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Old 02-21-2021, 09:59 AM   #1
alschnertz
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Default How far would you go with a bad alternator?

2001 S40 254,000 miles.

I have a failing alternator. Began charging intermittently over the past few days.
Have ordered a new one, but it won't be here for a few days.

So in the event of a total failure, I was curious how far I could make it on just the battery.
I've got a 20 mile highway commute to work and may have to use the headlights for about 15 minutes in the morning. Would keep all the accessories off.

Had a complete alternator failure in a carb'd 145 years ago and made it 50 miles easy. I think this more modern car would not make it that far before the battery was dead.

Anyone with real-world experience on how far they went on just the battery?

Edit: I can fully recharge the battery overnight.
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Old 02-21-2021, 11:39 AM   #2
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You're asking for trouble. Take a portable jump box with you in case you run out of juice. Make sure it is fully charged.
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Old 02-21-2021, 12:55 PM   #3
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I have a failing alternator.
Well, go start a GoFundMe...
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Old 02-21-2021, 02:19 PM   #4
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During the day you get about an hour of driving with a fully charged battery. The lights add a large load onto the electrical system. Night time I got about 20 minutes before the car wouldn't run anymore. This was on my 240. I'd say a 20 mile drive is pushing the limits since you have to use the headlights and it's probably going to take a half hour. Bring another battery that's fully charged and you'll be able to make it.
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Old 02-21-2021, 02:40 PM   #5
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I had an issue once with one of my 240s. Same deal with a bad alternator. Happen while I was going to work. Turned out to be bad brushes on the alt. I had enough power to make it to a parts store, bought a portable jump box then went to work. Took the jump box inside and fully charged it. When I left work I connected the battery charger to the battery, secured the hood so it wouldn't fly open and drove home, about 20 miles. Granted it was still light out, summer time. Made it home and worked on it a few days later. It's not just the lights that will drain the battery, computers, clock, and anything else electrical that makes your car run will be drawing current.
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Old 02-21-2021, 03:18 PM   #6
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If it isn't charging at all you will be lucky to make it 10 miles. The computers quit working correctly when the voltage drops below 12 volts. The fuel pumps draw quite a bit of power.
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Old 02-21-2021, 04:43 PM   #7
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Back when I had the alt go out on my old 2004 Fiat Stilo, I didn't even make 10 kilometres (about 6.5 miles or so) when the car shut off and I coasted the rest into my driveway, that was with no lights, radio, AC etc. on.
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Old 02-21-2021, 05:30 PM   #8
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Quote:
Originally Posted by dl242gt View Post
.
Quote:
Originally Posted by nel621 View Post
.
Quote:
Originally Posted by 2manyturbos View Post
.
Quote:
Originally Posted by Fa182 View Post
.
Thank you guys.
This is the real-world info. I was looking for.
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Old 02-21-2021, 06:41 PM   #9
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A good and fully charged battery will take you further than 20 miles. But why risk it if it's not a must.

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Originally Posted by Fa182 View Post
Back when I had the alt go out on my old 2004 Fiat Stilo, I didn't even make 10 kilometres (about 6.5 miles or so) when the car shut off and I coasted the rest into my driveway, that was with no lights, radio, AC etc. on.
Then it has not been working properly for a while.

Battery on the "B6284T" ran out of juice on a trackday and borrowed one from a friends car(got my battery charged enough to start his car). Drove ~106 miles without lights and still had good voltage.
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Old 02-21-2021, 11:53 PM   #10
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One time way back when, the alternator died in Brattleboro Vt in the early evening before dark. I hitched a ride to a local AZ and bought a battery in the hope it would get me home. At Bennington, about 40 mi away I got nervous and bought another battery at an Advance. Sure enough, the first one died at about the 50 mi mark and I slipped in #2. As I recall it began to fail just I pulled into the driveway in Albany. I was flush with batteries for the next few years.
After that incident I've always used a cigarette lighter voltmeter. More recently the voltmeter has a couple of usb ports. And that's the truth.

