Alternator high voltage when battery 100% soc

[BarryTheBoat]

I have an ISKRA95A alternator on a Beta 1903 connected to a bank of five 100ah lead acid batteries which is fitted with a Cargo 231044 (14.2V) regulator.  A second alternator charges the starter battery. When the leisure batteries are discharged overnight the alternator produces up to the max output of amps at around 14.2V, but as the soc of the batteries approaches 100% the charging current reduces to below 1A (as expected) but the voltages rises up to 14.8V.  Switching on some domestic load (cabin lights) totalling about 2A brings the alternator voltage down to about 14.3V.
When the boat is left and the batteries are kept near 100% soc (by solar panels or landline charging) on engine startup the alternator begins outputting an amp or two and within a few minutes (as the batteries warm up?) the voltage rises to 14.8V. This represents different behaviour from how it previously operated for 20 years.
The alternator has been tested three times by two different alternator repairers and found to have no problems!
In order to try and identify the issue i have directly wired the alternator B+ & B- to the otherwise fully isolated battery bank (leaving only the D+ connected via the engine wiring loom fed by the starter battery. I should say the panel warning light functions normally and goes off once the alt. stabilises).  Voltages have been measured (with several multimeters) at both alternator and battery and are identical within a mV or so.
At 14.8V the D+ to B+ voltage difference is around 0.22V whilst at 14.4V the difference is 0.02V.
This issue has been going on for some time and spans a replacement of the leisure battery bank. 
I'm pretty sure that 14.8V is very unlikely to cause problems for any 12V electronic components on the boat, but what about the batteries. 
I must also admit to confusion as to what constitutes battery overcharging. Many internet articles warn of the danger of too high a voltage, but quote radically different values for what constitutes too high, nor do these articles reference the current at these 'high' voltages making it sound immaterial and thus my 1A @ 14.8V would appear to be just as damaging as a much higher current at that voltage.   
Anyone comments on battery overcharging and any ideas on what i could look at next in order restore previous behavior of the alternator (and satisfy my OCD!)? 

[nicknorman]

1 hour ago, BarryTheBoat said:

I have an ISKRA95A alternator on a Beta 1903 connected to a bank of five 100ah lead acid batteries which is fitted with a Cargo 231044 (14.2V) regulator.  A second alternator charges the starter battery. When the leisure batteries are discharged overnight the alternator produces up to the max output of amps at around 14.2V, but as the soc of the batteries approaches 100% the charging current reduces to below 1A (as expected) but the voltages rises up to 14.8V.  Switching on some domestic load (cabin lights) totalling about 2A brings the alternator voltage down to about 14.3V.
When the boat is left and the batteries are kept near 100% soc (by solar panels or landline charging) on engine startup the alternator begins outputting an amp or two and within a few minutes (as the batteries warm up?) the voltage rises to 14.8V. This represents different behaviour from how it previously operated for 20 years.
The alternator has been tested three times by two different alternator repairers and found to have no problems!
In order to try and identify the issue i have directly wired the alternator B+ & B- to the otherwise fully isolated battery bank (leaving only the D+ connected via the engine wiring loom fed by the starter battery. I should say the panel warning light functions normally and goes off once the alt. stabilises).  Voltages have been measured (with several multimeters) at both alternator and battery and are identical within a mV or so.
At 14.8V the D+ to B+ voltage difference is around 0.22V whilst at 14.4V the difference is 0.02V.
This issue has been going on for some time and spans a replacement of the leisure battery bank. 
I'm pretty sure that 14.8V is very unlikely to cause problems for any 12V electronic components on the boat, but what about the batteries. 
I must also admit to confusion as to what constitutes battery overcharging. Many internet articles warn of the danger of too high a voltage, but quote radically different values for what constitutes too high, nor do these articles reference the current at these 'high' voltages making it sound immaterial and thus my 1A @ 14.8V would appear to be just as damaging as a much higher current at that voltage.   
Anyone comments on battery overcharging and any ideas on what i could look at next in order restore previous behavior of the alternator (and satisfy my OCD!)? 

 

It depends on the exact type of lead acid batteries and the temperature. Some types of battery relish 14.8v when cold. For other types, it is too high especially when warm.

As to why 14.8v, perhaps the D+ is connected directly to the engine battery supply (via the ignition switch) so the voltage measured by the the cargo regulator is influenced by the engine battery voltage. The correct way to wire the domestic alternator D+ / warning light is to use a relay that is powered by the starter battery via the ignition switch, and whose contacts connect the domestic battery postive to the warning light when the ignition is on.

