Current draw and alternator loading

[Mr Pid]

OK..... Done a couple of longish trips in the GTR over the last week. 200 miles to brooklands wings and wheels, the requisite high speed runs on the runway, and 200 miles back again.

There was a bunch of traffic including stop start for 40 minutes or so. Despite fairly warm weather (for the UK) the car could sit with the fans on indefinitely without a problem. However I did notice that with the a/c on the idle dropped and so did the battery volts. I was expecting this due to the direct load from the a/c pump, but the voltage drop was more than I expected.

So got to wondering which element was driving the big drain on the battery.

So just did a few tests in the garage. With no engine running I looked at the voltage drop caused by each of the elements.... Assuming voltage drop is proportional to current this seems a fair indication of load.

I saw...

Radiator fans (2 high power pacet profane) 0.4v drop

A/c fan (under rear clip) 0.2v drop

Heater blower setting 1 0.8v drop
Heater blower setting 2 1.1v drop (0.3v more than setting 1)
Heater blower setting 3 1.5v drop (0.4v more than setting 2)

A/c compressor clutch (just the actuator) 0.4v drop

So I am surprised by a couple of things.....

The heater blower on setting 1 draws twice the current of two rad fans.... And on setting 3 nearly 4 times the current !!!!!

Also just the clutch on the a/c compressor draws the same as the two rad fans !!

Interested if anyone else is surprised or can challenge my conclusions or have any measurements themselves..... I had always assumed the rad fans would be the biggest draw.

Stu

[Mr Pid]

No ECU this is old school blown SBC with carb.

Thinking the voltmeter measurement on the stack may be different to the drop at the battery due to losses in the loom. Will recheck the numbers with a voltmeter on the battery and report back.

[Mr Pid]

Mr Pid said:

OK..... Done a couple of longish trips in the GTR over the last week. 200 miles to brooklands wings and wheels, the requisite high speed runs on the runway, and 200 miles back again.

There was a bunch of traffic including stop start for 40 minutes or so. Despite fairly warm weather (for the UK) the car could sit with the fans on indefinitely without a problem. However I did notice that with the a/c on the idle dropped and so did the battery volts. I was expecting this due to the direct load from the a/c pump, but the voltage drop was more than I expected.

So got to wondering which element was driving the big drain on the battery.

So just did a few tests in the garage. With no engine running I looked at the voltage drop caused by each of the elements.... Assuming voltage drop is proportional to current this seems a fair indication of load.

I saw...

Radiator fans (2 high power pacet profane) 0.4v drop

A/c fan (under rear clip) 0.2v drop

Heater blower setting 1 0.8v drop
Heater blower setting 2 1.1v drop (0.3v more than setting 1)
Heater blower setting 3 1.5v drop (0.4v more than setting 2)

A/c compressor clutch (just the actuator) 0.4v drop

So I am surprised by a couple of things.....

The heater blower on setting 1 draws twice the current of two rad fans.... And on setting 3 nearly 4 times the current !!!!!

Also just the clutch on the a/c compressor draws the same as the two rad fans !!

Interested if anyone else is surprised or can challenge my conclusions or have any measurements themselves..... I had always assumed the rad fans would be the biggest draw.

Stu

Part 2:

Redone the measurements with a voltmeter on the battery (2 decimal places) rather than using the readings from the stack.

Rad fans 0.3v drop

A/c fan 0.15v drop

Heater blower setting 1 now 0.1 v drop
Heater blower setting 3 now 0.15 v drop

A/c clutch now 0.15 v drop.

This makes much more sense and goes to show that,depending on the cars wiring, you can't really trust the cockpit volt meter.

Stu