

If the capacitor bank is empty, this will appear as a dead short to the charging system (stator), will you be protecting it?

How do you intend to get the MAXIMUM amount of charge from the cap? Once you slip before 9 volts or so, the starter won't whirl.

At cold temperatures, the ESR will go WAY up. (But, these temps are likely colder than when you'd be riding.)



So, I'll take a crack at the equations.
You anticipate 35 amperes for 3 seconds, that's 105 Coulombs.
For a 13.8 volt system, that's 7.6 Farads. But, that seems low.

I think he's just about spot on in terms of the numbers.

The effective capacitance of six 50F caps in series is:

1/C = (1/50) x 6 = 8.33F

The starter resistance = V/I = 13.8/35 = .394 ohms

And the time constant = C X R = 8.33 x .394 = 3.28 seconds, at which point the cap voltage will be about 63% of nominal, or about 8.7.

So, you'd basically get a single shot at cranking the engine before having to resort to the kicker.

Your points about the apparent dead short and leakage rate are well-taken.

I don't know what these capacitors cost, but since Walmart batteries are pretty cheap, and the one in my Honda is nearly 10 years old and going strong, why bother?

Question for the electrically-savvy here: I still own an old trials bike with no battery, just a magneto. Would these capacitors allow it to keep the lights burning steadily, rather than rising and falling with the revs like it currently does?

And one more, just a shade different: If one owned a cranky old kickstart single like, say a Yamaha SR500, would capacitors instead of a battery be at all helpful in getting the old beast to cold start?

Regarding the trials bike question, these capacitors (or a battery) plus a voltage regulator would help. In such a system, changing bulbs to LEDs would also help, because the system would not need to supply as much energy. Depending on the age of the bike I wonder if it would want 6V or 12V? A popular solution in the past was to use a smallish NiCad or NiMH battery pack to keep the lights on. These are generally small enough to be hidden somewhere.

As for the SR500, ultra capacitors may help for a short time, during the first few seconds of cranking. This is because their low internal resistance may help get more current to the starter and more voltage to the coil. But if the bike didn't start quickly I suspect the capacitor output would begin to fall off, and that a battery might have more reserve power. Is the spark strong during cranking? If not, I wonder if the coil(s) are going bad.

I thought hard about installing one of these on my MB5:

https://www.treatland.tv/power-cap-2-5- ... 2.5amp.htm

It's a readymade version of what you are describing.

Ethan Allison wrote:Has anyone ever tried replacing their battery with an ultracapacitor bank? It looks like it might actually be cheaper than buying a new battery... You can get 50F caps from an electronics supplier for $5-6 each plus shipping, so the whole thing works out to about $40.

Does this look right to you all? Any suggestions?

I just bought a new battery from Walmart for $35 bucks. Even if you factor in the cost of two replacement batteries you'll only be ahead $40 bucks in 8-10 years. You also run the risk of frying your charging system that may not like the cap bank. Other than the fun science experiment aspect, I'd stick with a battery.

Southerner wrote:And one more, just a shade different: If one owned a cranky old kickstart single like, say a Yamaha SR500, would capacitors instead of a battery be at all helpful in getting the old beast to cold start?

OldGuy wrote: As for the SR500, ultra capacitors may help for a short time, during the first few seconds of cranking.

I did an iron butt run on a capacitor equipped SRX-6. It had no issues with the electrical system that I am aware of unless that is the reason it kept stalling out during the ride for no apparent reason. Seemed electrical in nature.

SR's are kick only of that vintage.

The electrical system struggled at or near idle.