+ feed for starter battery voltmeter

[LeeME3]

Hi I'm in the process of installing a voltmeter to monitor my main (ie vehicle starter) battery. Ideally I'd like it to show the battery voltage when the alternator is not supplying a charging current and the alternator output going to the battery when the alternator is running. I could just wire it across the battery terminals and this would give me just that info (by my logic anyway - feel free to correct me if wrong!) but I'd rather pick up a + feed in the dash if poss to cut down on wire runs and route it via a fused circuit for protection. Obviously I don't want the voltmeter itself to be a load on the battery so ideally it needs to be on a feed that is only live once the ignition is on although if for any reason that is tricky I have no issue with using one of my 'Push to Test' momentary switches recovered from an RAF Nimrod!

Any advice on which wires are easiest and best to pick up much appreciated just to save me poking around and risking disturbing something. Course I could just get out there with a multimeter and go through the fusebox but as ever keen to pick the brains of those who know more than me!

[nicq]

Stick with your original idea direct across the bat. Take a fused feed off the bat and a gnd from bat neg. Any other way you get volt drop down the cable as different thing are switched on and you won't know if it is the alternator output changing or its the volt drop down the cable.

[LeeME3]

Ok works for me! Simple is best!

[CovKid]

And remember, you only need a small positive wire from the battery, just ground the black wire for meter to nearest available spot under the dash - it cuts down on wires and as battery is grounded to body anyway, you shouldn't lose anything. You'll get a reading of around 14v when alternator kicks in and the residual battery voltage when the engine stops. Oh, and use a good multimeter to get an accurate reading and adjust your volt meter to get the same reading.

[ghost123uk]

use a good multimeter to get an accurate reading and adjust your volt meter to get the same reading.

Too true. I have 2 cheap Ebay panel type voltmeters and they were both way out of correct calibration. One of them still drifts well out very quickly after I calibrate it. Not good when 1/2 a volt out can be important.

[LeeME3]

Thanks all...only thing concerning me is that coming direct off the battery (via suitable fuse) I will be putting a constant load on the battery even when ignition is off. Think I'll put a switch in the circuit as well...just seems a bit messy...

[ghost123uk]

Most of these small voltmeters use less than 100mA so if you are using the van more than once or twice a week it won't affect he battery. Some car stereo's memory feed (yellow wire) use similar amounts of current. You can check the consumption with the multi meter you are going to use to calibrate the voltmeter.

[ghost123uk]

This one HERE on Ebay says it is less than 60mA consumption.

[LeeME3]

Gone for this one in the end: http://www.ebay.co.uk/itm/DC-0V-100V-10 ... 834wt_1436" onclick="window.open(this.href);return false;

Figured i may as well have an AMPERE reading as well.

The van is generally used every few days so a 'trickle' load on it shouldn't be an issue as ghost123 says. That said when you add in the relatively high draw the stereo places on it even when 'off' I may add a master battery cut-off in due course, always useful for security and longer term lay-ups (and switch the stereo across to lesiure of course!).

ETA: Of course the intelligent thing to do is add a 3 way (ON - OFF - ON) double pole switch into the set-up and then I can flick between main and leisure battery and have an OFF option.

[CovKid]

The switch is mentioned in original WIKI article. The only thing I would add, is don't get too anal about what the battery is doing. Its only of any real use when things actually go wrong or just to get some idea of whats drawing current. As always, looking out of the glass at the front is the number one priority. Too much instrumentation can be a real distraction.

[nicq]

If my interpretation of the diagram is correct current is read on the neg side which could be a problem as you don't want to read starter current. Please correct me if I am wrong ghost.

[LeeME3]

As always, looking out of the glass at the front is the number one priority. Too much instrumentation can be a real distraction.

Indeed! When driving it will routinely be in the OFF position - much more use when stationary / just after starting to check condition of both batteries.