Battery drain question

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Battery drain question

Postby Brooky » Sat Aug 18, 2012 8:58 pm

OK. This has been puzzling me for a while now, although not specifically in relation to VWs. Does anybody know how many ampere hours are drawn by an engine running a 12 volt ignition system, when running? Absolutely no other drains, just the ignition system, without any charge being added by a dynamo or alternator. If it depends on the number of cylinders, then it's just 2.

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Postby Globbits » Fri Aug 24, 2012 12:42 pm

I can't give you an exact figure, but you're looking at power being consumed by the ignition coil and the fuel cutoff valve on the carburettor

According to Mr Ratwell, coil specifications are:
Bosch Black coil (factory original Germany)
primary: 3.0 Ohm
secondary: 9.83 kOhm

Bosch Blue coil (Brazil)
primary: 3.4 Ohm
secondary: 7.79 kOhm

Ignoring the fact that the coil is shut off briefly to produce the spark, then you're looking at 12v/3ohms = 4amps being drawn by the coil
(If you want a more accurate figure, you will need to take into account the dwell angle, which will give you a percentage of the time that the points are open and the coil therefore not drawing current)
A little bit of Googling says that a typical fuel cutoff solenoid draws about 1amp

So you're looking at around 5amps, or around 60watts (12v*5) power being consumed
If you have a dynamo/alternator on there, the voltage will be 14.4v instead of 12v, so you'll be looking at approximately 6amps or 80watts being chomped
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