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Capacitor mod motor cycle battery.

Moneybags

New Member
I would like to replace my motorcycle starting battery with a bank of capacitors. Please don’t ask or tell why bother. I want reliable starting in remote area and reduced weight.

Other people have done this and I have found some people say the stator fails after a year. Is it possible that the initial demand of the capacitor charging is over working the stator? If so is it possible to throttle the charging with a resistance because the voltage regulator does not regulate the amperage?

Please no debate about why I want to do this I want to figure out how to do it reliably and need help figuring it out.
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
Other people have done this and I have found some people say the stator fails after a year.
In that case we would need to know WHY the stator failed, so that we could suggest a reliable solution.
As a starting point, a wiring diagram of your existing system would be helpful, together with the specification of the starter and details of the capacitor arrangements others have tried.
 

shortbus=

Well-Known Member
Most Helpful Member
I'd be willing to bet he heard about the old time Triumph "battery eliminator" capacitor. But back in those days it was also kick start and a generator.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
A 12V battery gives a typical voltage of 12 - 13V at mid charge, on load; down to about 11V dead flat and up to around 15V fully charged in a running vehicle.

A capacitor will discharge to zero; and a type genuinely rated to handle engine starting current will be extremely low internal resistance and appear pretty much a dead short to an alternator.

It would need a soft-start circuit, a resistor to limit the charge current until it reached eg. 12V, with a voltage comparator and relay to bypass the resistor once a suitable voltage was reached.

If you used eg. a number of these in parallel, to get the required starter current rating, with a suitable overvoltage protection circuit:

Assuming starter current 100A, so five units in parallel giving 325 Farads:
Energy storage at 15V: 36 562 Joules
Energy storage at 11V: 19,662 Joules

Usable capacity (difference between the two above): 16,900 Joules = 16,900 Watt-Seconds.

A capacitor will discharge at one volt per second, at one amp per farad. eg. 325 seconds to discharge by each volt at one amp.
That gives the equivalent capacity of 1300 amp-seconds or 0.36 Amp-Hour capacity for a bank of five of those 65F modules.
 

Pommie

Well-Known Member
Most Helpful Member
And those capacitors will only cost ~UK£800. Seems like a very expensive alternative. It would be nice to know why you want to do this?

Mike.
 

Diver300

Well-Known Member
Most Helpful Member
Alternators are generally current-limited by design. The inductance of the windings limits the current. As the engine speed increases, the voltage generated increases, but the frequency, and therefore the impedance of the winding inductance goes up in proportion, resulting in a constant current.

Alternators on motorbikes and dynamos on bicycles (which are also alternators, just always called dynamos) have permanent magnets so regulation is often done by bypassing the current to ground. That can be by limiting the voltage with a zener, but many will just short out the alternator for part of each cycle.

Some small motorbikes and bicycles running incandescent lights have the lights directly connected to the alternator, with no regulator. The bulb current is matched to the alternator short-circuit current. I had a motorbike with a 6 V system, running a 25W headlight and a 5 W tail light. I converted that to run a 12 V, 60 W headlight and I didn't have to make any change to the alternator, as the current was still 5 A.

I don't know why the stators of those alternators had failed after a year of having capacitors instead of a battery, but I don't think that it is likely that they are burning out due to too much current. Burning out would be much faster than a year, and the current is limited by the construction.
 

tonigau

Member
"I would like to replace my motorcycle starting battery with a bank of capacitors"
An ultracapacitor can be used to assist engine starting by reducing the peak current supplied by the starting battery especially cold climate starting, I don't think capacitance alone would be much use especially when I'm yelling 'start-ya-barsted'.

For non starting battery replacement, I had a big capacitor on my suzuki TS400 for over 20years, it lasted probably due to the 3phase rectified DC which has quite low ripple compared to 1 phase.

Just for reference, below is a capture of my 1100cc Vtwin starting, the battery was getting weak & can see the starter stalling at about 400A before inertia then assists the cranking process about 125A.
I did think about adding an ultra capacitor but there is nowhere to mount it, de-compression buttons would be the best.


Ehoa Crank-3 meas.gif
 

Moneybags

New Member
Wow lots to take in. I was thinking of using a resistor to limit charging and by pass with a relay on start.

I will do some digging and see if I can get values for the bike stator charging system.
 

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