CPAP device on a 12v battery: system???

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CPAP device on a 12v battery: system???

Postby russ on Fri Jul 02, 2010 1:19 pm

I have sleep apnea and require use of a CPAP medical device. It can be used with 120 vAC or 12 vDC. I’m befuddled by the array of alleged choices re batteries, chargers, and other potential components….and the seemingly misinformation about all. I have the adapter cable (plug in to cigarette lighter) and cigarette lighter-alligator clips for attaching the CPAP device to a 12v battery. The CPAP device, per its specs, uses 3 amps max if not in heated humidifier mode. If used 6-8 hours/night, it would consume 18-24 amps, according to the manufacturer. The CPAP device is a Respironics Remstar Pro M series.

I want to be able to car camp (2003 Chrysler minivan), sometimes having access to AC power, most times not. I may be traveling a fair amount of distance (thus time) daily or I may be largely based in an area with shorter excursions (10-60 minutes). The duration of the trips may be 1-2 nights (locally) or may be for several weeks at a time. If I have access to AC, I’ll use it. If not, I’ll have to use the 12 vDC. At 18-24 amps/night I’ll soon need to recharge the battery…..but how often, for how long, and by what method(s)?

I’m reasonably certain that an AGM battery is best for my needs (passenger compartment), but I’m having difficulty really deciphering which brands are truly the most “deep” cycle. The Sears Diehard Platinum batteries are attractive because of warranty, price, and “they’re everywhere”….but are they really a deep cycle battery? I understand they’re similar or the same as Odyssey which is a rebranded Enersys battery. It seems that there is a tradeoff in larger (more amp hours) vs battery weight (for shuttling back and forth to the van), ranging from 68-100 amp hours and 50-80 lbs.

I understand that an AGM deep cycle battery shouldn’t be discharged to less than 50% routinely. At 18-24 amps/night for the CPAP device, a 68 Ah battery (32 Ah available to 50%) would last me less than 2 full nights. A 100 Ah battery (50 Ah available) would last me 2+ nights. Note that there are reports of users regularly getting 3-4 nights before needing recharge, suggesting that either the CPAP devices in fact draw much less current (ie 1-1.5 amps), or other users have much more efficient machines, or users are running their batteries to far below the prudent 50% levels. I suspect that its approximately 1.5 amps/hr., but everything hinges off that assumption re calculating the rest of the “system”.

Reportedly the CPAP device will auto shutoff if the battery is below 12.4 vDC. I assume that voltage is well short of the maximum 80% discharge level of true deep cycle batteries. If its also well above the 50% discharge level I’m using to calculate maximum hours (and therefore days) of usage between necessary recharge, that could be a determining factor.

Recharging the deep cycle battery: my plan is to carry a recharging unit (yet to be determined which). Plan A is to overnight at a place (campground) where AC is available often enough to allow recharging of the battery used previously elsewhere. Plan B is to provide necessary recharging via the vehicles alternator, presumably thru a 400w inverter plugged into a cigarette lighter. Plan C would be no self contained charging, necessitating me to drop off the battery occasionally within my travel area for recharging (garage, etc)…potentially constraining travel/activity schedule.

1. Everyone claims to have deep cycle batteries, yet many refer to their specs primarily in CCA and “reserve capacity”…..indicative of starting batteries. How do I really know which ones are really capable of performing , ie deep and recoverable discharge many times?
2. Uncertain re the actual current draw of my CPAP, I’m going to either cautiously over design the capacity (using 3 amp) or endanger running out (battery too low) by being aggressive (1.5 amp). How can I better determine the actual draw?
3. If the CPAP auto shuts off at 12.4 vDC, where is that relative to the battery’s capacity?
4. I considered the possibility of replacing my car starting battery with an AGM but read that AGMs are more sensitive to heat, specifically engine compartment heat. I’m also reluctant to risk drawing down the battery using the CPAP and stranding myself because the battery is too low.
5. Additionally I read that AGMs shouldn’t be recharged at rates higher than 14v, lower than what an alternator produces (15+ v?) unless its set point can be adjusted. True?
6. Another possibility is using a cigarette lighter adapter to an inverter (400w) to a 167-220w(?) battery charger to the AGM while I’m driving…..I think??? I assume I’d have to ensure that the inverter/battery charger draw did not exceed the difference in what the vehicle otherwise was drawing and what the alternator was producing, or else it would pull from the starting battery as well. I do NOT want to unduly risk premature death of my alternator. Is this charging “system” feasible and practical?
7. Re sizing the charger, I understand AGMs charge rate can be up to 75% of the amp hour capacity. Using a 70-100 Ah battery, that’s 50-75 amps/hr….well above the expected campground circuitry (20-30 amp?).
8. I understand that my alternator is sized at 160 amps. Is there a possibility of wiring a 2nd battery (for the CPAP) into the vehicle’s system and letting the vehicle do the charging? I’d prefer to have the 2nd battery readily removed and only added when needed on overnight trips. I’d imagine that it would need to be housed in the passenger compartment as there’s no(?) room in the engine compartment and I have concerns about excessive(?) heat.

Ideally I could function without the CPAP and wouldn’t need any of this…but that’s not an option. I’m trying to optimize my freedom of traveling minimally impeded by this device at the same time cognizant of using a system that avoids failure to the device or, more importantly, to the vehicle’s electrical system. My head is swimming. Any help will be much appreciated.
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