[mudittandon]

Hi!

I smartly left a light on in my car, and went off on vacation. I returned to a completely drained battery. I called a mechanic over to start the car, thinking that the alternator will take over - but after half an hour of keeping the car running (idling), the battery is still not charged. The (neighborhood) mechanic claims that my alternator is now damaged. I cant seem to understand this - is it possible for the alternator to somehow die because the battery was drained?

It seems to me that its purely a case of the battery being completely drained and not being charged enough with 30 mins of idling. But the mechanic swears that the alternator is damaged (but he did not use a multimeter - he is inferring this from the fact that the battery is not charged after 30 mins.

I have told him to take out my battery and charge it fully - but meanwhile, am quite perplexed by this -- is it possible for a drained / draining battery to damage the alternator!?

Thanks for your help!

[mudittandon]

Sorry - should have mentioned. Its an Optra, and the battery dates to December 2009.

[tsk1979]

Never heard of this happening. I have used the alternator to charge a completely dead battery, and there was no bad affect on the alternator.
However, if battery is weak, it may not come alive again unless you go through a deep charge(2 days charging).
I suggest you get the specific gravity checked.
As for the alternator, check the voltage on battery terminals with engine running. If its below 12.9-13 volts at idle, it could indicate an issue with the alternator.

[R2D2]

Originally Posted by mudittandon

Hi!

It seems to me that its purely a case of the battery being completely drained and not being charged enough with 30 mins of idling. But the mechanic swears that the alternator is damaged (but he did not use a multimeter - he is inferring this from the fact that the battery is not charged after 30 mins.

I have told him to take out my battery and charge it fully - but meanwhile, am quite perplexed by this -- is it possible for a drained / draining battery to damage the alternator!?

Thanks for your help!

A battery not taking a charge could be for one of 4 reasons:

a) Battery dry - top it off with distilled water
b) Battery damaged - replace it, no other way out.
c) Alternator fault - Can be checked and repaired
d) Wiring fault - Can be checked and repaired.

That said, it is a common belief that driving with flat battery suffices to charge it. In theory it can. However do keep in mind a car's charging system is designed to maintain a battery's state of charge and not charge it from a deep discharge state.

Charging a flat battery loads the alternator heavily and puts the stator coil and regulator/rectifier at risk.

There are cases where alternators have failed trying to charge flat batteries but it is not common. If you have a flat battery, take it out, check water levels and connect it to the appropriately specced wall charger. Unless it is an emergency never drive with a flat battery.

So before your mechanic condemns the alternator, ask him to check it using a digital VM and fully charged battery. The alt should put out around 13.8-14.4v at idle with all lights and accessories switched off.

PS - A load test also helps in detecting alt faults. But you would need to take it to an larger garage for e.g. your dealer, Bosch, or Lucas for this.

Cheers!

[Sutripta]

Hi,
First answer, no.

Would suggest you bench charge the battery first, before taking any decisions. Though the alternator does provide output at idle rpm (unlike dynamos), it is certainly not enough to charge a deep discharged battery in 30 mins.

Regards
Sutripta

[DieselFan]

Alternator is nothing but a dynamo or a electric generator. A dead battery doesn't have any effect on alternator while a broken alternator brings down the battery very quickly.

[Mr.Boss]

Originally Posted by DieselFan

......
A dead battery doesn't have any effect on alternator......

Dead battery puts more stress to the alternator as it draws more power out of it. I am not an expert, but from theories it should not damage the alternator.

[bluevolt]

Since your car was behaving normally before and battery discharge resulted because of the light remained on, the alternator is not faulty. The battery may not be charging as it is deep discharged. Also only idling doesn't charge a battery at full capacity. The car should be driven for some kilometers so that the system generates enough current to charge the battery.

First make sure if your battery is OK or not by giving it bench charge. Install some other battery and see if it is charging or not. Sometimes these neighborhood mechanics tries to take customers for a ride.

[DieselFan]

Originally Posted by Mr.Boss

Dead battery puts more stress to the alternator as it draws more power out of it. I am not an expert, but from theories it should not damage the alternator.

Mr. Boss - this is a wrong theory. Battery doesn't draw anything from alternator. Alternator runs and charges Battery. It is alternator which charges the battery and hence battery health has not effect on alternator.

[dhanushs]

Originally Posted by R2D2

Charging a flat battery loads the alternator heavily and puts the stator coil and regulator/rectifier at risk.

