I want to fit a voltmeter in the cabin to keep an eye on the leisure battery state

[Rigger Mortice]

The plan was to fit two small toggle switches (I have those) with inline fuses so that I can check either battery. What sort of switch could do both? I assumed such a thing wouldn't exist.

Personally I would go for one of these. http://www.maplin.co.uk/p/sub-miniature-toggle-switch-g-fh06g

The latched switch for the house battery which you will want to routinely monitor.
The non locking for the occasional check of the starter battery.
And the "off" option so that you can switch the thing off at night when the light (assuming it is lit) may be irritating.

The advantage is that you can't accidently leave it on the starter battery and be fooled into thinking that your house battery is fine.

PS It's also worth getting one of these. http://www.maplin.co.uk/p/sub-miniature-toggle-switch-cover-jr79l

 

[Murv]

Personally I would go for one of these. http://www.maplin.co.uk/p/sub-miniature-toggle-switch-g-fh06g

The latched switch for the house battery which you will want to routinely monitor.
The non locking for the occasional check of the starter battery.
And the "off" option so that you can switch the thing off at night when the light (assuming it is lit) may be irritating.

The advantage is that you can't accidently leave it on the starter battery and be fooled into thinking that your house battery is fine.

That sounds a great idea, thanks for the link.

Just to confirm, as it's been far too long since I did any electronics, but these switches use a shared earth connection between the two batteries?
So, I just run 3 wires through to the batteries, one for each positive and a shared earth bridged between the two?

 

[Rigger Mortice]

Connect the +ve wire on the meter to the central terminal on the switch.
Connect the -ve wire on the meter to earth

Connect one battery +ve to one of the "outer" terminals on the switch.
Connect the other battery +ve to the other "outer" terminal.

If it is a double pole switch (6 contacts) ignore the terminals down one side. Just use 3 terminals down one side.

All assumes that the battery -v's are common, and the meter is a two wire one (some are four wire, some three)

Personally, I would connect up to the master switch and not direct to the batteries.

 

[Murv]

Connect the +ve wire on the meter to the central terminal on the switch.
Connect the -ve wire on the meter to earth

Connect one battery +ve to one of the "outer" terminals on the switch.
Connect the other battery +ve to the other "outer" terminal.

If it is a double pole switch (6 contacts) ignore the terminals down one side. Just use 3 terminals down one side.

All assumes that the battery -v's are common, and the meter is a two wire one (some are four wire, some three)

Personally, I would connect up to the master switch and not direct to the batteries.

Fantastic, thanks very much

 

[Amulet]

I've got one that taps into the supply a long way from the batteries - almost a complete waste of time. Under load, it gives a frighteningly low reading, which I just don't want to see when in reality, all is fine.

The only sensible way is to connect it to the battery. As has been said you tap into a functional circuit there will be voltage drop on that circuit depending on what's switched on that uses it. Even if you can and do calibrate it, its reading will vary depending on load. If you only read it when everything on that circuit is switched off you will get a correct reading - you are effectively using the existing wires instead of running dedicated wires. My recollection is that my NASA battery monitors even require that you even take their own power supply separately from the signal so that the tiny load they impose doesn't influence the reading.

There are advantages to being to read the battery voltage under load. It is useful to know how much the voltage drops.

 

[William_H]

Back to OP original question. Yes he can use say cabin lighting wires to supply the volts for the meter to measure. As said the volts will drop as cabin lights are turned on and the volt drop will be imediately apparent. he might be ok with that limitation depending on how hard it is to run new wires to the battery. If you run separate wires to the battery don't omit small in line fuses in the wires near the battery +ve terminal.
When buying these meters look out for the type of power supply needed. Some older style chips require a supply isolated from both the +ve and -ve of the supplu you are measuring. This obligates you to use a 9v battery and switch to turn the meter on or use an isolating power supply to generate he 9v. LCD types don't use mush current so battery is not really bad. Just make sure which you are buying or have bought. 4 wire is a give away that it needs a battery or isolator.
I think decimal place meter could be useful. Absolute accuracy is about 1% but small change in voltage can be quite accurate. You could easily see for instance if mast head light is working by small volt drop in battery when it ios turned on.
good luck olewill

 

[Murv]

Back to OP original question. Yes he can use say cabin lighting wires to supply the volts for the meter to measure. As said the volts will drop as cabin lights are turned on and the volt drop will be imediately apparent. he might be ok with that limitation depending on how hard it is to run new wires to the battery. If you run separate wires to the battery don't omit small in line fuses in the wires near the battery +ve terminal.
When buying these meters look out for the type of power supply needed. Some older style chips require a supply isolated from both the +ve and -ve of the supplu you are measuring. This obligates you to use a 9v battery and switch to turn the meter on or use an isolating power supply to generate he 9v. LCD types don't use mush current so battery is not really bad. Just make sure which you are buying or have bought. 4 wire is a give away that it needs a battery or isolator.
I think decimal place meter could be useful. Absolute accuracy is about 1% but small change in voltage can be quite accurate. You could easily see for instance if mast head light is working by small volt drop in battery when it ios turned on.
good luck olewill

Thanks for that, I have the inline fuses, just need to get the switch and get it all fitted (some LED striplighting switches are going on as well)
It's not that big an effort to run wires through to the battery, it's just that the new panel covers all the switch connections. busbars etc for 90% of the boats supply so it would have made sense to tap into something if it was going to work. So, I'll run them through to the batteries, and then it's as good a setup as I can achieve.
Good tip for checking on circuits working by measuring the V drop, it certainly reads out to at least one decimal point from what I can remember so even if the meter itself isn't hugely accurate, the relative readings should be OK.
I'll put a picture up when it's complete and in position, hopefully by the end of the week.