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Re: [ST] Flat Battery



The current (in amps) that is DRAWN by (i.e. flows through) any electrical 
LOAD is determined by two things.  One is the VOLTAGE placed across the 
load, and the other is the RESISTANCE (to the flow of current) that is 
presented by that load.  Hence Ohms Law that most of us are familiar with.

The load in this case is the motorcycle starter motor and, for the purposes 
of this discussion, this can be considered to be a constant.  The car 
battery/alternator supplies the starter voltage, which is no different than 
that provided by a motorcycle battery.

So the main thing you have to understand is that THE MOTORCYCLE STARTER 
DRAWS EXACTLY THE SAME CURRENT from a car battery as it would from the 
onboard motorcycle battery.  Of course the car battery must be capable of 
delivering the required current, but as has been pointed out already, a car 
battery is more than up to the task, even without the assistance of the 
car's alternator.

The dangers associated with running the car while jump starting the bike 
have more to do with the VOLTAGE transients associated with starting and 
stopping the car (i.e. heavy inductive load associated with car starter 
motor windings) while the cables are attached and, to a lesser degree, with 
connecting and disconnecting jumper cables while the car is running. 
Apparently the m/c ECU is susceptible to such transients.  Personally I 
believe that such susceptibility is overblown -- ECU's are designed for the 
hostile automotive environment and there are regulators in the mix.  But you 
will hear plenty of anecdotal data.

The current-delivering capability of a given battery is given in AMP-HOURS, 
and represents the amount of time that the battery can continuously deliver 
maximum current.  If less current is drawn by the load, the longer the 
battery will last.

Battery chargers control the current, and thus the time replace the battery 
charge, by varying the voltage applied across the battery terminals.  But 
applying higher voltages can cause excessive current to flow into the 
battery (i.e. to be drawn by the battery) and this can cause excessive heat 
build-up (mortal enemy to most batteries), so it is generally better to 
charge a battery for a longer period at lower current levels.

Oh and BTW, car batteries are usually in the range of 50 to 75 AH, not 500 
to 800.  You were probably thinking of the CCA (cold cranking amperage) 
specification.


----- Original Message ----- 
From: <drbrant@xxxxxxxxxxxxxx>
To: <ST@xxxxxxxxxxxxxx>
Sent: Wednesday, March 07, 2007 4:05 PM
Subject: Re: [ST] Flat Battery


> Neil,
>
> Actually Kevin explanation is a rather good.  As a basic rule you can 
> divided a batteries amp hours, the rating on the battery, by the charging 
> amps to determine the maximum total time to charge the battery.  Note 
> typical car batteries range from 500 amp hours to 800 amp hours.  Most 
> motorcycle batteries are in the 10-20 amp hours range.  I think the amp 
> the middle numbers on a Yuasa Battery.
>
> The major risk associated with using an running automobile to charge a 
> motorcycle battery is too many amps over heating the electrical system and 
> damaging components.  I sold a GS650 to an in-law many years ago who was 
> an automotive mechanic.  He attempted to charge the battery with an 8-0 
> amp hour automotive charger and destroyed the regulator rectified in the 
> bike.  An alternator in a car will produce similar results.  It is also 
> possible to short a battery out by exposing it to too many amps.
>
> If you don't believe us you can either study electricity some or go and 
> hook a high amperage charger to you motorcycles electrical system.
>
> Rick B
> Pfafftown, NC
> -----Original Message-----
>>From: Neil Lindsey <Neil.Lindsey@xxxxxxx>
>>Sent: Mar 7, 2007 6:42 PM
>>To: ST@xxxxxxxxxxxxxx
>>Subject: Re: [ST] Flat Battery
>>
>>Kevin -- 
>>
>>sorry, but you have no idea what you are talking about
>>
>>----- Original Message ----- 
>>From: "Kevin Dicks" <kdicks@xxxxxxxxxx>
>>To: <ST@xxxxxxxxxxxxxx>
>>Sent: Wednesday, March 07, 2007 12:14 PM
>>Subject: Re: [ST] Flat Battery
>>
>>
>>> Agreed.  What I should have said is that a car battery needs more 
>>> current
>>> to
>>> charge it compared to a bike battery given the same time period - 
>>> another
>>> effect of this is, as you point out, it has a higher AH.  If you leave a
>>> battery on a 20mA charger for long enough, given the correct voltage, it
>>> will probably eventually charge (as long as the battery is in good
>>> condition
>>> etc).  The point I was trying to make was that even though a car battery
>>> and
>>> a bike battery have the same voltage, they are very different when it
>>> comes
>>> to current (and therefore the AH each can provide).  Given this
>>> difference,
>>> the charging system of a car can (because it has to) provide far more
>>> power
>>> (Volts *  Current) than a bike charging system can.  Given that,
>>> jump-starting a bike from a car with the car engine running, 'may' lead 
>>> to
>>> problems with the bike due to the higher current the car charging system
>>> provides.
>>>
>>> -----Original Message-----
>>> From: st-bounces@xxxxxxxxxxxxxxxxxxxx
>>> [mailto:st-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf Of Eoin Kirwan
>>> Sent: Wednesday, March 07, 2007 3:08 PM
>>> To: ST@xxxxxxxxxxxxxx
>>> Subject: Re: [ST] Flat Battery
>>>
>>>
>>> On Wednesday 07 March 2007 03:23, Kevin Dicks wrote:
>>>> Current is not the same as Voltage. Just look at the size of a car's
>>>> battery in comparison to a bike's.
>>>
>>> But it's the same voltage. The larger size means more amp-hours, so it 
>>> can
>>> power a given load for a longer time - or a larger load for the same
>>> length
>>> of time - before it becomes discharged.
>>>
>>>> The car battery needs more current to
>>>> charge it - even though the voltage is the same as the bike's (give or
>>>> take).
>>>
>>> It doesn't need more current to charge it, it needs more amp-hours to
>>> charge
>>>
>>> it. Whether you do that by using a larger charge current in the same 
>>> time,
>>> or
>>> use the same charging current for a longer time, is up to you. The
>>> *maximum*
>>>
>>> recommended charging current will be larger, but you don't have to 
>>> charge
>>> it
>>>
>>> at the maximum rate, it'll just take longer.
>>>
>>>
>>> Eoin
>>>
>>> '04 ST955i
>>>
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>>
>>
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