Charging voltage

[pheran]

Both the engine and domestic systems on my mobo are 24 volt with both battery banks charged by a single altenator. According to the voltmeter, the charging voltage varies frequently between 26 and 28 volts but every so often it flicks over to 30 volts momentarily before returning to 26 or 28 volts. Do I have a problem with either the altenator or the voltmeter or am I worrying about nothing. Having recently had some £800-worth of new batteries fitted I am anxious not to cook 'em!!

ps as this boat is new to me, I don't know whether the charging voltage has always behaved in this way. Many thanks for any advice

 

[halcyon]

Check the voltmeter with a decent multimeter first, it may be reading wrong.

I would be more concerned with the 26 volt, when do you see 26 volt, when do you see 28 volt etc.

Brian

 

[William_H]

Yes check with another volt meter.

Digital volt meters can be misleading under some circumstances. The voltage is sampled for a brief period about 4 times per second so that if there are pulses or fluctuations on the line it can occasionally be reading at the time of a peak and again at a low point. This can be checked or overcome by using a resistor between the meter probe and the supply and fitting a largish capacitor across the meter. A 10K ohm resistor will reduce the reading by .1% so can be ignored and a 1 to 10 microfarrad capacitor will act like a mini battery to ensure the measurement is an average. Wait a few seconds for the reading to settle.

However back to the posters concern.... If the voltage is measured at the actual battery terminal it should be fairly smooth and accurate. Of course the charge voltage should be close to 28volts.

If it really is fluctuating that much then check the brushes and skip rings of the alternator these may be worn with low brush pressure resulting in dirty / burned slip rings and so unreliable contact.
good luck olewill

 

[roly_voya]

The fact that you needed new batteries may indicate there is an underlaying problem, what killed the old batteries?

 

[pheran]

Many thanks for the replies. To clarify -

- the voltmeter is the analogue 'dashboard' type
- at normal running speeds ie around 1500 engine revs, it reads either 26 or 28 volts and altenates between the two every couple of minutes or so.. Then there is this momentary, and infrequent, flick to 30v
- I assume it was old age that killed the previous set of batteries. They were 6 years old and my survey picked up the poor condition

 

[halcyon]

You have a problem, either voltmeter, or alternator ( could be worn or sticking brushes ), you should have a steady voltage. Maybe starting low 26 volt, but climbing to 28 / 29 volt area.

First monitor battery voltage with multimeter, against dash meter, if multimeter steady, dash meter changing fit new dash meter. If the voltage is changing get the alternator checked out by your local LSUK, or similar.

Brian

 

[shmoo]

If you have a smart (or smart-ish) regulator the voltage excursions may be due to that. We have an old, but excellent, TWC regulator that pulses high voltages at some point in its charge cycle. The only thing this causes a problem with is the Ebber, which seems to refuse to start if the voltage is out of range.