17-09-2017, 09:27 AM
(16-09-2017, 01:11 PM)Peter Naulls Wrote: 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