2003 Forester--Battery Discharge with Medical Device

I'm trying to determine how deeply I draw power from my battery when a medical device is attached to it, 12v dc. I don't know the amp-hour draw. I talked to the medical company and all they can tell me is on a 'normal' battery it should be OK for 10 hours. I've been running the device as a test for nearly 5 hours in my driveway. Is their some instrument that will tell me how far the battery is down? The Subaru says normal batter voltage is

12.5, and leaving the lights on for 1.5 hours will draw the battery down so that the car can't be started. Any way to calibrate the battery voltage with how far down the battery is. I want to avoid having a dead battery.

I'll be camping a lot, and will be using the device every night for 14 days. Comments?

Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA) (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time) Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

Reply to
W. Watson
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Buy yourself an inexpensive digital voltmeter and use the green line on this chart:

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As you can see, once the voltage drops much below 12.0 volts, you're in trouble. Common engineering practice is to design deep-discharge systems so that 12.0 volts equals 10% remaining capacity. Taking a car battery down this low more than a couple times will destroy it very quickly as it is not designed for deep-discharge service.

I think you need to seriously consider other alternatives if both car starting and the medical device are mission-critical for the trip.

It is possible to connect a large deep-discharge type battery in parallel with the car battery, but I'm not qualified to discuss how to do it.

Reply to
Jim Stewart

Good. Thanks for the chart.

Did I do myself in? After 7 hours the device started beeping. I turned off the electricity and couldn't start the car. I measured the voltage, and it was about 11.6. Then I jumped the cables. The fully charged battery was reading 13.65, which seems right. When I got the Subaru started, the lights came on. However, I know that I had turned them off at 11:00 am when I began this effort. My guess is that when I found the battery discharged. I hit the light control. Well, I'll go with 7 hours.

Fortunately, my life is not dependent upon the medical device. It's helpful but not critical, a CPAP machine that forces air through my air passages to prevent apnea. Tomorrow I'll be driving on my trip. In the evening, I'll run the device and keep an eye on the voltmeter. I'm taking the chart! If the battery causes more problems, I'll just cease using the device until I get back. I will be in motels a few nights, and I'll see if I can find camp sites that have electricity.

I'll consider the parallel battery approach with my dealer.

Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA) (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time) Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

Reply to
W. Watson

On additional note. I left the house for the store at 7 hours, and the device was pumping. 30 minutes later when I returned it was emitting and audible beep.

Reply to
W. Watson

How about this?

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HTH,

Greg in IN

Reply to
Greg in IN

Wayne, I've been camping for years with my Cpap, but limit trips to campgrounds that have a power hookup. The few times I did camp without power I hooked up the RV battery and inverter from my Pop-up and used that to power my machine. I got the battery recharged at the camp store each day. It ran the Sullivan 'V' I was using for about 5.5 hours on a charge.

J>

Reply to
Zeppo

Would that be Gold Rush Subaru in Auburn? That's where we got my wife's WRX. I'm in Davis.

-jim

Reply to
Jim Stewart

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