Join Date: Mar 2007
Location: Toronto, Ontario, Canada
Just a summary for those who may wonder why a battery that reads 12 volts may not start a bike.
All batteries have internal resistance, some high, some low and it acts just like a resistor in series with the battery.
Even if you short circuit the battery the internal resistance will limit the current flow per ohms law. Small batteries like those used in electronics have higher internal resistances. Lead acid batteries used for starting usually have low internal resistance but when they get old or have been let sit dischargedtheir internal resistance goes up, limiting the amperage they can deliver. Sulfating of the plates is usually the cause.
The voltage taken with a multimeter wouldimply that the battery was charged but if you took the voltage when it was under load you would find that the internal seriesresistance was hoggingmost of the power.
Some high output batterieshave such low internal resistances that their output currents are litteraly explosive.
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