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Old 3rd June 2011, 13:31   #1
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Default Car Battery / Alternator Capacity Query

Hi Folks
Saw this article that has got me worried

Car Battery Myths & Facts: Car Battery Myths (Part 2 of 4)
especially the following part

The vehicle's electrical load is first satisfied by the charging system, and any remaining power, if any, is used to recharge the battery. For example, if the total electrical load is 14 amps and the charging system is producing 30 amps at 2500 RPM, then up to 16 amps will be available for recharging the battery and take approximately six minutes. If the charging system is operating at say a maximum capacity of 80 amps at 5000 RPM, then the battery usually will be recharged within two minutes. Now let us assume that the engine is idling and the charging system is only capable of producing 10 amps. Four amps from the battery are required to make up the difference to satisfy the 14 amp electrical load. The battery is being discharged further. This example is why that during short trips or driving in stop-and-go traffic, the battery may never get recharged and may even “completely” discharged.
Using the example above, let's assume that you add an after-market, high-power audio system or lights that adds 20 amps of load. With a total electrical load of 34 amps, at RPM below 2500, the battery will never be recharged with an 80-amp system. During operation, the battery must make up the deficit. The solution is to upgrade the charging system to 125% or more of the new worst-case load. In this example, based on your stop and go driving habits you would need a high output charging system capable of 105 amps or more. High temperatures can further reduce the maximum output of a charging system.
Going by this putting a high power audio system could have disasterous results especially in city traffic. what are your thoughts on this ??

My Questions:
1. Is there any way to get the RPM versus output current rating of the alternators in our car.
2. How much power/current is typically required to operate the basic car components, spark plugs, lights, meters etc (not including ICE)
3. Is it advisable to upgrade the Alternator to a higher current output one. I know this is done in the US but havent heard about it being done in India.
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Old 3rd June 2011, 14:43   #2
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Default Re: Car Battery/alternator capacity query

Quote:
Originally Posted by jinu_joseph View Post
Hi Folks
Saw this article that has got me worried


My Questions:
1. Is there any way to get the RPM versus output current rating of the alternators in our car.
2. How much power/current is typically required to operate the basic car components, spark plugs, lights, meters etc (not including ICE)
3. Is it advisable to upgrade the Alternator to a higher current output one. I know this is done in the US but havent heard about it being done in India.
What that site says is true. Contrary to popular belief, it is the alternator that supplies electrical energy to run the electronic devices in the car and not the battery. The battery is used as an accumalator to be used only when the alt output (A) is less than what is required i.e. lower than the demand OR the engine is switched off.

To answer your questions:
a) You cant get this information as it is generally OEM confidential information. OEMS do not give out specs for auto parts whether manufactured or bought out. You can mount an alt on a test rig to check the volt/amp v/s rpm numbers.

b) This will depend from car to car. There is no general rule as such because the number type and specs of components varies from model to model and mfr to mfr. I would expect it to be in the 5-15A range...thats a pure 'guestimate' with the lighting circuit i.e. headlamps, indicators etc and any electrical accessories switched off.

c) You can get your alternator rewound to boost output (Amps). But I seriously don't recommend having it done here in India. You will compromise on the reliability and lose any warranty. Some people fix after market alts made for other vehicles after making changes to the mounting bracket, belt and belt tensioner, wiring, regulator/rectifier as required. Again - NOT recommended.

Cheers!

PS - If you aren't facing problems with the alternator and battery after having fixed accessories and ICE..I'd say leave it alone. Very, very few people need an alt upgrade and those are people with very powerful ICE setups (1000s of watts), winches etc fixed in their cars.

Last edited by R2D2 : 3rd June 2011 at 14:47.
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Old 3rd June 2011, 14:47   #3
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Default Re: Car Battery/alternator capacity query

Thanks for details. So what is that people do when they get High power high end systems installed in their cars ?
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Old 3rd June 2011, 14:51   #4
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Default Re: Car Battery/alternator capacity query

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Originally Posted by jinu_joseph View Post
Thanks for details. So what is that people do when they get High power high end systems installed in their cars ?
They go in for after market alts abroad ONLY if required. They add capacitors and better quality batteries (GEL cell/VRLA).

