As soon as the engine fires the starter draws nothing, and the alternator starts to recharge what has just been used.
What electrical components put the biggest load on the battery?
Patrick
The demand load any electrical component determines how thick its supply cable must be. So take a look. The biggest electric power guzzler - by far – is the starter motor. Its supply cable is as thick as an adult’s forefinger (most other wires are as thin as a pencil lead). The starter can draw up to 300 amps (3.6 kilowatts). That is the same as three dozen 100W headlamps on full beam!
Fortunately, the battery only has to feed the starter for a few seconds at a time. As soon as the engine fires the starter draws nothing, and the alternator starts to recharge what has just been used. Any draw of battery power generates heat (by chemical reaction), but in normal circumstances this is minor and is easily and almost instantly dissipated into the air around the battery.
But if the car is hard to start and requires more lengthy cranking of the starter motor, the heat build-up can be considerable, so you should never crank the engine for much more than 5 seconds at a time before pausing. And turn everything else “off” (especially headlights) so you don’t add to the starter load.
To put that starter load in perspective, if you cranked the engine non-stop even a fully charged battery would be flat after just a few minutes, and the battery and its terminals would get hot enough to start melting the cable insulation and boiling the electrolyte. Hot enough to warp the lead plates and potentially start a fire.
The need for that massive surge of electrical power is why cars still use lead-acid batteries, which are inferior to alternative battery materials and systems in every respect except…the rate at which they can deliver power for turning the starter motor. Lead acid batteries are champions at doing that. As an indication of how potentially powerful they are, if you pressed a steel spanner across their positive and negative terminals you would cause a direct short circuit and they would “weld” the metal. Instantly.
Not something you should ever! ever! put to the test. No other electrical accessory needs anything like the same surge of power. Look at the thickness of all the other wires. You might find one that is in the same league as the battery - if the car is fitted with an electric winch. This, too, has to turn a heavy load (albeit much more slowly and with the help of gears), but still uses enough power to flatten a battery very rapidly.
Whenever possible, the engine should always be running (at fast idle or a bit more, to support the battery) when the winch is in operation. The same applies when you are giving another car a jump start using jumper leads from your own battery. Start your car before they try to start theirs. The rate of power consumption of all other accessories can be judged by the wattage rating of each item (divide by 12 to get the Amps).
Headlights, wipers and the horn are not things to leave on for long when the engine is off, but everything else combined is a small fraction of the starter motor, and the average car’s alternator probably delivers somewhere between 40 and 70 Amps (enough to serve more than 800 watts) of recharge when the engine is cruising.
