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Alternator output


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I am in a confused state. My battery was not holding charge so I bought a new one and fitted it. I thought I'd check the charging voltage at the battery and found  17.5 volts. I therefore assumed the alternator was not regulating the out put. Rather than replace the regulator I ordered a new Alternator. (The old one came with the car 28 years ago). On checking the battery voltage at tick over with the new kit it was still 17.5 volts.  

Are there any sparkies out there who can explain why this should seem so high. Will it burn out the battery and have I bought a duff battery/alternator or both?

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It's probably more likely you have a voltmeter which is lying to you.   If you can, check it against another one before doing anything drastic.  

If that voltage was real your battery would be being drastically over-charged.  If your car has an ammeter, what does that indicate?

Edited by RobH
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Well there was a problem as your battery was going flat, that’s real. Two alternators being faulty in the same way to the same degree is so unlikely you can discount that. As the multimeter is saying two separate alternators, one new, are giving out 17 volts lt seems highly likely the meter is wrong. If the alternator was putting out 17 volts and the battery was at 12 there would be very significant current draw so you should see a high current on the ammeter, so again that points to the multimeter being wrong. Disconnect the battery and test it’s voltage with the multimeter, if it’s reading say 14 or 15 volts then you know where you stand, it would be consistently reading  2-3 volts high.
That would leave you with the issue of what was the original problem and that would be looking more like a dying battery incapable of holding a charge. 
 

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Could it something as simple as the multimeter range being set on ac rather than dc? I just used a cheap (<£7) multimeter on the dc range on my car battery and it read 13.0 Volts (20 V range) but on ac it read 27 V on the lowest ac range of 200 V, hence only 2 figure display.

Regards

Bill 

 

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Thanks for all the suggestions.

I traced the wiring back through the ignition line found the bulb on the dash failed and rechecked the connection from there to the alternator.  I made sure they were good. started the car and everything ok chagrinning at a nice 14.4 volts.  I think it came down to a poor connection. I can't believe a failed ignition bulb would stop it charging , however all settled.

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6 minutes ago, roughground said:

I can't believe a failed ignition bulb would stop it charging

Actually yes it will. The alternator needs the current through the ignition lamp in order to start up. There have been several instances of people replacing that lamp with an LED and finding they have no charging as a consequence.  It's quite possible that blown bulb was the cause of your old battery becoming discharged. 

NTC was right to ask the question.

Edited by RobH
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I have read that there can sometimes be enough residual magnetism in the field cores that the alternator can bootstrap itself without external field current. Certainly can't count on it, though.

Ed

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On 1/30/2021 at 10:01 PM, ed_h said:

I have read that there can sometimes be enough residual magnetism in the field cores that the alternator can bootstrap itself without external field current. Certainly can't count on it, though.

Ed

Every Alternator I have had in my car was/is able to self energise without DC current albeit above idle. 

I think I run a 47 Ohm, 11 Watt resistor in parallel with my LED ignition indicator. I have a series diode in circuit to prevent the alternator from keeping the main ignition relay live after engine switch off  

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17 minutes ago, SpitFireSIX said:

Every Alternator I have had in my car was/is able to self energise without DC current albeit above idle. 

I think I run a 47 Ohm, 11 Watt resistor in parallel with my LED ignition indicator. I have a series diode in circuit to prevent the alternator from keeping the main ignition relay live after engine switch off  

Can you please attach a simple sketch of the circuit you are proposing?

 

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