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Old 02-22-2021, 03:43 AM   #11
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Then it has not been working properly for a while.
Idiot light didn't trigger before but could be, given the short distance I had
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Old 02-22-2021, 04:05 AM   #12
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Quote:
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A good and fully charged battery will take you further than 20 miles. But why risk it if it's not a must.


Then it has not been working properly for a while.

Battery on the "B6284T" ran out of juice on a trackday and borrowed one from a friends car(got my battery charged enough to start his car). Drove ~106 miles without lights and still had good voltage.
Considering that a group 27 battery on overage is rated at 75 AH and a typical fuel injection equipped car draws 35-50 amps without lights or accessories running, I doubt your alternator was totally dead. The math doesnít add up.
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Old 02-22-2021, 07:37 AM   #13
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typical fuel injection equipped car draws 35-50 amps
Seems a bit high...for an OEM 240
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Old 02-22-2021, 08:12 AM   #14
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My 245 lasted about 1.5 miles when the alternator ground strap came loose.
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Old 02-22-2021, 10:43 AM   #15
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on my 240 with the mystery age napa battery it came with, I made it about 17 miles after I realized it wasn't charging. I had a friend jump me and I made it an additional 2 miles, about 100 yards from home. an additional jump got me in the driveway. Only the cluster exciter bulb burnt out

I still ran that battery for a year or two until rust in the battery tray ate through the bottom of it. That time ~50 miles from home
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Old 02-22-2021, 11:02 AM   #16
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A PV is a different sort of thing, for sure. But I had an alternator fail at mountain meet the year I took it. Something went wrong and the voltmeter was pegged around 19 volts as I started off on the 600 mile ride back. I figured voltage that high wasn't good, so I turned everything electrical on that I could, dropped the voltage down off the peg to something like 16 or 17 volts.

This lasted for about 45 minutes or so, then the voltmeter dropped down to 12.5-ish. Turned *off* everything I could (electronic ignition, electric fuel pump, M41 OD, it uses a bit more electricity than a stock PV would) and kept driving with an eye on the voltmeter. I got another 2, 2,5 hours or so before it was down to about 10 volts, to the outskirts of Nashville (about 300 miles from start). I pulled into a Wal Mart parking lot, parked at the top of a slope (just in case) and went inside and bought a cheap battery, about $60. FWIW I was also using a cute little 15 lb. Odyssey mini battery to begin with. Still another 300 miles to go so I didn't want to swap that battery in yet, so I just roll-started it and kept going. Refueled with motor running to avoid a crank. And out of curiosity more than anything I just kept on going.

The voltmeter kept on dropping, car kept running normally. 9, 8 , 7 volts. Still normal. 6 volts, still normal. Even the gas gauge was still acting normal. Then at a bit below 6 volts, the gas gauge went to zero. The car was still running fine though, but I decided to pull off on a rural exit in southern Illinois rather than finding the true lower limit and then changing the battery on the roadside. Pulled over, found a good parking spot, turned it off. Just for fun I tried to crank it, it was for too dead to even make the start click. Just nothing at all.

Swapped the batteries (had to remove my Odyssey mount and used a ratchet strap), started right up, and I drove the remaining hour or so into St. Louis. Battery was still showing 12.5 volts as I pulled into my driveway.
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Old 02-22-2021, 12:26 PM   #17
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Quote:
Originally Posted by 2manyturbos View Post
Considering that a group 27 battery on overage is rated at 75 AH and a typical fuel injection equipped car draws 35-50 amps without lights or accessories running, I doubt your alternator was totally dead. The math doesn’t add up.
I'm assuming the fuel pumps(K-jet copy) took 10-15A. Coils around 10A because it's just two coils firing at a time and low load/rpm. Injectors and ECU might be around 10A combined. That makes around 30-35A which is within the realm of possibility.

But you are right about the alternator. Didn't remember before that I did check the voltage with Tunerstudio while driving. Occasionally it would charge for a couple of miles and then stop again.

When the previous time alternator died I kept constantly track of the voltage and it'll still cruise with 8.8-8.9V but any lower and it's dead. Took a while to get that low too. Not the first or last one to not trigger the light.

Last edited by Lankku; 02-22-2021 at 01:06 PM..
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Old 02-22-2021, 01:06 PM   #18
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...Coils around 5A because it's just two coils firing at a time and low load/rpm...
Duty cycle has to be considered.

