Background to LEDs
LED (light emitting diode) bulbs for cars have been around for a while now, but only comparatively recently have they found their way into older vehicles. The reasons are obvious – far brighter lights for far less current consumption make them ideal for vehicles with charging systems and lamps designed for a by-gone age.
The actual wattage (current consumption) of LEDs is far lower than tungsten bulbs. LED car bulbs for older cars don’t seem to be wattage-rated; rather, they are sold by application. However, consider that you would buy a 5-watt LED bulb to replace a 40-watt tungsten bulb for your home and you get the idea.
However, the law has not caught up with this and, technically at least, owners could fall foul of it. This is because the actual letter of the law states minimum wattages for each type of light in order that they should be adequately bright. This was clearly a good idea in the time of tungsten bulbs, but today the wording prevents the legal fitting of LEDs in certain lamps whilst allowing them in others. Douglas touched on this in an earlier issue of this Magazine.
In the case of the Austin Seven this translates to LEDs being illegal in headlamps and indicators but quite OK in side, stop, tail and number plate lamps. Now, who said the law is an ass?!
If the law were changed so that LED bulbs are rated in light output (lumens) rather than power requirement (watts), the problem would largely be solved.
There is such a significant improvement in light output when LEDs are installed, clearly a huge safety feature when compared with the originals, that is hard to imagine anyone being prosecuted unless they are dazzling other road users. (This, of course, is my own view and not to be taken as authoritative). The bulbs must be fitted and aimed to avoid dazzle.
A Nightmare Journey
I fitted LEDs to Tattybogle after a dark, wet drive home from Portsmouth after the DA7C Spanish holiday a couple of years ago. By the time we got home, the headlamps were worse than candles and the wiper wouldn’t wipe any more. The reason, of course, is that all the lights plus wiper consume 92 watts for which 15 amps are required from the 6 volt battery (W = V x A, dredged up from the memory of my physics lessons). However, the 6-volt dynamo can only produce 8 amps, so it is not surprising that my battery resembled a pancake on arrival home.
LEDS dramatically alter this equation. A full suite of LEDs will only consume 3 amps, so the dynamo can easily cope and a healthy charge still show on the ammeter. A win-win situation? Well, no, unfortunately not.
This is because there is no voltage regulation balancing demand and output from the 3rd brush dynamo. Once set to the correct 8 amps output in the winter setting, that is what it will deliver all day long. So if you are only consuming 3 amps, the battery will continue to be charged at 5 amps whether it is needed or not. Under these circumstances, once the battery is fully-charged, the electrolyte will bubble and quickly evaporate, emitting hydrogen gas (most unpleasant if the battery is under the passenger seat, as in many A7s).

I thought that the answer would simply be to adjust the 3rd brush  to reduce the dynamo output.  However, I experimented with this for a long time but found that the correct balance could not be achieved. With the 3rd brush rotated back so that the demand from the lights was balanced, there was still too much charge with the lights off in winter setting and a large discharge in summer setting.

The Solution
After much discussion with other owners, particularly one very helpful fellow in Cornwall, my solution was to change the half-charge summer/winter resistor for one of a higher value. After trial and error I found that a 2-ohm cement, wire-wound one would fit unnoticeably in the little box atop the dynamo. The ammeter now shows barely an amp in summer charge, and 4 amps in the winter setting. Thus, under normal driving, the status quo of the battery is maintained. When the headlamps are on (and the winter setting automatically kicks in) the charge is 1 amp (i.e. 4 amps less the 3 required by the lights).
Now, that is a win-win situation! Terrific lights, greatly improved safety and a full battery.
Nuff said!
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Roger Bateman DA7C