Voltage Issues
On the way
back from Campeltown to Largs
in company with Tangram, Appuskidu’s
new Raymarine tiller pilot stopped working - shortly
after passing the southern tip of Arran. After having to go to court for
redress when, after little use, the previous Simrad
tiller pilot had failed permanently, I was not best pleased when the new Raymarine one quit, especially as it had been performing
splendidly up to now. According to Raymarine the
symptoms (continuous buzz and no action) suggested insufficient battery voltage
– BUT the battery condition meter on the distribution panel was off the scale
in the green for each of the two batteries thus reporting that both batteries
were fully charged. The cabin lights worked fine too. So was the meter lying?
After the cruise I towed Appuskidu back home to
Don’t buy one of these
In the winter, enquiries on the web forums
revealed that the battery condition meter on the low cost panel that I had was
widely regarded as unreliable. To prevent possible accidental current drain I
had already changed the battery test switch on this panel to one that was
spring loaded to return to the centre off position after testing one or other
of the batteries. Also, the cigar lighter type socket on the same panel had
rusted and had needed to be
replaced with a marine spec one. Now I had to replace the meter
on the panel as it was obviously utterly useless.
A posting on the PBO forum pointed
me to a low cost 0 to 20V digital panel voltmeter on EBay and so I bought one.
One forum posting suggested that the meter took a long time to settle if it was
powered by the source that it was testing. This was not my experience. I did
not 
plan to fit a
separate (dry) battery to power the meter and found it worked fine without.
However accuracy is important as there is only just over one
volt difference between a fully charged battery (12.7V) and one with
only 10% charge (11.5V). In fact for good battery life you shouldn’t allow the
battery to fall below (say) half charge (12.1V) in normal use and that gives a
useable range of only a bit over half a volt (0.6V) from full charge. Obviously
for this kind of measurement an accuracy of 10% would
be no good. So before installing the new meter I took some readings with it and
compared them with measurements from my multimeter.
There was a whole one volt difference! Which was
right? I needed a third instrument to see which of my two was wrong. After asking
everyone that I knew if they had a digital multimeter
I was finally directed to a local electrical engineer. His instrument confirmed
that the new voltmeter was acceptably accurate and my old multimeter
was not. The readings are reproduced below. Folk on the PBO
forum who suggested cannibalising a multimeter to use
as a battery condition meter would be well advised to check the accuracy of
their instrument first.
The new meter will be installed on Appuskidu in the spring.
METER
The readings below were taken under the following conditions –
No load (except for the meter).
Temperature 14 degrees centigrade.
One week after temporarily taking the batteries off winter maintenance charge for these tests.
New Digital Voltmeter 13.0V
Reference Digital Multimeter 12.9V
My old Digital Multimeter 12.0V
New Digital Voltmeter 12.9V
Reference Digital Multimeter 12.8V
My old Digital Multimeter 11.9V
The majority readings say that the batteries are fully charged at this time.
Info.
The batteries are 26Ah sealed standby batteries designed for deep cycle but low current (Max 7 amps each). They are four years old.
There is no engine start load as my outboard motor auxiliary is manual start. When it is running, according to cruising rpm, it presents 2 to 3 amps of charging current - not all of which of course gets into the batteries.
Purely out of curiosity, with the new voltmeter only, I checked the voltages under the 1.75 amp load from a 21 watt light bulb. The results were:
I don’t know if that is good or bad.