Installing voltmeter

spartacus · 25 December 2019, 13:22

No it's not an April fool's joke. My daughters have given me a genuine Tohatsu Voltmeter for Christmas. Honestly, the instructions look like they've been written by someone with a degree in nuclear physics.

Give me a spanner any day. Electrics are not my thing. I understand I need a positive + feed, etc. Trouble is the Voltmeter has three terminals? One positive +, negative -, and another one. Ground? There's also a spade connector, for positive + for illiminated display.

The electrics on my boat include Blue Sea fuse box (integral bus bar), Blue Sea battery isolator. Any pointers for connecting? I'll be using tinned cable., heat shrink spade and ring connectors, etc.

I know there's a consensus of opinion regards digital multimeters versus old school analogue gauges for accuaracy, but hey I like it. I just want to read that the battery is holding charge.

I have a multimeter if I'm in doubt. The original marine battery failed a couple of years ago. I charge the new one before every trip with a trickle charger.

spartacus · 25 December 2019, 13:27

No remarks regards my electrics. Everything works just dandy plus you need to by Houdini to work in the console of a Ribcraft 4.8m.

jeffstevens763@g · 25 December 2019, 13:54

I put mine straight across the battery that way it reads voltage when everything switched off and when it's on with the engine running it shows charging volts, -ve and ground are linked by the buzz bar so the same the back light is on the same buzz bar -ve the spade +ve for the light. Of course the battery switch should switch everything off or the voltmeter will drain the battery. If that all makes sense after a few sherbets

jeffstevens763@g · 25 December 2019, 15:59

By across the battery I mean across the fuse box for connection fused which is on the off side of the isolator switch on the +ve side

Last Tango · 25 December 2019, 16:01

Two of them are just "grounds" (-v) You only need to connect one of them.
The meter +v is the other stud and the spade is just for the illumination. I'd connect that to the same line as your nav lights. I'm guessing the bridge for mounting this is plastic otherwise it would cause a short. I've got a pair of batteries so the main engine battery voltmeter is connected directly to the "ignition control" and the backup battery is monitored via a reed relay switched on by "ignition control" and fed from the backup battery to the second gauge.
The voltmeters are high impedance so draw very little current and don't need heavy wiring.

Stevem · 25 December 2019, 19:01

As above. The two negs are tied together and ground the illumination lamp via the brass metal plate so doesn't matter which one you use.

HughMattos · 27 December 2019, 17:19

Here at Cornwall Boat Services https://cornwallboatservices.co.uk , we work on many boat types and the usual problem with voltmeters - after corroded wires and terminals - is when the installer has used too light a rating of cable. Thus the voltage seen at the voltmeter reads low, due to cable resistance. If you have a significant voltage drop along the cables, then a 12.8 volt fully charged battery shows as a 12.4 volt nearly flat battery. Sea water ingress into the cable will also cause a high resistance as the copper breaks down.
It is also worth calibrating the gauge during installation by temporarily connecting the new voltmeter with short wires directly to a charged battery, and comparing the resuling reading with a good voltmeter.
Hope this helps
HM

GuyC · 27 December 2019, 17:50

I'm struggling to understand that.

A voltmeter e.g. a Bluesea 1733 has an operating current of 15mA i.e. 0.015A.

1.5mm cable has a resistance of 0.013 ohms per metre.

Voltage drop = Current x length of cable X resistance

Assuming a 5m run, which is probably excessive, the voltage drop would be around 1mV (0.001V) which is nothing.

I think the damage to cable and connectors is valid and maybe a bigger cable would allow more damage before it became an issue however I'm not sure if cable size alone is an issue. I am happy, however, to be corrected.

jeffstevens763@g · 27 December 2019, 18:07

Quote:

Originally Posted by GuyC

I'm struggling to understand that.

A voltmeter e.g. a Bluesea 1733 has an operating current of 15mA i.e. 0.015A.

1.5mm cable has a resistance of 0.013 ohms per metre.

Voltage drop = Current x length of cable X resistance

Assuming a 5m run, which is probably excessive, the voltage drop would be around 1mV (0.001V) which is nothing.

I think the damage to cable and connectors is valid and maybe a bigger cable would allow more damage before it became an issue however I'm not sure if cable size alone is an issue. I am happy, however, to be corrected.

Me too volt meter minimal current

spartacus · 27 December 2019, 20:50

Thanks for the replies. I hope I haven't started anything.

The technical boffins (Jedi masters) at Tohatsu suggest stranded insulated wire not lighter than 18AWG.

I don't even know what I'm typing any more!

Last Tango · 27 December 2019, 22:33

To calculate voltage drop in a copper wire, use the following formula:

Voltage drop = Length x Current x 0.017 Divided by area

Cable length 10 meters (max run both ways)
Current absolute max with cheap mechanical gauge 0.3 amps
18 swg cable (1.6mm)

10 x 0.3 X 0.017 divided by 1.6 = 0.032 volts.

All absolute worst case scenarios.

You'll not see that change in a automotive type analogue gauge.

Mechanical damage and corrosion is a separate issue but potentially effects all electrical wiring on a boat.

GuyC · 28 December 2019, 06:42

You've not started anything at all and most of the posts are in agreement with each other.

In summary I'd suggest connecting the -ve side of the gauge to the -ve on the battery or as close to as possible. Connect the +ve side to the switched side of the battery isolator.

Connect the blade by the light to a +ve feed from one of the other instrument lights. If this is a PITA to do, you can either not connect up the light or connect it to the +ve side of the gauge. This will mean the light is on whilst the gauge is on.

You can use 1.5mm cable which is at least equivalent to 18AWG (American Wire Gauge). Use tinned wire if you can - it's much more corrosion resistant.

Connect it up using heat shrink connectors. Again they are better at resisting water ingress.

I'd suggest putting a 2A fuse in the +ve line to the gauge.

I've included a few links below although I know there are other suppliers e.g. Kojaycat.

Almost finally - I'm not an electrician.

Finally - whilst I think the voltage drop is a bit of a red herring in this discussion, it is a worthwhile calculation to do if you are fitting anything that does draw appreciable amounts of current.

https://www.12voltplanet.co.uk/singl...1-5mm-21a.html

https://www.12voltplanet.co.uk/stand...ith-cover.html

https://www.12voltplanet.co.uk/stand...ade-fuses.html

https://www.12voltplanet.co.uk/adhes...onnectors.html

spartacus · 08 February 2020, 15:52

Finally getting around to installing the Voltmeter. A circular cutting disc and grit flap attachment for the drill makes this an easy job. Forgot I didn't have Sikaflex 291 to seal it (it fits like a glove), so have called it a day until that arrives.

GuyC · 08 February 2020, 17:06

FWIW on my console instead of Sikaflexing the instruments in, I've either made a neoprene foam gasket out of neoprene sheet or where there isn't much room for a gasket I've made O-rings out of neoprene foam cord and superglued the ends together.

When it leaks I'll Sikaflex them in!

mikehhogg · 08 February 2020, 17:26

I am with GUY C on this one , short wire runs and fuse.

spartacus · 16 February 2020, 15:44

Job done. Installing wiring in the Ribcraft 4.8 console is a job for inside given its blowing a gale. Absolutely no room to move, let alone swing a cat. Now lights up like a NASA shuttle.

A1an · 16 February 2020, 19:39

Tidy!