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Message: <blockquote data-quote="t2ribena" data-source="post: 350987" data-attributes="member: 3847">brader...u use the wrong ohm's law la... Yes V=IR but R is load/resistance and it is in ohmic value and not watt... watts is Power and it is express by the equation, P=IV, so based on your example, 55W = I x 12V, rearranging you will get, I= 55/12 = 4.58A next your assumption on the battery current ratings and the alternator is also not correct ler...when u use a 40Ah battery with a car with 90Ah alternator it is ok as the charging current depends on the battery, in fact with higher current availability the battery will charge up faster...the only thing is that when u use smaller battery for say 40Ah, when ur car requires 80Ah let say for ICE show off without engine runnning meaning alternator not running, ur battery will be fully drain in 30min... for the starter, 300W translate to current rating of about 25A...so battery that do not have enuf charge may be too week to supply this as the inrush current for an induction load could be as high as 6 times the nominal currrent...in this case when u start the starter motor, it could require up to about 150A of current! the 12V system in cars has reach its limit actually versus the cables thinkness since the electrical and electronics grows trumendously in cars...in EU they have been talking about 48V as it offers better solution to the current needs but till now no one (car manufacturer) is starting it yet...</blockquote>

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