2003 Forester--Battery Discharge with Medical Device

[W. Watson]

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
--
U.S. Deficit Forecast: A. U.S. Treasury--$760B (std. audit
method), White House--$318B (method??), Full Audit--$3.5T!!
(Corp. method) -- Source: U.S. Today (current forecasts)
Guess which one is most often given in the news? WH
Web Page: <home.earthlink.net/~mtnviews>

 

[Jim Stewart]

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?

Buy yourself an inexpensive digital voltmeter
and use the green line on this chart:

http://xtronics.com/reference/batterap.htm

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.

 

[W. Watson]

Buy yourself an inexpensive digital voltmeter
and use the green line on this chart:

http://xtronics.com/reference/batterap.htm

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.

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
--
U.S. Deficit Forecast: A. U.S. Treasury--$760B (std. audit
method), White House--$318B (method??), Full Audit--$3.5T!!
(Corp. method) -- Source: U.S. Today (current forecasts)
Guess which one is most often given in the news? WH
Web Page: <home.earthlink.net/~mtnviews>

 

[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.

 

[Greg in IN]

How about this?

http://www.igloo-store.com/product_detail.asp?T1=IGL+8040&HDR=thermoacc

http://www.amazon.com/Igloo-8040-Battery-Guard/dp/B0006H4X46

http://www.skingcompany.com/detail.asp?product_ID=8205

HTH,

Greg in IN

 

[Zeppo]

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.

Jon

 

[Jim Stewart]

W. Watson wrote:

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

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

-jim