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Old 06-22-2002, 12:50 PM   #1
Fevre
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Default Can a low battery cause poor idle and rough running?

I went to the gym this morning and left my electric fan on for about 45 minutes will I got a little pump in. When I got done and started my vette it fired up just a touch slower than normal and idle like crap. It ran crappy when I got going and did not smooth out until the ammeter dropped below 20 from the 40 it started at. I figured it has to be the low battery drawing all the current from the alt and the msd 'starving' for power. What do you think?

BTW I have a 100 amp alt.

Thanks

Chris


[Modified by Fevre, 9:51 AM 6/22/2002]


[Modified by Fevre, 2:45 PM 6/23/2002]
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Old 06-22-2002, 02:58 PM   #2
bigvette1
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

Really kind of doubt it . Once you got started the alt. puts out 13.8-14.5vdc. The MSD box will function and fire down to about 8vdc. Unless you are going below this the box will still put out a good pulse/spark. However, stranger things happen. Put a charger on the battery and then try. PS alternators make poor chargers on a drained battery.
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Old 06-22-2002, 11:28 PM   #3
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Default Re: Can a low battery cause poor idle and rough running? (bigvette1)

As far as I know, yes a low battery will make it run lousy. It will still run but acceleration and power will suffer. This is due to the fact that the car runs off the battery and the charging system charges the battery.
Some cars with a very good charging system might not suffer at all but older cars such as our Vettes and any other older hot rod I have owned have been like that.
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Old 06-23-2002, 02:13 AM   #4
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Default Re: Can a low battery cause poor idle and rough running? (MotorHead)

I noticed that Miranda ran much better after I put a new battery in - many moons ago. No MSD though, just stock stuff.
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Old 06-23-2002, 03:21 AM   #5
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Default Re: Can a low battery cause poor idle and rough running? (mayberg)

Motorhead, I don't know about that. I've changed bats from cars while the engine runing, I used to have a good bat to start all the cars and then take the bat off to another car. Never did it on my vette though, it was a toyota and a ford.
Paul
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Old 06-23-2002, 04:03 AM   #6
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Default Re: Can a low battery cause poor idle and rough running? (inferno-vette)

Like I said the cars will run just not as good, you will have power loss etc.
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Old 06-23-2002, 08:33 AM   #7
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Default Re: Can a low battery cause poor idle and rough running? (MotorHead)

I have different view on that. As the battery tries to charge, it takes a lot of the alternators power. If the battery is bad it can take almost ALL of the available current leaveing very little if any current for anything else. Sometimes you may have noticed that when you try to jump a car, It won't start until you take the old battery out. Same applies for the reason you have to wait for the battery being jumped to charge up a little when you are starting a car with a good battery that has just run down by leaving something on. With any electronic stuff, low voltage can make them not work or work real poorly. When our Lumina Mini-van's battery went south last time we could start it with a jump but it would die EVERY time I took the cables off. Batteries do all sorts of strange things.
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Old 06-23-2002, 10:22 AM   #8
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

I know it sounds like a split decision. My vote is YES... a low battery WILL cause it to run ruff...'specially if maybe your alternator might not be up to par...even thought it sounded like you had one with good output.
Is your MSD box properly wired to the 12v + battery source? It's not hooked up to...say... maybe the alternator output? I'm being repetitive, but the + side of the battery terminal...say...at the starter solenoid is where it needs to be connected for the correct power source.
Hope this helps!
Weasle :D
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Old 06-23-2002, 04:42 PM   #9
Ingar, Norway
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

Even a properly regulated 100amp alternator will have problems to keep a totally drained battery voltage close to the regulation point (should be 14.4V at room temp), it may drop a volt or more until the alternator catches up.
This will cause lower spark voltage out of the coil.

Seems to be some confusion on this subject, guess most of it is related to terminology:

>PS alternators make poor chargers on a drained battery.
This is sort of correct, but the problem is not with the alternator, but with the regulator.
A proper regulator will drive the alternator at max until you reach the regulating voltage. At 100amp, the battery charges pretty fast.

>This is due to the fact that the car runs off the battery
The car does not run off the battery. The sole purpose of the battery is to deliver power when the motor isn't running.
(Starting a motor only consumes a couple of Amp/hr, so this cause very little drain on the battery)
When the motor is running all power comes from the alternator.

>If the battery is bad it can take almost ALL of the available
>current leaveing very little if any current for anything else
If the battery is totally drained it might, as said, pull down the system voltage for a while.
But it will never "steal" any current from it.
Unless the power consumption of the car is larger than the alternator capacity. But then you have a serious problem, the battery will become completely discharged after a while.

I have a tech article on charging at my page.
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Old 06-23-2002, 05:30 PM   #10
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

Sounds like most agree with me as far as the battery pulling all the charge from the alt until it gets charged enough to draw less, then the msd system gets what it needs. I had the same thing happen the next day. Stopped to eat and left the electric fan running for about 20-30 minutes. Car started fine but the ammeter, not voltameter, was ready +40. The engine ran crappy until the ammeter read just below 20 and then it ran fine again. I have to admit I have a wimpy battery so it probably is draining pretty quickly Guess I'll just turn the elec fan off when I shut the car off from now on.

