Battery voltage, switches and cut out

[Peter Naulls]

I have a Ruby with a standard 6V charging system. I'm finding there is a voltage drop when the headlights are on which is sufficient to dim the rear lights (especially the LED ones which almost go out). Its not an earthing problem.

The voltage at the battery is 6.2v (measured with an analogue multimeter). There is a drop of about 0.2v across my new battery master switch and another drop of around 0.3v across the headlight /sidelight switch. Perhaps another small drop elsewhere giving an insufficient 5.6v at the rear. With the headlights off I get the full 6.2v back again. The voltage measured at the battery itself doesn't ever change.

Several questions. Firstly is 6.2v normal for a fully charged battery? - I always understood that a healthy battery should be nearer 7 volts. It's only a year old and turns the engine over for ever and a day without getting tired. Second question, is a 0.2 or 0.3v drop normal for a switch? Thirdly, if 6.2v is too low, can I adjust the cut out in anyway?

I'm coming from a position of electrical ignorance so simply-worded replies would be appreciated!

Peter Naulls.

[Charles Levien]

Paul,

Are you using LEDs in your headlights?

Charles

---

Mean Peter!

[Derek Sheldon]

My 38 Opal has L.E.D all around and at 30mph with all lights on I have set the dynamo to charge at 2.5 volts. All of my light units are directly earthed by wire to the negative terminal. I still suspect a earthing problem on your car

[Zetomagneto]

My 38 Opal has L.E.D all around and at 30mph with all lights on I have set the dynamo to charge at 2.5 volts. All of my light units are directly earthed by wire to the negative terminal. I still suspect a earthing problem on your car

Derek, do you mean 2.5 amps?

[Derek Sheldon]

My 38 Opal has L.E.D all around and at 30mph with all lights on I have set the dynamo to charge at 2.5 volts. All of my light units are directly earthed by wire to the negative terminal. I still suspect a earthing problem on your car

Derek, do you mean 2.5 amps?

As a matter of fact yes I do mean amps

[Peter Naulls]

Hi Charles, and Derek,

Thanks for your thoughts, but no, it has halogen bulbs in the headlights. The charging system works fine.

I know it's not an earthing problem because if I connect a lead from the earth terminal of the battery to the earth wire at the tail lamp it doesn't make any difference. If however I connect a lead from the supply side of the battery to the supply wire of the lamp it cures the problem.

Peter.

[Colin Reed]

Hi Peter,
a fully charged battery should be between 6.37 and 6.25  the latter figure is an 80% charge
and a fully discharged reading would be 5.82
as you will see there is  just over half a volt difference and the reason the use of a digital for this test is recommended

hope this is of help
Colin

[Bob Culver]

The battery voltage does seem somewhat modest, esp if immediately after chrging, but analogue meters are notoriously inaccurate. Meter tolerance is percentage of full scale and even for new very expensive AVO was quite considerable especially for part scale readings. Can be checked against a good digital.
Voltage drops across switches, lengths of wiring, across earth leads direct to battery etc, etc are often instructive. Some drop unavoidable at the higher currents. Do the switches improve with repeat operation to improve contact? Switches intended for only 8 amps originally.

On charge voltage is not regulated on a Seven. Will be quite high on Winter with a full charged battery. With lamps on the charge voltage is modest, and reduces as the battery flattens!

The cutout simply connects and disconnects the battery to the dynamo.

With your investiagtion so far a read up of Ohms Law will likely prove interesting!

There was a recent post about problems with LED tail/stop lights which may be applicable to your case. Unfortunately the solution, as with most modern elcctronics, involves an understanding far beyong Ohms Law!.

[Peter Naulls]

Thank you Colin, that's very helpful and suggests the battery and charging voltage is ok. I did for comparison try charging the battery with a mains 6v charger and it charged it to 7v - but this is probably academic. Once charged to 7v the lights are considerably brighter but still dim when the headlights are turned on

Bob, I'm not clear from what you are saying if the charging voltage is governed by the battery or the cut-out? I would have expected it to be the same on low and high charge or am I missing something?

Peter.

[Bryan]

Hi Peter,
I recommend you update the headlights to LED as well. You will find most LED tail light bulbs are made to work on 6&12v supplies, so when you have the halogen headlights on the voltage drops down to the low end of the lLEDs working range.