I havent heard of anyone doing this here on TBHP or in my friends circle. Someone may correct me if there are guys on TBHP with ultra powerful ICE fixed (for .e.g. low_bass_makker) who may have done an alt upgrade.
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Old 3rd June 2011, 19:31   #5
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Default Re: Car Battery/alternator capacity query

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Originally Posted by R2D2 View Post
They go in for after market alts abroad ONLY if required. They add capacitors and better quality batteries (GEL cell/VRLA).

I havent heard of anyone doing this here on TBHP or in my friends circle. Someone may correct me if there are guys on TBHP with ultra powerful ICE fixed (for .e.g. low_bass_makker) who may have done an alt upgrade.

Remember, nothing is free and so is the electrical power from the alternator.
Alternator takes power from engine (<--from the fuel you fill). So higher capacity alternator means more fuel consumption, and even lesser power available to engine to do the necessary thing.

OEMs like Mercedes will have an extra battery for situations where the main battery runs out!!
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Old 3rd June 2011, 21:21   #6
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Default Re: Car Battery/alternator capacity query

@jinu joseph
How thick are the power wires to your system? And the fuse rating?

Regards
Sutripta
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Old 3rd June 2011, 21:29   #7
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Default Re: Car Battery/alternator capacity query

I don't think new generation cars and alternators have their output directly linked to RPM. Reason why I feel so is that I've scan gauge installed in my car and the voltage output is always between 13.5 to 14.4 volts and doesn't increase with RPM.
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Old 3rd June 2011, 21:48   #8
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Default Re: Car Battery/alternator capacity query

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Originally Posted by saurabhkum View Post
I don't think new generation cars and alternators have their output directly linked to RPM. Reason why I feel so is that I've scan gauge installed in my car and the voltage output is always between 13.5 to 14.4 volts and doesn't increase with RPM.

That happens to be normal range of operation regardless of whether you are idling the engine or revving it up. The regulator ensures the voltage stays within range.

Cheers!
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Old 3rd June 2011, 22:17   #9
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Default Re: Car Battery/alternator capacity query

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Originally Posted by cicero View Post
Remember, nothing is free and so is the electrical power from the alternator.
Alternator takes power from engine (<--from the fuel you fill). So higher capacity alternator means more fuel consumption, and even lesser power available to engine to do the necessary thing.
Absolutely! Which is precisely why car manufacturers clearly advise owners to lower electrical loads for max mileage.

Also, folks who upgrade their alts generally feel a drop in the low end response. One of the reasons to stay away from alt upgrades till you absolutely need it.

Cheers!
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Old 3rd June 2011, 22:45   #10
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Default Re: Car Battery/alternator capacity query

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Originally Posted by R2D2 View Post
Also, folks who upgrade their alts generally feel a drop in the low end response.
Hi,
Load should matter, not the (max) rating of the alternator.

Regards
Sutripta
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Old 3rd June 2011, 23:25   #11
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Default Re: Car Battery/alternator capacity query

Quote:
Originally Posted by Sutripta View Post
Hi,
Load should matter, not the (max) rating of the alternator.

Regards
Sutripta
Yes Sutripta, you are right. I should have been clearer with my response.
It is not the max output or rating of the alternator that causes the drop but the output at low RPM. Kind of similar to what we feel when the AC is switched on but not that obvious.

High power alts provide a lot of power at relatively low engine RPM..where it is most needed to provide sufficient amps or current to keep the electronics/electricals running plus maintain the battery. This is most useful in city traffic. In fact this characteristic is one of the USPs of an after market high power alternator, something that alternator mfrs harp upon.

Regards,
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Old 4th June 2011, 00:42   #12
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Default Re: Car Battery/alternator capacity query

Ok so lets take a typical example.

The method for calculating the Current requirement for running the amps is taken from

Car Amplifiers FAQ

Assume that i have a Focal Solid 1 Mono Bloc and a Focal Solid 4 Amp powering my ICE. My battery is a 55ah rated battery and alternator is a 75V-A rated.