There are cases where alternators have failed trying to charge flat batteries but it is not common

Originally Posted by Mr.Boss

Dead battery puts more stress to the alternator as it draws more power out of it. I am not an expert, but from theories it should not damage the alternator.

I seriously doubt it. AFAIK, the 'load' on the alternator for charging a battery is the same, regardless if the battery is flat.

Only time taken should be more.

[Mr.Boss]

Originally Posted by DieselFan

Mr. Boss - this is a wrong theory. Battery doesn't draw anything from alternator. Alternator runs and charges Battery. It is alternator which charges the battery and hence battery health has not effect on alternator.

I was about to say the same but gone wrong. Thanks for correcting me.
BTW alternator charging a dead battery should have more stress (or what ever we say technically) than a alternator charging a live battery (charged battery) rite? Please correct me if I am wrong.

[DieselFan]

Mr. Boss - Simply put battery alive or dead doesn't have any effect on the alternator. It is alternator (Alive or dead) which will have erious repurcussions on the battery. Hope I have made my point clear. Alternator Drives battery and not vice-versa.

[R2D2]

Originally Posted by dhanushs

I seriously doubt it. AFAIK, the 'load' on the alternator for charging a battery is the same, regardless if the battery is flat.

Only time taken should be more.

Apologies if I sound (unintentionally) rude but you couldnt be more wrong.

Electrically speaking (for a battery of a given capacity) there are two aspects to a battery being charged - voltage and amps (current) and the third factor being time. There are also three states of charging - bulk, absorption, float and in some cases an equalizing charge.

Please read about it at www.batteryfaq.org which is a very comprehensive site. All you wanted to know about lead acid batteries, their maintenance and charging. A very useful read for all vehicle owners.

I use a Fluke 177 DVM and a clamp meter to check the health of my car's charging system.

Regards,

[dhanushs]

R2D2 , I would be more than happy to learn what I do not know, or correct my knowledge.


Well, the site you have provided is very comprehensive indeed, and hence could you please point out where it mention's that battery puts a load on the alternator?. I could not find it.

R2D2, AFAIK, unlike electrical equipments, which put a 'load' on the alternator, the battery does not do that.

What you mentioned is correct. There are mainly two factors influencing the charging of a battery. Voltage and Current. Assuming the voltage is ~constant (btwn 12 and 14), its the current that varies. If the battery is flat, then more current means less charging time, and vice versa.

Simply put, a flat battery will not demand more current from the alternator, but then, it will just accept what it gets and charges accordingly. - Battery does not have(Or negligible?) electrical(internal?) resistance to put load on the alternator.


P.S - Please feel free to correct me, as and when required.

[R2D2]

Originally Posted by dhanushs

R2D2 , I would be more than happy to learn what I do not know, or correct my knowledge.


Well, the site you have provided is very comprehensive indeed, and hence could you please point out where it mention's that battery puts a load on the alternator?. I could not find it.

R2D2, AFAIK, unlike electrical equipments, which put a 'load' on the alternator, the battery does not do that.

I could have typed out the reply. But a link and a relevant copy/paste are so much easier.

Here is the relevant section (note that last sentence in the para that I have quoted):

Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 14

"14.2. Will driving a vehicle will fully recharge it's starting battery?
There are a number of factors affecting a vehicle charging system's ability to recharge a battery, such as how much power and charging voltage from the alternator is diverted to the battery, how long the power is available, and the temperature. Generally, idling the engine or short stop-and-go trips during bad weather or at night will not fully recharge a car battery. It will leave your battery undercharged which causes sulfation. When a dead battery needs to be recharged, it is best to use an external battery charger because you could overheat and damage your vehicle's charging system and you will save a lot of gas and wear and tear on your engine. Please see [COLOR=#0066cc]Section 5[/COLOR] and [COLOR=#0066cc]Section 9[/COLOR] for more information on vehicle charging systems and charging.
If jump starting is required to start an engine, the battery should be fully charged by an external charger and then tested for latent damage. Assuming that a car battery has a 50 amp hour capacity and the vehicle's charging system is capable of recharging it at 50 amps at highway speeds, it would take approximately 120 minutes to fully recharge a good battery. If the battery is frozen, install another fully charged battery until the original battery can be thawed out, fully recharged and tested or tow the vehicle to a heated garage. Vehicle charging systems are not designed to recharge fully discharged batteries and doing so may damage the stator windings or the diodes (from overheating)."

Hope this helps.

Cheers!