# Question about voltage and current relationship

How can 12v push hundreds of amps, let's say 500amps at 0.024 ohms? clearly the starter motor can't have that little of resistance to turn the engine rapidly. Is there a some sort of boost converter included?

• Sort of, the pinion on the starter motor is very small compared to the ring gear attached to the crank shaft giving you a mechanical boost =-D – Tyler Mar 1 '18 at 16:03
• Clearly it can't? Clearly it must. – Brian Drummond Mar 1 '18 at 16:07
• where are you getting these numbers from? – Trevor_G Mar 1 '18 at 16:35
• manufacturer sticker on battery, 12v. Numbers vary depending on battery type. The ohms are calculated if 500amps were needed to start the engine, cold start in this case. – bujashaka Mar 2 '18 at 4:05
• Have you ever tried turning an engine over by hand? It requires a great deal of energy, hence a powerful motor that draw lots of current. Resistance is only part of the equation as a lot more of the energy you put into it produces movement than produces heat. – Finbarr Mar 2 '18 at 14:54

clearly the starter motor can't have that little of resistance to turn the engine rapidly

The starter motor does have a low enough resistance to draw the current it needs to turn the engine straight from the 12v battery. And it doesn't need to turn the engine 'rapidly', just fast enough.

• @bujashaka, for low resistances, you can't just use a normal meter to measure. You have to use specialized meters (with 4 wires) or other techniques. – mkeith Mar 1 '18 at 16:49
• If you need to draw 500 amps from a 10v battery (the voltage sags under load, and it keeps the sums simple) then the total resistance of the starter motor AND the cables and contactor must be less than 10/500 = 20m ohms. This is why the cables are so thick. 1m of 100mm2 cable has a resistance of 170u ohms, so you can start to see that it's possible. Generally, if it happens, it must be true. – Neil_UK Mar 2 '18 at 6:12
• To further clarify mkeith's comment, with a normal meter (well, mine anyway, and I'll bet yours too) on the minimum resistance scale with test prods, the reading accuracy is about +/- 1 ohm. Much of this is the variable contact resistance of the prod to device under test. So if the wire resistance is << 1 ohm, the meter is well within its specification if it says anything in the 0 or 1 region. Car starter cables and other low resistance items are measured using a '4-terminal' method. You shove a test current though it, and measure the voltage across it with a different set of probes. – Neil_UK Mar 2 '18 at 6:53
• so the cable doesn't actually have that 0,5ohms but much more less? then this start to make sense! – bujashaka Mar 2 '18 at 15:00
• I used some online wire calculator resistance for copper wiring. It's no where near 0.5ohms, actually it's fractions. So my multimeter half of ohm confused me. – bujashaka Mar 2 '18 at 15:04

Some vehicles will draw 1000A and more - the large HGV's (Heavy Goods Vehicle or also known as TIR - Trans International Routiers) for example, so cars in the range of 2 to 3 litres are usually around 400 to 600 Amps - just look at the thickness of the cables linking battery to starter...

However, newer starters have better technology and the current is coming down, but, if you think of the power needed to get an engine turning sufficiently fast to start then the current draw is significant.

If you take a starter motor apart - most of its weight is due to the amount of copper used in its construction.

And, a battery terminal can disappear in a puff of smoke if it is left loose and someone tries to start a 6 litre inline diesel .... I had to get the terminal on the battery replaced... Oh and it was not me - I was helping a mate out - replace terminal or new battery...

• Heavy Goods Vehicle ... – Solar Mike Mar 1 '18 at 17:34