[siddartha]

in theory , whether its a diesel engine or a petrol engine , it will run without a battery , but in the context of cars , i think it depends on how it is "wired".

so instead of fighting/argueing we all should examine wiring diagrams .

[amitk26]

Originally Posted by Samir Taheer

why are people with half knowledge confusing others here.

All cars can run without a battery once started - I wouldnt go as far as ALL Cars. I tried taking out the +ve lead on my Suabru and it died.

two possibilities
1. Your subaru might have some Battery monitoring system which kills the Engine I remember some one mentioned something to this effect already
2. The alternator is kaput.

Originally Posted by Samir Taheer

Its the alternator which is powering everything in a car - The Alternator powers almost everything. Why do SUV's have 2 batteries? Possibly to relieve some of the load from the alternator? They still start with one battery. So why put the second one in?

I am yet to see an SUV with 2 battery but that is not the point, There are few vehicles with more then 10 batteries in India used in marriage processions / Election campaigns. The battery is a storage device and it is alternator/generator which charges battery, The potential difference across battery terminals is always lower then PD across alternator coil by design.

So if you remember 2 basic laws of physics things will be crystal clear.
1. charge flows from higher Potential to lower : So it is always alternator that would power not battery.
2. Law of conservation of energy : It is alternator that is primary source and charging the battery.

Originally Posted by Samir Taheer

Battery cannot power the car if alternator is producing 13v and above - Read point 2 + What if the alternator is generating more than 13V but the load is higher? I think you are contradicting you're next point here.

Higher load means more current drawn which in turns mean drop in potential difference ( voltage) , when this drop is significant enough to bring voltage below the battery voltage current will be drawn from battery.

Originally Posted by Samir Taheer

Battery comes in parallel in overload situation weather momentary or continious - Agree

OK good now couple this agreement with the fact that current flowes from higher to lower potential.

Originally Posted by Samir Taheer

you can push start without battery as long as alternator rotates to develop a voltage sufficient to run the ecu and ignition circuit - Maybe in older cars. Thats why you had hand cranks. Maybe some newer cars with older tech in them. But not with most modern cars.

You can easily add a hand crank by adding a rope and pully across alternator and check it out. Modern cars don't have hand crank because
battery reliability and car service network has improved to such an extent
that people call helpline to jump start rather then sweating out hand cranking. Also a customer handcranking gives a negative image to brand so car companies no longer provide a provision to hand crank.

[esteem_lover]

Originally Posted by gigy

why are people with half knowledge confusing others here.

That means you are the only one with 'FULL' knowledge about this. AFAIK, I spoke from my experience and I am here to know better. So if someone says something contradicting my belief, i will argue till I am satisfied that I was wrong or the other person was wrong.

Coming back to topic, I feel that there are 2 types of cars/electricals. One will run without the battery & the other cannot run without the battery. I have also seen diesel engines run without the battery, I will check with my car today.

[Samir Taheer]

Anup - Sorry i was quoting what gigy was saying and forgot to take that part off. Read further down my post - i didnt mean to say people have half knwoldege but its peoples opinion.


But Anyways ignoring the rudess - Just spoke to a mate who works at Delphi and this is what he had to say '
The battery runs as a parallel circuit to the alternator, its mearly there to start the car and take and excess load. All the work whilst the car is running is done by Alternator. Yes the car can be run without the battery, once they start the engines on the test bed the battery is disconnected. If the alternator is weak, the engine wont stay on.'

So i decided to just try removing the +ve cable from my car again...but this time made sure i had turned everything off inside the car (Music System, Climate Control etc..) and guess what - The car did stay off.

So 'my' (and yes only mine) opinion is - I was wrong the car can stay on without the battery - Yes it can. And the Juice is powered by the alternator, with the battery there only to take off any excess load.

TSK1979 - I wasnt generalising - I was mearly stating what i know!!!
I have admited i was wrong with point 2. But most SUV's i have seen (Hilux, Land Cruiser, Pajero - Here in the UK & India) have 2 batteries - and thats mainly to swallow excess load. I dont know much about the Safari.

[anupmathur]

Originally Posted by Samir Taheer

Why do SUV's have 2 batteries? Possibly to relieve some of the load from the alternator?

IF any SUVs have two batteries, the reason is certainly not to relieve load on the alternator!!

[Crazy_Eddy]

Originally Posted by amitk26

The point was about load on a engine put by a mechanical fan running by a belt from engine vs load on engine due to an electrical fan of same power consumption. Anup Mathur rightly pointed out that there is no reason to belive that an electrical fan will not load the engine.
Law of conservation of energy holds true everywhere isn't it ?

ideally if there are no energy losses in the system then both of them should load the engine equally because alternator is ultimately drawing then energy from engine.

