Current draw for running a petrol fuelled car

It's not possible to give this figure as battery will be charging to recover from starting.

In use there should be no battery drain. The alternator regulator keeps output above the battery voltage 13.5-14.7V is typical. A fully charged battery is 12.7V so while running it should be "on charge".

All charging tests I've seen use system voltage to determine alternator health.

I cannot help you with the current draw but there is simple way of establishing a current in any circuit protected by blade fuses.

Each fuse has a small test point each side of the fuse wire, measure the voltage drop on relevant circuits with a DMM while the engine is running, refer to a voltage drop chart (google the search term "fuse voltage drop chart") for the size and rating of blade fuse then sum the currents.

AJH

Something like 10-20 amps.

Thanks Dave, That's something in the area I was guessing but didn't want to prompt a figure

So maybe an endurance of 3 to 6 hours on a modern 70Ah battery neglecting the start, which could, if the vehicle electronics permitted, be a push or hill assisted start giving less than half the endurance of what a carb equipped car with a mechanical fuel pump might do on a more typical of the period 55Ah battery.

I seem to recall (long ago) a 20Ah sealed lead acid feeding just the distributor based ignition and basic instruments (rev counter+oil pressure+oil temp+ water temp) would be around 5A total draw and the battery would keep a small engined competition vehicle going for a complete weekend of practice sessions and race (maybe two hours total) if it were always started with the assistance of a plug in portable Red Top 30 equipped battery pack.

The charging arrangements on the vehicle did bugger all and were deliberately arranged to look as if they did to keep the scrutineer happy :)

It won't without an alternator :)

Yes agreed, normally their shouldn't be any battery drain but this is not normal

The voltage would be falling by the second.

I've a plug in adaptor to measure the current directly via a DMM for the ATO type fuses but not the mini fuses nor the maxi fuses, plus the 10A range on the DMM is limiting hence the mention of the DC current probe (100mV/A) which I since discovered has a knackered PP3 battery connector so until that is fixed I can't even check the battery and alternator cables on a sample vehicle.

It's the general ballpark figure I was after for fuel and ignition, which Dave Plowman suggested a typical range.

Like many things this all stemed from a pub discussion, in this case around the use of stop start electrics

It's going to vary quite a bit by make, I'd say.

Some cars have two fuel pumps. The type of ignition too. Wasted spark is going to be more power hungry than sequential.

Are you sure it's just 12.7V? Where did you get this figure from?

A stable float charge is generally 13.7V

13.8v here. ;-)

A fully charged 6 cell lead acid battery has a voltage of 12.7V. Maybe

12.9V immediately after taking it off charge.

As I said on charge it's 13.5 to 14.4 volts.

But I missed the bit about him running this battery in total loss on a race car.

If you are running around the pits with a battery cart it's completely obvious that the car doesn't have functional charging system. The race organiser needs a rule change banning battery carts from pit/start lane. Or scrutineers to test for self starting and > 13.5V running.

Both my cars, one old, one newer, stabilise at 13.8v engine running battery fully charged. It's the textbook figure.

It will take hours to charge a lead acid beyond 80% capacity at 13.8V Does the charging voltage start higher and then drop off?

Mine also increases voltage when cold. The hotter the battery the lower the charging voltage needed.

Of course. That's why I phrased the above as I did.

I gather, deviously so. Fully charged can mean so many things to different people given its a level that can never actually be reached.

Eh?

Eh?

We are talking 'real world' here right?

Cheers, T i m

I suspect you might consider that a battery is "fully charged" where for a 70Ah battery the charging current has dropped below?

If you have a look at figure 4.28 of

your level might be 100%, but when you can still put a further 10-20% into the battery the term "fully charged" becomes blurred.

You may also see that time here is measured in hours. How many journeys do you make that last several hours? Most of mine are generally less than 1 hour and I volunteer that is the norm.

I can assure you asymptotic behaviour is real life. Pulling a vacuum with a pump is another I frequently come across, a vacuum is something you never quite achieve however hard you try.

Just meant that the voltage had stabilised. It never drops below that - except with a heavy load that the alternator can't balance at low revs.

No, I'd consider a battery fully charged when my quality smart charger tells me it is (when it finishes all it's tests and goes into maintenance mode)?

What you seem to be doing here is quoting some specific example as if it reflects what actually goes on in the real world and for 'most people'.

Eh?

But the scenario you quoted isn't how I (or 'most people') would use, charge or determine a battery to be fully charged.

Now you are changing the subject?

'Most people' would determine a rechargeable battery to be fully charged once it no longer accepted any charge (or appeared to be but was liberating that excess charge in gas or heat etc). Now, given that most rechargeable have some level of self discharge, as you say in your theoretical world, it would be difficult to find a rechargeable that was *still* fully charged, once taken off charge.

But that wasn't what you said.

I could tell when my electric Moke was fully charged with my nose. ;-)

'Hmm, there's nothing like the smell of hydrogen in the morning'.

Cheers, T i m

Doubt it was the hydrogen you were smelling given that it?s odourless...

Tim