How comes a battery becomes flat when cranking a vehicle for some time if an engine fail to start. I understand even during cranking, an alternator must be charging a battery.

 

The battery when cranking gives out around 150Amps for a couple of seconds, this will only last 5 mins at best,and while it's cranking the battery voltage is probably down to 10V.

The alternator wont charge unless its running at around 950- 1500rpm and it needs an exciter voltage to provide charge, which comes from the battery,

 

How comes a battery becomes flat when cranking a vehicle for some time if an engine fail to start. I understand even during cranking, an alternator must be charging a battery.

No over unity discussions sorry.

 

The battery when cranking gives out around 150Amps for a couple of seconds, this will only last 5 mins at best,and while it's cranking the battery voltage is probably down to 10V.

The alternator wont charge unless its running at around 950- 1500rpm and it needs an exciter voltage to provide charge, which comes from the battery,

Or the simple answer is that it is impossible for the battery turn the starter which (indirectly) turns the alternator and have the alternator return 100% of the energy being used.

 

The short answer is, any electrical system is going to have big losses to heat, not to mention the mechanical losses to friction/heat inside the motor that is being cranked. That energy is gone, and even if the alternator was perfect, that energy cannot be recovered. So even if the alternator was perfect (lossless) and could work at cranking speeds, for every 1000w you put into the starter there would only be a few watts at best available from the alternator. Also @Dodgydave is being conservative with the current required to start a modern car. I tried measuring the start current on my truck (small 2.8L diesel) but it took more than my 400A clamp meter could read, so more than 400A to crank it.

 

I tried measuring the start current on my truck (small 2.8L diesel) but it took more than my 400A clamp meter could read, so more than 400A to crank it.

Due to their high compression ratio (15:1 or higher) and taking in a full charge of air at each stroke, Diesels require significantly more starter current than a gas engine.

 

That is why my Duramax has TWO batteries connected in parallel.

 

Due to their high compression ratio (15:1 or higher) and taking in a full charge of air at each stroke, Diesels require significantly more starter current than a gas engine.

Yep. The old rule for starter current draw was ~ 1 amp per cubic inch of engine displacement for a gas engine and ~2 amps per cubic inch for a diesel engine and both could be considerably higher if it was an engine with a high output starter in a cold winter condition.

It's why most of the modern semi trucks with 12+ liter engines that have 12 volt electrical systems will have 4 huge batteries each capable of 1000+amps each in parallel and a 2/0 or larger cable sets from each of them, for both the postive and negative leads, directly to the starter. Seeing 1500 - 2000+ cranking amps at any time is normal for them.

BTW, the biggest electric starter system I ever saw was a old mine portable genset rig that put out ~ 2 - 2.5 megawatts that they used as a portable power source for walking the huge electric mining shovels and like machines from site to site in places where they didn't have utility line power to run off while en route.

It used 10 4D size 12 volt 1000 amp batteries in series and the single starter was close to the size of two 5 gallon buckets end to end and was rated for something like 100 KW (~135 Hp) cranking power!

As a junior electrician peon working through the college student co-op program, I got the job of taking out the 10 old batteries and putting in the new ones when it wouldn't start one time.