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Old 11th January 2016, 21:02   #1
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Default Why does my bike need a battery to run while kick-starting?

A little background to my question.

Few days ago, a friend suggested taking a short ride. As the work year is still not at its peak, my energy levels were strong enough to give an emphatic yes !!
So that night after work, took the cover off of my beloved Thunderbird and tried to kick-start it.
Not surprisingly, it refused to start even after 100 kicks (it sure felt like 100 kicks ). It had been under covers for about 3-4 months now.
Next step, I removed the battery and got it charged at a local auto-electrical shop. Once the battery was fully charged and plugged back in, Thunder started roaring after just the second kick.
Now this got me thinking which eventually led to this query:

What is the role of battery to kick start a bike?

Now before posting here, I did research this topic but haven't found a satisfactory answer.
I now understand that battery provides the 12 volts input which is stepped up to around 400-600 volts by ignition coil which is responsible for producing the spark. That is why it's impossible to start the bike with a dead battery.
Which is fine as a standalone statement but it doesn't explain why a bike with dead battery can be push started?
Where is the input 12 volts coming from during a Push start?
To best of my understanding, there are no moving parts/coils in CDI system which could generate the current during push start.
Is it due to the alternator? I understand that most bikes used fixed magnets stators.
Is during push start, alternator is able to provide the trickle charge to produce sparks?

By extension, the same confusion is applicable on cars. What provides the power to produce sparks in cars?
As cars with carburetor can be push started. Is battery's role limited to providing power to generate compression during the ignition cycle?
Also talking about MPFI systems, I believe they cannot be push started as battery is required to operate the injection system. Is that correct?

Pardon my ignorance on such a basic topic.
Till I understand this correctly, I won't be at peace.
Appreciate your inputs

Dear Mods: I tried searching for a similar thread but could not find one. If one exists, please merge.
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Old 11th January 2016, 22:49   #2
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Default re: Why does my bike need a battery to run while kick-starting?

Disclaimer: My knowledge of electrical systems is basic, far from being a guru.

CDI systems are pretty old and reliable, pretty much every bike uses a CDI to generate the spark, and it does not require a battery since it's driven by the ignition coil, which rotates with the crank.

So the FI system is the probable bottleneck. I have push started my EFI Pulsar 220, but then it did have a battery, the problem was with the starter motor/clutch.
Also, TCI systems, which some bikes now use, uses a semiconductor that can only run on 5V or 12V , requiring the use of a battery. Check if your bike has a TCI. Now a dead battery still holds some charge, which might be sufficient for the ECU, these electronics require very little current, but are voltage sensitive.
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Old 11th January 2016, 22:56   #3
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Default re: Why does my bike need a battery to run while kick-starting?

Battery has nothing to do with Capacitor Discharge Ignition (CDI). Hero Honda bikes have been using it for ages; you can remove the battery and throw it away, the bike will still start and run!

Last edited by Gansan : 11th January 2016 at 22:58.
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Old 11th January 2016, 23:35   #4
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Default re: Why does my bike need a battery to run while kick-starting?

Sorry for missing the critical piece. The bike in question is Royal Enfield Thunderbird Twinspark 2009 model. I believe it has TCI ignition system. If it requires starting voltage to power transistor, how does that voltage is provided during push start? Last time the battery was dead, I was able to push start successfully.
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Old 12th January 2016, 10:09   #5
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Default re: Why does my bike need a battery to run while kick-starting?

Your Thunderbird not wanting to start with a bad battery has most likely nothing to do with CDI.

Your Thunderbird has an ECU that amongst other things, controls fuel injection and ignition. It needs a steady voltage to work properly. Kick starting (with a bad battery) won't provide that steady voltage. If you had used a jumper cable, it most likely would have fired up properly.

Push starts might work a little bit better as you tend to get several firing cycles in sequence with the alternator running along as well. A kick start is just to short a duration.

Modern cars suffer from the same problem. Low battery voltage often leads to problems starting. Not necessarily because the engine won't crank sufficiently, but because the electronics don't work properly at a too low voltage.

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Old 12th January 2016, 11:28   #6
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Default Re: Why does my bike need a battery to run while kick-starting?

Quote:
Originally Posted by simplynitin View Post
What provides the power to produce sparks in cars?
As cars with carburetor can be push started. Is battery's role limited to providing power to generate compression during the ignition cycle?
Also talking about MPFI systems, I believe they cannot be push started as battery is required to operate the injection system. Is that correct?

Pardon my ignorance on such a basic topic.
Till I understand this correctly, I won't be at peace.
Appreciate your inputs
Now that Jeroen has explained the cause, I guess you can be at peace!

A jump start connection, even with a small 12v battery (for e.g. a motorcycle battery connected to a car's terminals) is enough to allow one to push-start a car and it will fire - because the electronics receive a stable voltage.

