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Usual voltage when alternator's working? - Engine & Ancilliaries/Gearbox - The Lotus Forums Jump to content
soldave

Usual voltage when alternator's working?

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What's the usual voltage across the battery when you've got a good alternator?  Is it the usual 13.8-14.3V when you rev it a little?  Just want to confirm this.  I know on the dash gauge mine usually hovers just below centre, and am wanting to check the battery to see if it needs a rebuild over winter.

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At the very minimum... the alternator should be working at around 13v to be able to charge the battery.  Lower than this, i.e 12 -12.5v you are not charging the battery and are likely to be seeing the voltage of the battery just across the terminals.  Higher than 14.5V and the regulator on you alternator is probably kaput.

 

Measure it using a digital multi-meter.

13.5 - 14.5v is a good range to be looking for.

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At the very minimum... the alternator should be working at around 13v to be able to charge the battery.  Lower than this, i.e 12 -12.5v you are not charging the battery and are likely to be seeing the voltage of the battery just across the terminals.  Higher than 14.5V and the regulator on you alternator is probably kaput.

 

Measure it using a digital multi-meter.

13.5 - 14.5v is a good range to be looking for.

Thanks for that Kieran.  I'm getting 13.8V across the battery but only once I've blipped the throttle over about 2,000rpm.  Before then it's just at 12V.  Strange...

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Just noticed my appalling spelling on that thread title.  Apologies for that.

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Thanks for that Kieran.  I'm getting 13.8V across the battery but only once I've blipped the throttle over about 2,000rpm.  Before then it's just at 12V.  Strange...

 

That points to a faulty voltage regulator. We've had the same in a Range Rover, upon starting the alternator wasn't charging, the reading of about 12V coming from the battery alone. Once revved passed 2000rpm the alternator 'woke up' and would continue charging even when dropping back down to idle. The charge light indicator should also come on as long as the voltage remains low.

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That was going to be my reply.  It either indicates a slow reacting voltage regulator or your car has a ridiculously low idle.

 

the only caveat to the above is if you are underdriving the alternator with a larger pulley.  The charge light indicator will only illuminate if the overall battery voltage dips below a certain threshold. Probably around 12.5v -12v

 

1. The light on mine stays on until I reach a certain rpm (until it begins charging).  i.e blip the throttle to extinguish the light.

2. It extinguishes after the circuitry begins charging (as can be seen on the digial guage rising above 14v).

3. It will then stay off when the engine idle is dropped to normal even though the battery is NOT being charged.

4. It only illuminates once again when the overall battery voltage begins to dip due to the ancillaries pulling down the residual charge levels left in the battery and the engine is at an idle state where it isn't being charged.  This takes quite a while.

 

N.B - The above for me is with a new battery and alternator but with an under-drive alternator pulley.

 

Could it also be your battery is FUBAR??  Winter shows up these weaknesses with aggressive intent.

Maybe borrow a brand new battery and see if it still does it before forking out on anything.

Edited by Fueltheburn

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First thing I'm going to check is changing the IGNITION bulb on my dash back from an LED to a normal bulb.  If I remember right the problem started after changing the bulb to an LED and the ignition bulb is actually an important part of the circuit (or is at least with Minis).  Might just be that causing it.

 

If not I'll investigate further...

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I'd get a decent multimeter rather than relying on the voltage shown on the instruments.

 

You own a lotus - a continuity tester is a must.

 

Maplin do their precision gold multimeter on special at the moment at 19.99

 

It is the best 20 quid you will ever spend on your lotus toolbox, and when you put it across the battery with the engine running, you have a fair idea of what is actually happening.

 

:smoke:

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Already got one :).  Knew it was going to be a vital tool!

 

Once the car starts up (and IGNITION light stays on) voltage is 12.03V across the battery.  Blip the throttle to 2,000rpm and it goes up to 13.8V and stays there charging.

 

So I'll try changing the bulb back to a standard one, start the car up and see what voltage I get across the battery then.

Edited by soldave

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How new is your battery?

 

12.03 v across the battery would suggest either a seriously discharged state or the plates are heavily suphated.

 

Nominal voltage for a fully charged 12V lead acid battery is around 12.5v

 

.... Just saying :sofa:

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How new is your battery?

 

12.03 v across the battery would suggest either a seriously discharged state or the plates are heavily suphated.

 

Nominal voltage for a fully charged 12V lead acid battery is around 12.5v

 

.... Just saying :sofa:

 

Agreed, but not if this is the voltage measured with the engine running but the alternator not charging.

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Agreed, but not if this is the voltage measured with the engine running but the alternator not charging.

Good point.  Will check it when the engine's not running over the next couple of days too.

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FYI. the battery in old Banana would now be about 2 years old or maybe just slightly over, but definatly not more than 3 and it was a pretty good quality battery that was fitted at the time.

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I have an Esprit S3. For the first few miles its 14.5V then it settles at around 14V. If I switch on the radio it drops back a small amount .

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Sometimes the globe is essential to excite the charging circuit, so a led would almost certainly not work properly. Depends on regulator configuration however.

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Sometimes the globe is essential to excite the charging circuit, so a led would almost certainly not work properly. Depends on regulator configuration however.

I have seen led's used, but they must have a shunt resistor across the led/ resistor setup to make sure it draws similar current to what a bulb would. Led on it's own wouldn't draw enough current for the alternator.

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My 72 MGB does the same thing and it has a new alternator in it. Light is on till I blip the throttle, then it's good to go from then on. Never comes back on till I restart it. Charges perfectly.

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Changed back to a standard bulb and alternator kicks in as soon as the engine starts.

 

Problem solved.  Crisis averted :)

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The simple answer,... it's fine.

 

The answer answer,...

 

alternators, unlike generators, do not have permanent magnets so they need the field coils energized (to become electromagnets) so that the rotor can spin in a magnetic field and produce electricity.

 

There is another sort of alternator that is called a 1-wire or self exciting alternator.  It only has one wire and often used in boats, generators, and hot rods, because it is easy to install.  They act like this, they produce no voltage below 1.5k rpm or so, then presto they start charging.  

 

They work like this because even though they have no proper magnets, the metal in alternator becomes slightly magnetic over time. These sad accidentally magnetic magnets will produce enough voltage to run the alternator but only if you first rev it to above 1.5k ish.  When it falls back below the 1.5 threshold, it continues to charge because the now energized field coils continue to be energized enev at ow rpms.

 

The secret is that most alternators will do this.  I am only running the two brown charge wires from the Lucas alternator in my Elite.  I disconnected the alt light and replaced it with a voltage meter directly from the fuse block.  Now it reads 12v or just under when the car starts and starts to charge when I blip the throttle above 2k or so.

 

As I think about it my 81 mk1 vw has always done this.

 

So, you might have an alt light out, maybe a voltage sensing wire broken, who knows.  Mend it if it bothers you, leave it if it doesn't.

 

Ross

 

 

 

 

 

 

 

 

 

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