Sunday, Nov 22, 2009 at 07:50
Bushy,
The current drain ( 0.6 amps) of the cpap 200 seems pretty high, but it does include a humidifier which probably accounts for a fair bit of that power. At 240 volts that's about 150 watts. If this power is drawn continuously for say 8 hours, that's about 1200 watthours required overnight. To supply this much power using an inverter (probably about 80% efficient) from a 12 volt battery will require from the battery a current of about 15 amps. That 1200 watthours is equivalent to about 120 amphours at 12V.
Your Xpower pack is rated at only 1/6 of that, but it gets worse - although the power pack is rated at 20 Ah, that will assume using all the energy in a perfect fully charged battery. No battery is perfect, they are rarely 100% charged, and have a very short life if repeatedly 100% discharged.
IF (and this is the critical if) the 0.6 amps at 240 volts is correct and IF it is a continuous drain, then you will require much more battery capacity than the Xpower unit. If all the above is valid, you will need about 120 Ah each night. To get reasonable life from the battery it shouldn't be discharged beyond about 2/3 of its capacity, and should be recharged fully each day.
Consequently, you will need about 200 Ah of battery capacity. That's about the size of two of the batteries you'll find under the bonnet of a big 4WD. The other problem is charging them. Putting 150 Ah in every day (120Ah plus margin for inefficiency) will call for a fair amount of driving, probably a special charger (eg Arrid or better) and/or a lot of solar capacity. (150 Ah from say 6 hours of good sunshine will require 25 amps - that's a lot of expensive solar capacity, probably about 400W of panels, and of course the sun doesn't always shine.
It's not good news. As already suggested, you might find an alternative machine, one that is intended to run from 12V a more viable option. Any machine though is going to be pretty heavy on power requirements.
A final point to dampen any remaining enthusiasm! - The machine may not like the modified sine wave from the inverter. I would expect it to be a bit noisey and this will indicate inefficiency resulting in heating.
Not good news but I hope that helps. You might find our blog
Electricity for Camping interesting as it discusses power requirements and solutions, though not on your scale.
John | J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein
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