As a rule of thumb, for ECU-equipped vehicles, a totally flat battery that won't even let you sound the horn strongly, won't start the car (or bike) either.
Quote:
Originally Posted by Jeroen View Post
Your Thunderbird has an ECU that amongst other things, controls fuel injection and ignition. It needs a steady voltage to work properly.
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Old 12th January 2016, 11:57   #7
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Default Re: Why does my bike need a battery to run while kick-starting?

The 2009 Thunderbird is a carb one with a TCI system for ignition.
It does not have an ECU.

Enfield sold few models with CDI units for a span of 4 years in early 2000. These run and start without a battery/dead battery.
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Old 12th January 2016, 13:01   #8
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Default Re: Why does my bike need a battery to run while kick-starting?

When you do a kick-start, the alternator runs for that short period. But the issue with the alternator is that it doesn't produce a constant voltage. Many devices in the electric circuit (including ECU) need a constant voltage (without spikes) to work properly. Battery is used to stabilize this voltage.
More info at: http://www.bcae1.com/charging.htm

Last edited by jinojohnt : 12th January 2016 at 13:02.
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Old 12th January 2016, 17:32   #9
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Default Re: Why does my bike need a battery to run while kick-starting?

Quote:
Originally Posted by tharian View Post
The 2009 Thunderbird is a carb one with a TCI system for ignition.
It does not have an ECU.

Enfield sold few models with CDI units for a span of 4 years in early 2000. These run and start without a battery/dead battery.
Spot on! My 2004 T-Bird has been running without a battery for over 3 months now. Still "Half kick start"
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Old 13th January 2016, 11:44   #10
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Default Re: Why does my bike need a battery to run while kick-starting?

Quote:
Originally Posted by SS-Traveller View Post
As a rule of thumb, for ECU-equipped vehicles, a totally flat battery that won't even let you sound the horn strongly, won't start the car (or bike) either.
Thanks!

Therefore, the only solution in this case are jumper cables? I keep a set handy and they've been used multiple times in the last year or two.
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Old 13th January 2016, 11:56   #11
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Default Re: Why does my bike need a battery to run while kick-starting?

Quote:
Originally Posted by GTO View Post
Thanks!

Therefore, the only solution in this case are jumper cables? I keep a set handy and they've been used multiple times in the last year or two.
It's the safest bet, you need that steady sufficient voltage. With a weak battery its not so much the fact that the engine might not crank swiftly enough as that the subsequent reduction in voltage makes your ECU (or any electronics) go wonky.

It's also the safest way. Some modern cars don't do well on push starts either.

The one thing you might want to be check in your manual is how to actually jump start. (yes, the manual again!) Most people will connect the two batteries in parallel and fire up. There is some risk to this practice.
You will find that many car manufacturers advise to connect to the car chassis directly rather then the battery negative. Also ensure the right sequence in connecting!

http://www.wikihow.com/Jump-Start-a-Car

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Old 13th January 2016, 13:18   #12
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Default Re: Why does my bike need a battery to run while kick-starting?

Quote:
Originally Posted by Jeroen View Post
It's the safest bet, you need that steady sufficient voltage. With a weak battery its not so much the fact that the engine might not crank swiftly enough as that the subsequent reduction in voltage makes your ECU (or any electronics) go wonky.
Thanks!

Quote:
There is some risk to this practice.
We have a pictorial guideline right here - link (How to Jump Start your car (dead battery)).
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Old 13th January 2016, 14:43   #13
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Default Re: Why does my bike need a battery to run while kick-starting?

Question - can I start a CBR 250R with a jump start cable from an Impulse? Do battery ratings matter here?
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Old 13th January 2016, 14:47   #14
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Default Re: Why does my bike need a battery to run while kick-starting?

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Originally Posted by naveenroy View Post
Question - can I start a CBR 250R with a jump start cable from an Impulse? Do battery ratings matter here?
Yes you can, Just keep the (Im)pulse runnning(:P) before you start your CBR
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Old 13th January 2016, 14:50   #15
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Default Re: Why does my bike need a battery to run while kick-starting?

Quote:
Originally Posted by naveenroy View Post
Question - can I start a CBR 250R with a jump start cable from an Impulse? Do battery ratings matter here?
As long as they have the same Voltage (12V, some old bikes might still have 6V) you should be fine. In essence if the Impulse battery has a too low rating your CBR just wont start. As long as the old battery isnt completely dead it really only needs a bit of boost and a battery of a lower rating might well do.

The one thing you do need to be cautious of is prolonged starting attempts as it might damage your (underrated) battery (and it might draw to high amperage. So as long as it seems to crank fine and fire up within a few seconds you should be fine. If it doesnt fire and the battery is of a lower rating I would look for a different battery with the correct rating to be used.

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