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thstone · 08-24-2016, 06:08 PM

Parasitic power draw.

I typically don't drive the Boxster very often in between racing events so it can sit for up to 60 days without being driven. I've never used a battery tender and so I have always worried a little that the battery might run down while sitting for that long.

So, this evening I decided to figure out how long the battery should maintain enough charge to start the car.

I started by measuring the parasitic draw on the battery by placing a multimeter in series with the battery negative terminal (disconnect neg battery cable and place the positive probe on the neg battery terminal and the negative probe on the neg battery cable). This will measure the amount of current being drawn by the car. Car should be off with key removed and doors closed and so on - just like it would be if it were parked and locked.

I measured around 2mA and converted this to 0.002 Amps (x1000)

Then I looked up the "C10" capacity of my battery online which was 27Ah (amp hours). C10 means the "10-hour rating". Then I multiplied the 27Ah capacity by 30% to get 8.1Ah. This is how much battery capacity could be used by the car over time and still leave 70% of the battery capacity to ensure that the car will start.

Last, I divided 8.1 Ah of capacity by 0.002A to get 4,050 hours. This is how long it will take the car to draw down 30% of the battery capacity. Then I divided 4,050 hours by 24 hour/day to get 169 days.

Even if I am wrong by a factor of 100% (due to battery capacity variation with temperature, age of battery, poor battery cable resistance, etc), the battery should still start the car even after 84 days (169 divided by 2).

This is consistent with my experience that the car has always started in between events of up to 60 days.

Important Note: I measured 2mA for a Boxster that has been stripped of most electronic components. The only components in the car that might use power are the ECU and immobilizer. Thus, I would expect this value to be much higher for a stock Boxster.

And yes, this is what I do in my spare time after getting home from a 10-hour work day.

08-24-2016, 06:08 PM	#642
thstone Certified Boxster Addict Join Date: Nov 2010 Location: Los Angeles Posts: 7,669	Parasitic power draw. I typically don't drive the Boxster very often in between racing events so it can sit for up to 60 days without being driven. I've never used a battery tender and so I have always worried a little that the battery might run down while sitting for that long. So, this evening I decided to figure out how long the battery should maintain enough charge to start the car. I started by measuring the parasitic draw on the battery by placing a multimeter in series with the battery negative terminal (disconnect neg battery cable and place the positive probe on the neg battery terminal and the negative probe on the neg battery cable). This will measure the amount of current being drawn by the car. Car should be off with key removed and doors closed and so on - just like it would be if it were parked and locked. I measured around 2mA and converted this to 0.002 Amps (x1000) Then I looked up the "C10" capacity of my battery online which was 27Ah (amp hours). C10 means the "10-hour rating". Then I multiplied the 27Ah capacity by 30% to get 8.1Ah. This is how much battery capacity could be used by the car over time and still leave 70% of the battery capacity to ensure that the car will start. Last, I divided 8.1 Ah of capacity by 0.002A to get 4,050 hours. This is how long it will take the car to draw down 30% of the battery capacity. Then I divided 4,050 hours by 24 hour/day to get 169 days. Even if I am wrong by a factor of 100% (due to battery capacity variation with temperature, age of battery, poor battery cable resistance, etc), the battery should still start the car even after 84 days (169 divided by 2). This is consistent with my experience that the car has always started in between events of up to 60 days. Important Note: I measured 2mA for a Boxster that has been stripped of most electronic components. The only components in the car that might use power are the ECU and immobilizer. Thus, I would expect this value to be much higher for a stock Boxster. And yes, this is what I do in my spare time after getting home from a 10-hour work day. __________________ 1999 996 C2 - sold - bought back - sold for more 1997 Spec Boxster BSR #254 1979 911 SC POC Licensed DE/TT Instructor Last edited by thstone; 08-24-2016 at 06:16 PM.