Voltage Issues
This is a
story about how I came to fit a digital voltmeter.
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.
I was
impressed that with modern cell phone technology I was able call them up from
the middle of the clyde
estuary. 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 as usual but before putting her away I took
the batteries into the garage and checked them. They were flat! The batteries
were then recharged at home after which the tiller pilot worked fine again.
Don’t buy one of these
In the winter,
enquiries on the web forums revealed that the battery condition meter on my low
cost panel 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 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 with the one that I
bought. 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 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.
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 and my old Digital
Multimeter is garbage.
Info.
The batteries
are 26Ah sealed standby batteries designed for deep cycle but low current (Max
7 amps each). They were four years old. (nine now - Feb 1012)
There is no
engine start load as my outboard motor auxiliary is manual start. When it is
running, according to cruising rpm, it presents a couple of 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.