Voltmeter location and voltage drop

[Ian Geary]

Afternoon,

after an unfortunate lack of electricity issue, I am fitting a voltmeter gauge to my bike. According to my multimeter, the battery reads 12.5v engine off, and 14.5v with the engine on (thaks to a newly repaired charging system).

I plan to attach my voltmeter to the sidelight +ve feed (switched with ignition) and earthed to the frame, which is convenient in terms of gauge location.

However, voltage being displayed in this location is something like 11.7v and 13.7v, with the engine off, and engine running respectively.


I realise I need to cross check the cheap ebay gauge against my multimeter first, but generally, would people "expect" a gauge to be reading nearly a whole volt lower due to the wiring location into the lights?

The cable run is negligable, but would other loads on the circuit affect the reading this much?


I'd rather see battery voltage, but this would need me to run a (fused) +ve wire up the length of my bike, which is a bit fiddly.

Thanks

Ian

[GreenV8S]

It's easy to lose a tenth of a volt in a cable run or connection, and these can add up to the sort of drop you're seeing. Since you have a meter, you could simply connect it to the various points in the circuit between the battery and your planned connection point and see where the drop occurs.

One way to look at this is as long as you know what's normal, the absolute reading doesn't particularly matter. An accurate reading would be better, of course.

[Ian Geary]

GreenV8S said:

It's easy to lose a tenth of a volt in a cable run or connection, and these can add up to the sort of drop you're seeing. Since you have a meter, you could simply connect it to the various points in the circuit between the battery and your planned connection point and see where the drop occurs.

One way to look at this is as long as you know what's normal, the absolute reading doesn't particularly matter. An accurate reading would be better, of course.

Fair point, but I did want the satisfaction of seeing a good 14.5v going into the battery.

I'll have a poke about tonight, and see if I can get at any more of the wiring loom (which is buried inside the frame)

Ian

[Mr2Mike]

Ian Geary said:

Fair point, but I did want the satisfaction of seeing a good 14.5v going into the battery.

You may be able to reduce the voltage drop a bit by cleaning connectors and switch contacts etc. but any wiring or connector with current flowing through it will suffer a voltage drop so the only accurate place to measure battery voltage is at the battery.

[Penelope Stopit]

Are the sidelights permanently on when the ignition is on or are they on when you switch them on with a side/head switch?

[Ian Geary]

Morning all,

Just thought i'd finish this post off with an update.

The sidelights are permanently on with the ignition. I checked the gauge against my multi meter, and it was (surprisingly) accurate.

So I did a limited amount of testing.

Power going into the junction (ie fusebox and relays sealed unit) (with ignition off) is the 12.4 the battery has. With ignition on, voltage into the junction box is now only 12.2v. And the feed out of the junction box only 11.9v.

The sidelight and gauge get 11.7v, so the majority of loss is in the junction box, with a bit in the loom.

Realistically this is a 14 yr old second bike, and the loom is way past it's best and doesn't warrant any further work.

But from what I read up in the mean time, likely culprits are corroded connectors (which poster above highlighted) and perhaps broken strands within the loom.

I don't need to measure the battery voltage as such, and would need a relay to turn the gauge off when the bike wasn't in use, which seems overly complicated.

The gauge is just there to alert me to lack of charge, as the bike doesn't have the red charging light that cars tend to have.

Thanks for your help / attention.

Ian