Electrickery

[Ladybird]

I would appreciate any guidance from those more knowing than I.......
I'm quite familiar with the electrics on old motorcycles but have no experience with the three brush dynamo on our car, a 1936 A7 Opal running on 6 volts.
When purchased the dynamo had no output but a good clean seems to have sorted it, however I'm getting some very strange ammeter readings as follows:
With engine NOT running, ignition only on (briefly) approx 5 amp discharge, with ign and sidelights approx. 8 amps discharge, with ign., side and headlights approx 20 amps discharge.
With engine running, dynamo on low setting, ign. only, slight discharge,  approx 1 amp charge when revved.
With engine running, dynamo on high setting, ign. only approx 8 amp charge, with ign. and sidelights 8 amp discharge, with ign. side and headlights 6 amp discharge. 
Side and headlights are all standard fittings.
These readings make no sense to me, and imply that any running with lights will just discharge the battery. Am I missing something? Any help gratefully received..

[Bob Culver]

Is there an error in your figs for charge/discharge on ign and sidelights?
The cars originally had headalmps of just 18w, 3amps each. The systsem will balance full load, iecorresponding  about 10 amp charge with lights off. Any greater output and there is severe risk of cooking the dynamo (many run with the cover off to assist cooling). I dunno if 18w or near bulbs are still available; many run 36w 6amp as yours apparently  are and it leads to feeble lights and a limited range at night , the latter often acceptable.
If you cannot source small bubs, LED bulbs are the modern solution and also a variety of dynamo mods, voltage conversions  and substitutes.
Many cars have run merrily for umpteen thousands of miles with the original system, but you may need to eat a lot of carrots.
(large bulbs drag the sytem voltage down and often give less light than the correct)
Do not attempt to increase the charge rate above 10amp net cahrge no load.

[andrew34ruby]

Also Chris, originally our cars had just one back light, now 2 or 3. And back in the day headlights were only used out of town. And also drivers would not sit in stationary traffic with a foot on the brake. Brakelights use more electricery.

[John Cornforth]

Hi Chris and Hilary

Some extra information for you.

The Ammeter is not a precision measuring device, but it will reliably tell you if there is a net charge or discharge assuming the needle is properly on zero when everything is off.

The PLC lighting and charging switch is arranged to automatically give full charge (by shorting out the half charge resistor) when sidelights or headlights are selected. You might like to check that it is wired correctly. See my Ruby wiring diagram on the Dorset A7 club site.

The ignition draws about 4 amps with the engine stopped and the points closed.  When running, this reduces to an average of about 2 amps as the points are only closed for 50% of the time.

As Bob says, the original bulb wattages were rather low (and only a single tail light).  Most will have been fitted with higher wattage over the years.

The handbrake actuates the footbrake, so the brake lights may be on when stationary.

Given a battery capacity of 80 Amp Hours, a discharge of say 4 amps when running with lights on is acceptable. It corresponds to 10 hours running to 50% capacity, which is quite long enough for the majority of people ! Just charge it up when you get home.

["Slack Alice" Simon]

It is odd that the discharge is less with headlights on, otherwise the figures are sort of in line with what I get.

You have to consider battery life when driving at night.   I have only once run out of juice, but even then the ignition still worked and I inched home, using a torch, on roads I knew well.

When the car was the main transport, I never used the starter, only the handle.

[Bob Culver]

As the system voltage dwindles so does the dynamo output so get into a decline. In an emergency with reasonable lamps can disconnect one headlamp. I an recall occassions when dipping the (two) solenoid dip lamps or sounding the horn would cause the ignition to cut! I knew little about battery maintenance at the time; ran the car for periods with no functioning dynamo. Life was about 2 years.....

[Ladybird]

Thanks everybody, your replies are most helpful. I'm going to do the additional checks you've suggested, porbably switch out the bulbs for lower wattage items and then stop worrying......   Thanks again, this forum is brilliant, hopefully one day I'll learn enough to be able to make a helpful comment rather than just bombarding you with questions.  Keep safe!!

[PaulS]

As John mentions, the current draw with ignition on and points closed should be around 4 amps.
Sorry if you already know this but you can turn the starting handle a little until the points are open, then the current draw will be very little and you can see what the lights are drawing without factoring in the current drawn by the ignition (which is the resistance of the low tension side of the coil).

[Bob Culver]

With a rudimentary understanding of circuits Johhn Cornforths diagram which includes the innards of the rotary switch should enable you to resolve. Various circuits can be disconnected . The dynamo shoud not be run with the output lead disconnected anywhere. Do not prod the cutout or it will lock. Then have to promptly pull it open or disconnect battery.
The info is a puzzle. Almost as if stops charging in the sidelight position. Perhaps the field resistor is not being shorted out in that positon. Removing the field fuse stops all summer charging. The output and field wires can be linked directly at the dynamo to lock the system onto winter charge (but maintain the external connections)