[sueanddaren]

I had exactly the same issue with a Iskra alternator (100A) on a Nanni 3.30N (Kubota 1305). The high voltage triggered an alarm on the ignition panel, and like you I learned to switch a load on (cabin lights). Like you I had it checked out by an alternator/starter motor repairer. Eventually I got tired of forgetting to put the cabin lights on and the alarm going off so just put a new alternator on. 

[pearley]

My Beta 43 with Iskra 175a alternator regularly regulated at 14.8v. The only thing it it did was trigger the high voltage alarm on the Smartguage. We had FLA batteries so just ensured they were topped up.

[BarryTheBoat]

16 hours ago, sueanddaren said:

I had exactly the same issue with a Iskra alternator (100A) on a Nanni 3.30N (Kubota 1305). The high voltage triggered an alarm on the ignition panel, and like you I learned to switch a load on (cabin lights). Like you I had it checked out by an alternator/starter motor repairer. Eventually I got tired of forgetting to put the cabin lights on and the alarm going off so just put a new alternator on. 

Yes, its frustrating that you get told that your alternator is being regulated at the correct level only to remount it on your engine and immediately find it exceeding that voltage.  It is fair for the Alternator Repairs shop to point to factors on the boat over which they, of course, have no knowledge - batteries, wiring, connectors, earthing, solar regulator, shunt etc etc.
However i can't get the niggle out of the back of my head that 99.9% of an Alternator Repairers work will be on vehicle alternators charging engine/starter batteries.  The regime under which these operate is subtly different to charging leisure-only battery banks particularly when the soc reaches 100% and there is no other load on the alternator. I am not an electrician or a car mechanic, but i doubt whether this condition ever exists in vehicles - there are always some lights, fuel pumps and ignition loads.
When the repairer tests the alternator i would expect them to want to 'stress' test the device to check if it can handle the designed output current. When i have tried to address the no-load circumstances i get met with a wall of misunderstanding - "well there's always some load". Equipped with nothing but an absence of knowledge i dont want to malign this fine body of men, but i wonder how sure can we be that the testing equipment they are using actually emulates the almost miniscule (<1A) loading experienced at times on the boat? 

On 10/03/2025 at 15:24, nicknorman said:

 

It depends on the exact type of lead acid batteries and the temperature. Some types of battery relish 14.8v when cold. For other types, it is too high especially when warm.

As to why 14.8v, perhaps the D+ is connected directly to the engine battery supply (via the ignition switch) so the voltage measured by the the cargo regulator is influenced by the engine battery voltage. The correct way to wire the domestic alternator D+ / warning light is to use a relay that is powered by the starter battery via the ignition switch, and whose contacts connect the domestic battery postive to the warning light when the ignition is on.

I should perhaps have raised the more philosophical question of what constitutes too high a battery charging voltage as a separate thread.
I agree that the type of battery and temperatures are big considerations, as might be the particular make and model, but it seems difficult to find any statements or graphs from the battery manufacturers of what constitutes best charging practices.
External chargers (mains or solar) refer to fairly sophisticated charging regimes involving such things as float, bulk and absorption levels whilst the engine mounted alternator seems to take the bulldozer approach of outputting as many amps as it can until the battery screams back in submission!

 

Regarding the exciter voltage to D+, all i can say is that it comes from the Beta installed loom and control panel and has worked OK until recently although component failure is always a possibility. As i understand it the voltage from the control panel via the D+ line becomes countered by the internal voltage being generated in the alternator destroying the potential between control panel and alternator and thus extinguishing the warning light as a result.  The engine battery and its alternator operate at 14.4V so if that voltage were being fed 'raw' to the D+ it would still be 0.4V lower than the ISKRA output.  

 

Thanks for your comments anyway.

[Tony Brooks]

It seems to be a nine diode machine and also machine sensing, so the charging voltage is being sensed from the output of the field diodes which is the same voltage that puts the warning lamp out, as you say. There is no separate battery sense wire to cause problems. Even in Nick's scenario of the amp being fed from the engine battery, I can't see how that can interfere with the voltage sensing, if there was a difference between the domestic and engine battery voltage then the warning lamp would attempt to glow, but probably not enough to see, even in the dark.

 

I wonder if this voltage really originates from any solar charging rather than the alternator. If you have solar, then try covering the panels or turning the panel to controller feed off and putting the load on for a short while. That should rule out the panels being the culprit.

 

If it is the alternator then the question is why is the regulator seeing a lower voltage from the field diodes than it should, because if it sees a lower voltage it will increase the charging voltage to compensate. So what can cause that? The answer is a blown field diode or a bad solder/crimp joint between the stator and the regulator. The path is stator > field diodes > regulator. If this were mine, I would have it apart and resolder all the field diode connections, plus and convections on the lead/spring that connects the diode pack to the regulator - if it is a spring it is definitely worth cleaning the contacting surfaces and maybe re-tensioning it a little.

