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High Charging Voltage Problem

Our boat is fitted with an Iskra 100a. alternator and an Adverc management system. Since fitting, five years ago, the alternator has charged as it should - cycling between about 14.1v & 14.5v through the Adverc system, measured across the starter battery terminals on the boat's built-in digital voltmeter.
Late last summer, the charging voltage abruptly changed to 14.7 - 14.8v - the boat's meter agrees with my hand held multimeter and with two others, one used professionally, so no question of faulty meter.
Disconnecting the Adverc made no difference - in any case, the Adverc will control up, but not down.
The alternator went away for a replacement regulator, but this too has made no difference.
The boat is also fitted with a Sterling 3-stage charger : the first stage 'boost' charge should be 14.4v, but is now 14.8v.
The batteries (1 x 96ah + 2 x 110ah) are only 3 years old, and hold charge very well, but are tending to need topping up regularly.
Do I have a problem ? - and if so where is the likely culprit, and how do I confirm ? I do not want to spend nearly £300 on new batteries on a hunch that they may be the cause.

Asked by: Richard Smith  | 2.06pm, Friday 27 January

WW says:

I think that it might be your batteries, and that the rise in voltage could be down to a change in resistance in the plates.
It might be worth checking the individual batteries voltage- both resting, and charging. This might show a culprit somewhere. The other option is to take a look at the cells- is one cell in the battery taking more water to top up than the others?
If it is, then it suggests that the plates might have started to thin or collapse.
#The raised voltage might be increasing the gassing, and so consumption of water, but I think that internal problems in the battery are moer likely. This seems to be borne out by your voltages registered using the charger as well as the alternator output.
Incidentally, what types of batteries do you have? Thicker, pure traction batteries are less likely to fail, but no battery is, alas, immune. At least with open cells you can top up the electrolyte.
For some people, 3 years (if they are used hard) is quite good going for standard "leisure" batteries; others might eek out over 6 years with them.
Let us know what is going on with an individual batteries, and we will put our thinking hat on further.

Mark Langley  | 4.21PM, Friday 27 January

I am afraid that I also think that the batteries are failing, due to the gassing (loss of electrolyte) and the raised voltage.
The chargers (the alternator and the Sterling) are both raising the voltage because the resistance of the batteries have increased and they are trying to "push" the charge in, the only common factor is the batteries themselves!
Disconnect the individual batteries from each other and measure the voltages. They might be slightly different; leaving them for a day or so will probably magnify the difference as they will self discharge at different rates. This will highlight the faulty battery, unless they are all failing equally. It is however a false economy not to replace all the batteries connected in parallel, although it is most likely the cabin batteries that are failing as the engine start battery has a much easier life.
It is always worth checking the wiring connections and the split charge relay as these can cause charging problems, but usually of the "not charging properly" variety.
Rupert Smedley

Rupert Smedley  | 4.52PM, Friday 27 January

Another option has been suggested that might be worth considering; the field coil in the alternator is powered via 3 small diodes that also drive the warning lamp. This voltage is also used by the internal regulator. If one of these diodes has blown the voltage will be lower than normal; the regulator will compensate by increasing the alternator output voltage until it sees the correct voltage on the warning lamp terminal.
In this situation the warning lamp often does not go out fully when charging, and glows slightly. Check by shielding the light when the alternator is charging. Also measure the voltage between the warning lamp terminal and the positive battery terminal. A difference of more than a volt or so indicates a blown diode.
Although the alternator has been for a replacement regulator, it might not have been giving a full health check.

Rupert Smedley  | 2.50PM, Monday 30 January

Readers say:

Thank you both very much. I will do the checks you suggest, but the boat is in for painting at the moment, so it will have to wait - but I will come back in due course. I have already checked all the wiring, battery terminals and the split charge relay, all proved ok.

Richard Smith  | 9.21AM, Saturday 28 January

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