Example: 115 Coil will draw around 16 amps with 3 ms dwell time.

"So, let's do the math on 3ms/16 amps. The average current when charging is 1/2 of the 16 amps, or 8 amps. Let's say you ran this coil on a 12,000 RPM 4-cycle engine (ie, so the coil fires 6000 times per minute, or 100 times per second) -- that is 10 ms per firing. So 3ms/10ms = 30% duty cycle of charging.

30% of the 8A average, is 2.4A, well within the capability of a 20 AWG wire. Multiply this times the number of coils to figure the current requirement for a feed wire that is spliced to to all the coils.

If you don't run 12,000 RPM, then this number would be proportionally smaller. If you only charged the coils at 2ms/10A, then the number would be 33% less."
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Old 02-22-2021, 01:17 PM   #19
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Originally Posted by 2manyturbos View Post
If it isn't charging at all you will be lucky to make it 10 miles. The computers quit working correctly when the voltage drops below 12 volts. The fuel pumps draw quite a bit of power.
Came in here to say this.
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Old 02-23-2021, 01:06 PM   #20
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Fun little story: i once bought a cheap V70-T5. The owner said it had a dead new battery. So i was thinking it must be a charging problem. So my friend and me took 3 full batteries and 2 sets of jumper leads and went to go get the car.
For the first cold start we hooked up 2 extra batteries in the cargo space, all in parallel off course and presto, it started right up. And as assumed, the car didn't charge.
So i started the 70km gentle drive back home. Sketchy? Yes, very. In the end i didn't even need the 3rd battery. i made it without a problem. This was during daytime though.
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Old 02-23-2021, 03:38 PM   #21
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I can't offer advice that hasn't already been given so here are my Volvo Alternator stories instead.

1989 245 hooptie with stock low output alternator - I for a while I was trashing and returning alternators faster than I was burning through gas, but the car always gave me fair warning when it was about to crap the bed. All the lights would be dim, the car wouldn't accelerate very well, ect. I went through FOUR alternators before thinking "Hey, maybe I should question the car instead of the part." It turned out that I had a 4V drop between the alternator and battery. A 4G amp wire from my junk pile fixed the issue permanently after replacing the alternator one more time.

2010 Volvo C30 T5 with roughly 90k miles and all stock - I never had issues with the car. In fact I was already about 50 miles into a trip when when suddenly my brake failure warning message came on the dash. That's scary when you haven't touched the brake pedal for miles and you're cruising at 75mph. I tested the brakes and they seemed fine, so I kept going. Only a couple miles later, the dash just lit up like a Christmas tree, and the car went into a limp. I pulled over, shut the car off, turned it back on, and kept going. All the lights were off. I'll look into that when I get home. Before I could even get up to speed, the charging system failure message came up and the dash started to go nuts again. When I pulled over again, the car would barely start again. I knew that the alternator is trashed, but there was no way that I was going to be able to find parts and change it myself without tools on a Sunday in Gary, Indiana.

I went from perfectly fine to completely incapacitated on a six month old battery in a matter of 10 miles or less. I had the car towed to the nearest Volvo dealer which was luckily not far, and had my parents meet me there with a car I could borrow. After a week and a $1000 alternator replacement bill, I had experienced getting bent over by the Volvo dealer for the first and (hopefully) last time.
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Old 02-26-2021, 03:28 AM   #22
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No one has commented that an automotive lead-acid battery really doesn't like deep discharges... if you are actually planning to tempt fate, you'll be much better off installing a marine battery for said voyages.
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Old 02-26-2021, 08:27 PM   #23
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I don't know about how far you will get with a bad alt, but." If you have a two engine airplane and one fails the second one will get you to the crash site."
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Old 02-26-2021, 09:13 PM   #24
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I don't know about how far you will get with a bad alt, but." If you have a two engine airplane and one fails the second one will get you to the crash site."
Thatís so bad it actually made me laugh.
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Old 02-27-2021, 11:24 AM   #25
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Thatís so bad...
Unless a pilot reacts quickly, and correctly, crashing is quite possible.


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