Volt meters

Post by **T'Onion** » 30 Apr 2007, 08:08

ok i have a couple of voltmeters that i want to wire up so i can see how my main and leisure batteries are doing , i know how to wire them up , but was thinking of either just hard wiring them in or putting them via a switch so i can turn them off

so the question is do they draw any current ? if so is it loads ..

i suppose i could just fit one via a three way switch ?

Post by **Mocki** » 30 Apr 2007, 08:13

if you mean the 50mm round car type, yes they use power up, more than a LED, less than a 3w bulb, so switching is good.
I have one in a 5pin relay,( automatic three way switch if you like?) so it shows the leisure when the engine isnt running, and the main switch to the leisure is on, and the main battery when the engine is running, ever so handy, you cn tell if the split charge is working, the fridge is on, ect from the readings...... dont bother with an amp meter tho, you wont be able to tell anything from it that is usefull to the job, only what is pulling what.

Post by **T'Onion** » 30 Apr 2007, 08:51

aye little round car type

Post by **geordie skydiver** » 30 Apr 2007, 09:28

That 3 way switch is a great idea, be a good 'how to' for the wiki too?

Post by **Westy.Club.Joker** » 30 Apr 2007, 15:19

Just bought a Zig volt gauge that does this, switch is 3 way type.
off , main batt, leisure batt. Cost about 20 quid. You are right about the gauge needing to be switched off when not required,or it will deffo flatten the batts.

Post by **irish.david** » 30 Apr 2007, 16:27

Any idea what the resistance of the coil is on the meters?

In order to measure voltage accurately voltmeters have to have an equivalent circuit as close to an open circuit as possible. To do this voltmeters have a really high resistance to stop them from interfering with the voltage you're trying to measure.

As an example, if the voltmeter you've got has a coil resistance of 10k ohm (very low for a voltmeter) it will only draw 1.2mA. This means that it would take about 6 years to flatten a 63amp/hr battery (i know that in practice it would be about half this as the figures i've used are ideal). I can't say for sure but the coil resistance is probably higher than 10k ohm which will mean it will draw even less current and affect the battery even less.

As far as the gauge goes, if the coil resistance is high then I think you can leave it connected permanently. If it's backlit then you'll need to wire that to the sidelight switch on your dash cause a bulb will flatten your battery much faster.

Dave

Post by **Dan Wood** » 30 Apr 2007, 16:33

Lidl, Netto, Poundland etc. occasionally have digital meters for, well, a pound!

These have their own batteries (watch type). If you butcher them and stick an ordinary AA battery in, the meter will run continuously for a year or two.
The good thing about these digital ones is

a) They are high impedance, so don't drain anything from the leisure/main battery you have them hooked up to.

b) They are quicker to read when drunk

Post by **HarryMann** » 30 Apr 2007, 16:58

By nature and design voltmeters have a near infinite resistance and ammeters near zero... otherwise they'd misread and ammeters get hot

Post by **Dan Wood** » 30 Apr 2007, 17:31

<pedantic>
An ammeter is actually a voltmeter. It's just that it gets connected across a rather small resistance!
</pedantic>

Post by **irish.david** » 30 Apr 2007, 17:42

<double pedantic>
These days high current ammeters use a Hall sensor (magnetism wizardry) and a bunch of electronics to measure currents so you don't need a rather small resistance capable of carrying 90A (expensive and large)
</double pedantic>

Post by **T'Onion** » 30 Apr 2007, 17:52

oh yes tiffen time

anyway i nipped down to maplins and got a three way rocker switch , for a couple of quid !!... so i can check either leisure or main and turn it off when i'm not using it

Post by **Rozzo** » 30 Apr 2007, 18:24

T'Onion wrote:oh yes tiffen time

anyway i nipped down to maplins and got a three way rocker switch , for a couple of quid !!... so i can check either leisure or main and turn it off when i'm not using it

in that case you'll only need one of the meters so i could blag the other one maybe

Post by **Westy.Club.Joker** » 30 Apr 2007, 19:50

Not sure on the gauges resistance etc Dave. I bought one of the those cheapo 32mm round gauges (about a tenner) for the leisure battery on my BOAT, fitted it into the battery-box wall so I could see it. looked nice, not backlit or anything flash! It was showing about 13V.

Left the boat alone for approx 2 months, came to check the batt ready for going fishing in Norfolk, and the battery was pancaked, flat as it gets. Luckily (or maybe not) it is a proper leisure deep-discharge batt, so I plugged in my modern current sensing batt charger, with selectable battery types, and it brought it back from the dead, and seems to be none the worse for it. I then fitted a switch to it so I can check it then turn it off again, and it`s been fine since.