In case of a mechanical fan losses are due to fan belt slippage where as in in case of electrical fan the loss is due to efficiency of conversion of mechanical energy to electrical by alternator. Which of them is greater ?

I would say if the belt and pulley is new the loss will be less in case of mechanical fan but we all know it is no longer preferred the reason(s) are

1. Electrical fan can keep on running after engine is switched off to cool the engine down , This happens in most modern Diesel and Petrol Engine.
Car manual tells you not to worry about fan running for some time once you switch off these days.

2. Over a period of time due to wear and tear the loss tends to increase in case of mechanical fan where as in case of electrical fan this is almost constant through out life of the fan / alternator

anup and you have nailed it perfectly !
Law of conservation of energy is the key term here that would explain things. A battery doesn't magically create energy; its just storing energy supplied by the alternator which once again gets it from nowhere else but the engine. Regardless of having 2~3 batteries in the car, where does the battery ultimately source its stored energy from?


Mechanical or Electrical, your engine is still the original source for the power, and its wrong to assume an electrical load would be lower than an equivalent mechanical load.

Take your fan for example :-
A Mechanical connection would be :
Engine -(mechanical)-> Fan
An Electrical connection :
Engine --(mechanical)--> Alternator --(electrical)--> Motor --(mechanical)--> Fan

As you can see, in both cases the fan would need similar equivalent power for the same work (neglecting the conversion losses). Since the Electrical route involves converting energy twice (and you know conversion is never 100% efficient), at optimal conditions the mechanical route would be more efficient. Practically though, the electrical route is a lot more convenient.

headers, I have a carb esteem so I guess that should technically be pretty close to your Gypsy. A few days back, I had a fuel pump issue and after hours of senseless cranking by the Maruti chaps the battery was kinda dead. Since they were lazy to go pull out their jumper cables, they just swapped the battery with a charged one, started the engine and while it was running they disconnected the battery and re-inserted the old one again; all this while the engine ran fine.

As per your query about a battery 'completing' the circuit.. its not so, since the battery is connected in parallel. The attached image should help explain things. HTH

[Samir Taheer]

Anup - My Hilux and Pajero both had 2 Batteries. If not to take off excess load , i can't think of anything else. Both had loads of gizmos (Adjustable Suspension.. etc etc..). I think some of the bigger luxury cars have 2 batteries too. Can anyone confirm this?

I'd honestly like to know why there were two.


EDIT: Thinking about it - 2 Batteries = 24V system? OR Its there incase your powering up something - you might drain one battery and leave the primary battery to power up the engine? That would explain why they would start with one battery but still have two

[Crazy_Eddy]

Originally Posted by Samir Taheer

Anup - My Hilux and Pajero both had 2 Batteries. If not to take off excess load , i can't think of anything else. Both had loads of gizmos (Adjustable Suspension.. etc etc..). I think some of the bigger luxury cars have 2 batteries too. Can anyone confirm this?

I'd honestly like to know why there were two.

I'm guessing two reasons:
- One, to have enough power to crank the relatively larger engine (for eg: trucks have larger capacity batteries even though there are hardly any gizmos on board). I suppose this is the primary reason.
- Two (and this is a wild guess), in a gizmo laden car, the additional batteries are probably to cope with load spikes. In a way your battery behaves like a capacitor - charging and discharging as per the load demands. Say for example your alternator is rated to supply 10A of current continuously, and a particular gizmo needs 15A for a very short burst (all hypothetical numbers); your battery would supply the additional juice for that short period and charge itself back up to original capacity after that.

[suman]

Originally Posted by anupmathur

IF any SUVs have two batteries, the reason is certainly not to relieve load on the alternator!!

So what is the reason for the two batteries Anup, can you kindly enlighten us?

Also, I thought it was a knowledge sharing thread so why the ridicule - if Samir is talking about SUVs having two batteries, I suppose he's speaking from personal experience?

[scooby05]

My R230 Sl500 has 2 batteries. One for the starter in the front and the second in the boot for all other functions.

[anupmathur]

Originally Posted by suman

So what is the reason for the two batteries Anup, can you kindly enlighten us?

Also, I thought it was a knowledge sharing thread so why the ridicule - if Samir is talking about SUVs having two batteries, I suppose he's speaking from personal experience?

Suman, I am not even aware of any SUVs that have two batteries as stock equipment. Hence, please note that I put the 'if' in caps.
It stands to logic that if there are two, they would be in parallel.
IF not, then one is meant more as a standby or to run other auxiliary gear that might have been retro-fitted.
It is conceivable that two batteries might be in series to create a 24V electrical system for the car. Everything then would be sized accordingly.
If vehicle demands are high, the logical route is to fit a larger alternator or even two! Yes, there are some people who fit two alternators, though the 'modifications' required for this are rather expensive.