 

Re, the batteries. Open cell, lead acids will probably be happy with 14.8 volts at all but zero amps as long as they are topped up regularly, but I am not so sure for any sealed battery. The gassing voltage of lead-calcium batteries is very close to 14.8V so probably not a problem.

 

To address the testing. Typically, the test rig is just an electric motor to drive the alternator, a battery, and a large variable resistor to load the alternator so if the alternator achieves the rated output it would be deemed serviceable. I doubt the test rig would stand passing 95 amps for a prolonged period without overheating the load resistor. Once the alternator's ability to produce its rated output is confirmed, then the load is reduced to ensure the regulated voltage is maintained (does not go high). I am not sure how often the last step is actually carried out.

 

I am not sure how helpful this is, but with a new regulator it is likely that any fault would be in the alternator stator > field diode > regulator  circuit.

 

 

 

 

[BarryTheBoat]

3 hours ago, Tony Brooks said:

It seems to be a nine diode machine and also machine sensing, so the charging voltage is being sensed from the output of the field diodes which is the same voltage that puts the warning lamp out, as you say. There is no separate battery sense wire to cause problems. Even in Nick's scenario of the amp being fed from the engine battery, I can't see how that can interfere with the voltage sensing, if there was a difference between the domestic and engine battery voltage then the warning lamp would attempt to glow, but probably not enough to see, even in the dark.

 

I wonder if this voltage really originates from any solar charging rather than the alternator. If you have solar, then try covering the panels or turning the panel to controller feed off and putting the load on for a short while. That should rule out the panels being the culprit.

 

If it is the alternator then the question is why is the regulator seeing a lower voltage from the field diodes than it should, because if it sees a lower voltage it will increase the charging voltage to compensate. So what can cause that? The answer is a blown field diode or a bad solder/crimp joint between the stator and the regulator. The path is stator > field diodes > regulator. If this were mine, I would have it apart and resolder all the field diode connections, plus and convections on the lead/spring that connects the diode pack to the regulator - if it is a spring it is definitely worth cleaning the contacting surfaces and maybe re-tensioning it a little.

 

Re, the batteries. Open cell, lead acids will probably be happy with 14.8 volts at all but zero amps as long as they are topped up regularly, but I am not so sure for any sealed battery. The gassing voltage of lead-calcium batteries is very close to 14.8V so probably not a problem.

 

To address the testing. Typically, the test rig is just an electric motor to drive the alternator, a battery, and a large variable resistor to load the alternator so if the alternator achieves the rated output it would be deemed serviceable. I doubt the test rig would stand passing 95 amps for a prolonged period without overheating the load resistor. Once the alternator's ability to produce its rated output is confirmed, then the load is reduced to ensure the regulated voltage is maintained (does not go high). I am not sure how often the last step is actually carried out.

 

I am not sure how helpful this is, but with a new regulator it is likely that any fault would be in the alternator stator > field diode > regulator  circuit.

 

 

 

 

There is nothing so sweet in life as having someone with experience tell you what you have been suspecting all along!

 

Everything i read, including your own contributions to this site, seemed to point in the direction you state, but the old "not knowing what i don't know" syndrome made me doubt my reasoning given multiple assurances that the alternator tested OK.  Because of this i was keen to keep an open mind and try to find other possible reasons which is why i posted the original topic.

 

As an aside, i discounted the solar panel issue (and many others) by directly connecting the alternator and battery in a 'closed' circuit with no connection to any other part of the boat wiring - a simple alternator-battery and nothing else circuit - with the same result. 

 

To add to your comment about how often repairers test with a reduced load i would add "and how often the equipment is calibrated."

 

I now have the problem of trying to convince the original repairer who has twice said there was nothing wrong with the device that there probably is!

[Tony Brooks]

1 hour ago, BarryTheBoat said:

There is nothing so sweet in life as having someone with experience tell you what you have been suspecting all along!

 

Everything i read, including your own contributions to this site, seemed to point in the direction you state, but the old "not knowing what i don't know" syndrome made me doubt my reasoning given multiple assurances that the alternator tested OK.  Because of this i was keen to keep an open mind and try to find other possible reasons which is why i posted the original topic.

 

As an aside, i discounted the solar panel issue (and many others) by directly connecting the alternator and battery in a 'closed' circuit with no connection to any other part of the boat wiring - a simple alternator-battery and nothing else circuit - with the same result. 

 

To add to your comment about how often repairers test with a reduced load i would add "and how often the equipment is calibrated."

 

I now have the problem of trying to convince the original repairer who has twice said there was nothing wrong with the device that there probably is!

 

I think time for another repairer, Lancaster rotating electrics is often recommended on here, no personal experience though.

 

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01772 623600
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