Thanks for all the input.

chris
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Old 06-23-2002, 06:43 PM   #11
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

Ingar-->If the car runs off the alternator then you should be able to remove the battery and the car will run fine at all speeds and loads. I am not 100% sure the car runs off the battery but I am going to find out. I have run a car without the battery and it ran lousy, also with a near dead battery the car ran crappy.

I thought everthing ran off the battery, the battery is charged by the alternator that is constantly charging the battery because it is being drained by the car's electrical components.

You could say the alternator is running everthing because they are in the same circuit. What happens when the battery is fully charged ? The regulator shuts off current from the alternator to the battery. Now does that mean the alternator is running everthing now or the battery ?

I hope someone who knows the answer to this debate chimes as I am interested because of the work I do and if I designed the system I think I would have the alternator charge the battery until fully charged then shut off as it would be less wear on the alternator
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Old 06-23-2002, 06:56 PM   #12
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

You must have been pumping iron with your head. Why would you leave the fans on for 45 minutes. They draw at least 15 amps that is a lot of juice. The reason your idle was rough was due to the fact that your alternator was running at max placing a lot of drag on the engine, once your batt was recharged that places less of a drag hence smother idle. It always amazies me on how manay stupid Bubbas on the board.
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Old 06-23-2002, 07:25 PM   #13
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Default Re: Can a low battery cause poor idle and rough running? (73-454)

That was another tactful comment left by 73-454. Way to make this an enjoyable place.
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Old 06-23-2002, 07:42 PM   #14
Ingar, Norway
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

>If the car runs off the alternator then you should be able to remove the battery and the
>car will run fine at all speeds and loads.
Don't do this! Disconnecting the battery while the alternator is charging will most probably kill your regulator and/or diode bridge in an instant, due to the heavy transients.

The car might run lousy without a battery, not because the battery needs to deliver any current, but due to transients since the battery also acts as a big capacitor.

>I thought everthing ran off the battery, the battery is charged by the
>alternator that is constantly charging the battery because it is being
>drained by the car's electrical components.
The battery is only charged a short period after start (as said only a few amp/hrs are used). Later there is only a trickle current flowing into it to keep the voltage at 14.4V.

The critical thing when it comes to charging is the voltage loss from the regulator sensing point to the battery.
If there is significant loss here you will never be able to utilize the full capacity of the alternator. (see that article)

As long as you don't have an electric water pump it makes no sense to leave the fans running after stopping.
You should power the fan-relay control from the ACC circuit to avoid this problem. (also have an article on this)
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Old 06-23-2002, 08:52 PM   #15
Fevre
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Default Re: Can a low battery cause poor idle and rough running? (73-454)

I 'll refrain from posting what I would like to say.

I'll just ask one question:

Are there not OEM fans that run after the car shuts off?

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Old 06-23-2002, 09:04 PM   #16
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Default Re: Can a low battery cause poor idle and rough running? (Fevre)

Yes. I have had a few.
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Old 06-23-2002, 09:05 PM   #17
Fevre
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Default Re: Can a low battery cause poor idle and rough running? (Ingar, Norway)

Ingar

My car runs a little hot so I left the fan on so when I started the cool fluid in teh rad would cool the engine. I believe this to be the reason that there are OEM fans that act in the same fashion. I am going to overhaul my cooling system when I switch engines so I am trying to deal with it in it's current state without spending any $ until I have to.

BTW I left the fan on the second day on purpose to see if it would do the same thing.
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Old 06-23-2002, 09:41 PM   #18
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Default Re: Can a low battery cause poor idle and rough running? (73-454)

George, Did you forget to take your meds again? Turrets Syndrome is a terrible thing :yesnod:
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Old 06-23-2002, 09:51 PM   #19
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Default Re: Can a low battery cause poor idle and rough running? (MIKER)

Yes a car will run without a battery but if you have a bad battery all of the alt. current will go to the battery and not the ign. My 79 had a 10 volt batt(one bad cell) it ran but not well. One day it wouldn't start so I put the charger on it. I couldn't get the batt to come up so I looked at the cells one was completely dry and appeared to have been that way for a while. A new battery made a complete difference in the engine running the HEI requires a full 12 volts with the alt. trying to make the battery a 12volt when it wasn't possible the HEI didn't get all of its voltage. You MSD is as bad or worse about voltage as a HEI.
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Old 06-23-2002, 10:22 PM   #20
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Default Re: Can a low battery cause poor idle and rough running? (MIKER)

Quote:
George, Did you forget to take your meds again? Turrets Syndrome is a terrible thing :yesnod:
Oh oh, I'll be the first to duck. :boxing
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Old 06-23-2002, 10:22 PM
 
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