And please, I did not intend to ridicule anyone! If it came across like that, sorry!

[star_aqua]

Originally Posted by Samir Taheer

TSK1979 - I wasnt generalising - I was mearly stating what i know!!!
I have admited i was wrong with point 2. But most SUV's i have seen (Hilux, Land Cruiser, Pajero - Here in the UK & India) have 2 batteries - and thats mainly to swallow excess load. I dont know much about the Safari.

having two batteries doesn't mean it is for running the engine. You can have as many batteries as you want. SUVs come with two batteries, yes for luxury purpose. let me try to explain you like this, imagine your house has a sump as a water reservior and you get the city corporation water into it. the rate of water supply to the sump is constant or fluctuating. but you get the water inside your house at what ever rate you want it.
Having an extra battery is just expanding the reserviour capacity. and why two seperate batteries instead of one single huge battery? its again due to space constaints in the car, weight and expensive if you have to replace the battery.
Here i have seen a maruti zen with two batteries. the additional was installed just for the ICE and the Amp.

When you have additional battery, you dont really need any changes in the alternator. you can still use the same to charge both, but it will take long time to charge. We have old trucks with two batteries (TATA 1210SE). this is because of the 24v system.

[Samir Taheer]

Anup - Most SUV's i've seen have 2 batteries - as standard!! And i guess scobby05 just confirmed that his 500SL has two batteries too.

So if, as Star_Aqua says - the 2nd battery is used as a reserviour - Whats the purpose of having this 2nd battery if everything is powered by the alternator? I can understand in an SUV the 2nd battery is a backup, but what about in an SL500? Your never going to be stuck in a desert in one of those. If its the 24V system i can totally understand the 2nd battery.

I am just curious - I always believed the alternator powered everything and thought the battery was only there for starting the engine and maybe relieving the alternator of excess load. But (using the Zen as an example since not everybody is familiar with 2 batteries as stock), why would the owner need the 2nd battery?

I am not saying anyone is right or wrong here - Just seeking an explanation!!! I am not an Electronics Genius so the knowledge would be useful.

[amitk26]

Originally Posted by Samir Taheer

Anup - Most SUV's i've seen have 2 batteries - as standard!! And i guess scobby05 just confirmed that his 500SL has two batteries too.

So if, as Star_Aqua says - the 2nd battery is used as a reserviour - Whats the purpose of having this 2nd battery if everything is powered by the alternator? I can understand in an SUV the 2nd battery is a backup, but what about in an SL500? Your never going to be stuck in a desert in one of those. If its the 24V system i can totally understand the 2nd battery.

I am just curious - I always believed the alternator powered everything and thought the battery was only there for starting the engine and maybe relieving the alternator of excess load. But (using the Zen as an example since not everybody is familiar with 2 batteries as stock), why would the owner need the 2nd battery?

I am not saying anyone is right or wrong here - Just seeking an explanation!!! I am not an Electronics Genius so the knowledge would be useful.

Samir 2nd battery is installed to run ICE as in the example zen. The assumption is that you do not run full load 100% time but peak load requirement is high.

A battery is nothing but an array of cells , so 2nd battery is same as adding more cells in same battery ( bigger battery)

The battery voltage is 12V and alternator is around 14 V and two batteries and alternator are connected in parallel, When the system is not loaded current will be drawn from higher potential source and when load increases there will be voltage drop and battery will be utilized.

Thus if you are not using full load 100% of time in lean period the two batteries will be charged. but the total power drawn over a time period
can not be higher then what alternator produced.
Some people fit expensive biger batteries of higer Ampere rating as well
but 2 batteirs in parallel is a more economic choice.

[suman]

Originally Posted by star_aqua

its again due to space constaints in the car, weight and expensive if you have to replace the battery.

No offense but do you know this for a fact or are you just assuming?

Originally Posted by star_aqua

this is because of the 24v system.

This is what looks more plausible to me.........but I'm no expert.

Originally Posted by Samir Taheer

If its the 24V system i can totally understand the 2nd battery.

Exactly! Now, perhaps the experts would tell us one way or the other - because so far I can just see surmises, not confirmations

Originally Posted by amitk26

Samir 2nd battery is installed to run ICE as in the example zen. The assumption is that you do not run full load 100% time but peak load requirement is high.

Amit & all others - can someone please explain the stock second battery in the SUVs & the Merc? Lets leave the non-stock ones out of it for the time being? It would help us understand better