It takes much less than half the charge of the battery to start the engine.
Usually you can start the engine lots of times in a short period without
time for recharging.

I don't know for small cars, but tractors and light trucks with 4 or 6
cylinders diesel engines need around 800A of current at 12V to start.

I think the capability to properly start an engine is more related to
the internal resistance. While the battery is able to source the needed
current (hundreds of Amperes) with an acceptable voltage drop (anybody
wants to investigate?) it will be able to start the engine.

Suppose a 5s start-up: 5s * 800A / 3600s/h =3D 1.11Ah

Regards,

Isaac


Bob Blick escreveu:
> It's all relative, but my rule of thumb is that car batteries are sized
> such that there is a 50% reserve when the battery is new. A medium-sized
> car has a battery that is about 50AH, so you can take 25AH out of it and
> still start the car. So your 100mA load can go 250 hours and still leave
> enough to start the car.
>
> But after the car battery is two years old in a rough environment, the
> battery capacity has dropped to perhaps 30AH so 50 hours is all you can
> expect to run your device and still leave 25AH to start the car.
>
> These are just my rules of thumb because cars and batteries are real
> systems and not calculable. The reality is that it's not the AH that
> start the car. But this simplification is as good as any other.
>
>
> Vitaliy wrote:
>   =

>> Let's say you have a device that you leave permanently plugged in to the =

>> vehicle, and that draws about 100 mA. How do you calculate (or estimate)=
 the =

>> time it would take for the device to drain the battery to the point wher=
e it =

>> won't have enough power left to start the engine?
>>
>> To clarify, I am more interested in determining the number of hours it i=
s =

>> safe to leave the device plugged in, rather than the number of hours it =

>> takes to kill the battery for sure (although both values are useful). :)
>>
>> Vitaliy =

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