At 17:23 +1100 7/11/97, Trevor Boicey wrote:
>Trevor Jordan wrote:
>> When the car is being started, the battery drops to 6V (or
>> thereabouts) under load
> Well, voltage at the starter can drop pretty far, but 6V
>is a bit of an extreme number.
> However, it is worth noting that the goal is not
>to "compensate" for starting issues to get the same spark
>as always. The goal is actually to get a better spark
>during starting to help the engine catch when the combustion
>chambers are cold and fuel isn't well atomized.
> A ballast system allows you to have big sparks at
>startup when you can use them, without running those
>big sparks (and high coil currents) at all times to give
>reasonably point life.
I am sure that both effects (compensation for battery voltage drop and
improved spark) occur, but I was trying to keep it simple rather than
precise. Additionally, I had never been sure which was the predominant
effect, so I made some measurements over the weekend.
On a TR6 (US Spec 7.5:1 compression ratio) and with a good battery (about
one year old and lightly used), I got the following results.
Battery terminal voltage
unloaded - 12.5
lights on (low beam) - 12.2
cranking engine (no lights) - 9.5
My conclusions are that with an average mid-life battery, the two effects
are about equal and that the coil will receive around 9V during starting.
This also means that the engine will still have an adequate spark when the
battery is near the end of its life.
Higher compression engines will probably place a greater load on the battery.
74 TR6 CF29281U