Total RMS wattage of the amps would be as follows:
Focal Solid 1 - 470W
Focal Solid 4 - 75X4=300W
Total Amp RMS Watts = 770W

Since these are class AB amps account for efficiency of only 50% so power required increases to 770X2 = 1540W

Since the average music signal requires about 1/3rd of the average power in a test tone, divide by 3 (1540 /3 = 513W)

Now divide by the average output Voltage of an alternator, 13.8 volts. This will give us a current requirement of 37Amps

Assuming that regular Car electrical load (lights etc) is about 30A for night driving. That would mean that if driving at night with music i would require an average supply of 67A. For a alternator rated at 70 or 75V-A, this would be practically impossible in bumper to bumper traffic, when the average RPMs are low. SO in effect i could be draining my battery during this drive

This does not consider the user of the Wiper during rainy season as the wiper motor itself could draw an additional 12-14Amps.

Your views please.

Last edited by jinu_joseph : 4th June 2011 at 00:43.
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Old 4th June 2011, 10:44   #13
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Default Re: Car Battery/alternator capacity query

Quote:
Originally Posted by jinu_joseph View Post
Ok so lets take a typical example.

The method for calculating the Current requirement for running the amps is taken from

Car Amplifiers FAQ

Total RMS wattage of the amps would be as follows:
Focal Solid 1 - 470W
Focal Solid 4 - 75X4=300W
Total Amp RMS Watts = 770W

Since these are class AB amps account for efficiency of only 50% so power required increases to 770X2 = 1540W

Since the average music signal requires about 1/3rd of the average power in a test tone, divide by 3 (1540 /3 = 513W)

Now divide by the average output Voltage of an alternator, 13.8 volts. This will give us a current requirement of 37Amps

Assuming that regular Car electrical load (lights etc) is about 30A for night driving. That would mean that if driving at night with music i would require an average supply of 67A. For a alternator rated at 70 or 75V-A, this would be practically impossible in bumper to bumper traffic, when the average RPMs are low. SO in effect i could be draining my battery during this drive

This does not consider the user of the Wiper during rainy season as the wiper motor itself could draw an additional 12-14Amps.

Your views please.
Two things:

a) Firstly A is always calculated at nominal voltage i.e. 12v so your power requirement jumps from 37.8A to 42A. And assuming your car requires 30A during nighttime driving then the total consumption is ~72A. Max output of an alt is generally around 3000-4000 engine RPM while operational testing is done at 2000 RPM.

Therefore if you are not driving at highway speeds you may drain your battery. But there's a twist - engineers design alternators to take a 10-20% extra load, so there's every possibility your 75A alternator can push ~83A or even slightly more.

b) Ergo, you will drain your battery in stop and go traffic where engine RPMs are relatively low between 1000-1500. With the wipers switched on the drain on your battery is even faster.

I have faced this issue myself, pouring rain at night, wipers, headlamps )(100/90w) and AC switched on plus stop and go traffic. This was in my Esteem..which to my knowledge had a ~40A alt (not sure). The battery almost died that night. Marutis petrol engines generally have low power alts. Diesels in general have high capacity alts compared with the petrol counterparts.

Most people dont take alt, battery and wire sizing into consideration before accessorising their cars. They like to upgrade their batteries in the mistaken belief that the battery supplies the energy first, not the alternator.
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Old 4th June 2011, 14:13   #14
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Default Re: Car Battery/alternator capacity query

BUt what i have mentioned above is not exactly a very powerful setup. There are TBHPians with much more amplis and much more powerful setups..

What are the options available to ensure that the battery does not get drained out,
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Old 4th June 2011, 19:18   #15
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Default Re: Car Battery/alternator capacity query

Quote:
Originally Posted by jinu_joseph View Post
BUt what i have mentioned above is not exactly a very powerful setup. There are TBHPians with much more amplis and much more powerful setups..

What are the options available to ensure that the battery does not get drained out,
Oh yes there are people who put in thousand watt plus ICE setups and but do not upgrade their electrical systems. They generally get by thru draining the battery faster especially at low engine speeds. As a result battery life is affected. You dont get to run your mega expensive audio without paying a price in one form or the other.

Options:
a) make sure your dont run your ICE at idle, low engine speeds or even engine off conditions for extended durations unless you have upgraded your system.
b) Upgrade the alt, battery and wiring. Abroad it is easy and a DIY operation for many who dont want to pay for expensive labour.
c) Resist the temptation to install ultra powerful ICE or too many ICE components.
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