Battery voltage, switches and cut out

[Ian Williams]

Peter, it sounds like the issue is resistance through poor connection, poor earthing or undersize cable. Think of electricity like water, cables like hose, current (amps) like pressure, and resistance like a restriction in the hose. The bigger the hose (cable) the more easily the electricity can flow, place a restrictor in the hose and less water (electrickery) can flow, not so much of an issue if you only need a trickle but with a big demand the resistance will cause a problem. This is very simplified layman's explanation but you don't need to understand all the science, leave that to those who like to tie you up in technicalities.
Try turning all your lights on, and then with a supplementary cable run a direct earth from the battery to the head and tail lights. this will eliminate any doubt about the earth path, but do both head and tail. Then if still an issue run seperate supplementary feeds direct form the battery to both, head and tail lights, if this fixes it you obviously have a resistance somewhere in the supply side. Then try to narrow it down, try a supplementary feed to the light switch, try simply bypassing the switch, remove the battery isolator etc, etc eventually you should narrow it down. I will reiterate that sound connections, adequately sized cable's and clean switch contacts are vital, and often the cause of problems with 6v wiring.

[Bob Culver]

The cutout simply isolates the dynamo when its output is too low to be useful.
The battery and state of influences system running voltage rather than governing it.
On a modern the charge voltage is regulated/governed to a fixed say 14 volts and this is achieved under most running conditions, whereas the Seven can range from 8v to 6v or less in commonly achieved extremes. The owner is expected to use the Summer charge to avoid overcharging and hence the high volts.
Immediately after charging, a full battery may read 7v or more with no drain taken. The “true” voltage only shows after a significant drain
None of which is really relevant to your “problem”

I am a bit surprised at the measured voltage loss thru the master switch as this presumably handles hundreds of amps of starter current. .2 v at 10 amps would be 4 volts at 200 amps!
The drop across the light switch also seems considerable. If the voltage remains near 5.7 across the taillamp, you seem to have proved that earthing is not the problem. What drops do others measure directly across light switch?

[Roger J]

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.

Several issues at once to deal with with ! Firstly the battery. Battery voltage is a reasonable way to judge the state of charge and possibly the condition of a lead acid battery. A charging system will need to raise the voltage across it to about 7.1 volts to ensure it gets fully charged. When the charging voltage is removed the battery will still show 7.1V for a while if it is providing no current to a load. This is called the surface charge effect and will slowly decline to about 6.3 to 6.4V if indeed it is fully charged and in good condition, All measured voltages can vary slightly due to battery design characteristics but the range 6.3 >6.4 is typical.
So..6.2V is possibly low. But as Bob has already said only the best, calibrated digital volt meters can be relied on to be dependably accurate to 0.1 volts.

The volt drop across the battery cutout switch is worrying. What current was passing through when you measured that ? If it was with only ignition coil current with engine not running it is excessive. With such high contact resistance The starter motor might not turn. But it does !

You could try bypassing the headlight switch with a short length of wire to see is the headlights brighten up. If they do, the contacts there do need attention.

The cutout has nothing to do with the charging voltage/current. It simply disconnect the dynamo from the battery when the dynamo output is too low. This is to stop the battery driving the dynamo like an electric motor..The "third brush" is the charging control on Austin 7's

That's as brief as I can manage. Check out the internet for copious details of charging LA batteries..Hope it helps..Roger

[John Cornforth]

Hi Peter

It would seem to me that you have a combination of factors.  With 6 volts there isn't a lot to play with to begin with, so the cumulative effect of several small voltage drops can be significant.

I would be unhappy with any switch that dropped more than about 0.1 volts at its maximum current.  For a battery isolator that means 0.1 volts at about 200 Amps when cranking, so the drop when just running headlamps should be miniscule.  I don't know what type you have, the "Red Key" isolators were originally made by Hella, but there are also some look-alike ones which may not be as good inside.

For the Lucas PLC2 lighting switch, this was designed in the days when switches had to take the full lighting current, so it is of the "chunky brass" school of construction and has low resistance when working properly.  It's possible to dismantle and clean these, but only for the brave !

Your LED rear lights seem to be particularly fussy about voltage, whereas a filament lamp would carry on glowing even at 5 volts, albeit not so brightly.  I wonder if replacement LED designers aren't fully familiar with the vagaries of 6 volt electrics?   Maybe a different brand of LED would work better,

It's also worth checking the wiring, not much to go wrong with wires themselves but where they join at each end must be clean and tight.

[Peter Naulls]

Thank you all for your replies both on and off the forum, very much appreciated. I have a much better understanding now.

Ian, your analogy is a good one. I was hoping to find a resistance in one component in the system but the apparent numerous small drops have left me flummoxed. All measurements were taken with the headlights on. I will try to borrow a digital voltmeter this week and repeat the exercise.  I'll check terminals again but screws seem to be tight and all the wire ends have been soldered to stop them splaying - so I don't expect there are any issues there.

John, The isolator is indeed a red key type but I couldn't  tell you the make. It is easily replaced though. dismantling the plc switch fills me with dread though - memories of a previous car, but I can see it might be helpful.

Roger, your explanation of battery voltages is excellent and reassures me that the battery and charging system are not to blame ... and Bob that also backs up what I am finding as being normal. It doesn't seem as though I have any need to look further at that.

It does make me wonder why the LED units aren't designed to run on (say) 5v upwards rather than 6 but I guess there are technical reasons which I wouldn't understand!

Peter.

[Robert Leigh]

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.

Peter
I suggest that you run the engine at a speed to give you a good charge with lights off, and maimtaining that speed check the voltage at the battery and at the rearlamps. If the voltage at the rearlamps is appreciably lower you know that there is resistance somewhere in the switch or the wiring. If that is the case, turn off the engine, and with the lights still on check the voltage to a good earth at all connection points between the battery and the rearlights. Old wires can have broken strands and connections can have resistance due to dirt and corrosion. Incidentally the voltage during running will be slightly higher in the Winter than the Summer position, and the state of the battery has considerable influence on dynamo output voltage, with load of any sort reducing the voltage.

Robert Leigh

[Peter Naulls]

Thank you Robert. I'll spend some more time on it later this week and let you know what I find.

Peter.

[Charles Levien]

I have to say I think Bryan is right -- change your heads to LED. My Dynamo is set to charge around 2.5 amps on "high" and this is plenty to keep the the battery well charged. Interestingly, I do find less battery terminal corrosion using LEDs but you have to watch electrolyte level in your battery if running a lot of the time with lights on, as we did recently on a five week European tour.
Hope this is helpful

Charles

[Nick Salmon]

I may have dreamed this but somewhere I think I read that if LED bulbs are in the same circuit as filiament bulbs then the LEDs get rather upset and won't play nicely.

[Peter Naulls]

No you didn't dream it Nick, I seem to remember certain types had a diode or something missing. The voltage drop I have is nothing to do with that though as it is present even if I disconnect the LEDs.

Once I find a workable solution I'll post more news.

Peter.