Charge Battery from Car

I want the ability to run my Respironics Remstar Auto M Series CPAP machine from a battery backup (less humidifier). After reviewing some of the fine articles on the homemade solutions, I decided that the light weight approach of the Respironics battery pack made the most sense for me. However, it appears the battery recharger operates from line voltage (120 VAC) and since the rated current draw of the CPAP (3 A by spec, 500 mA conservative test data found on this community)

Current Pressure
mA cm
~320 4.0
~360 7.0
~390 10.0
~420 13.0 (estimated)

compared to the amp-hour rating of the battery (7.8 A-Hr), I figure I can get nearly two nights usage per charge. Since I may need more than two nights and can't be guaranteed to be near a line power, I was wondering whether I could recharge the CPAP battery from the car battery via a cigarette lighter connection using a custom (home made) wire harness? Would I need something like a resistor (10 ohms as a guess) to limit surge currents into the battery. Perhaps a diode to ensure the CPAP battery doesn't try to charge the car battery?

I'm no electrician, but if your cpap battery is run down the current will flow to the weaker battery ... Best to charge with the car running too probably ... Since I'm not familiar with the cpap betters you have ... If it is a 12v wet cell type lead acid battery, just hook up wires + to + and - to - ...

These are things I wonder about:
1. Will a standard alternator be capable of charging a deep cycle battery and your regular car battery at the same time without putting undue stress on the alternator and/or the car battery?
2. Assuming you have proper control circuitry to prevent car battery or deep-cycle battery drainage/damage, how long would you have to leave the car running?

Here's some other stuff I found:
http://www.cloudynights.com/ubbthreads/ ... er/4704161
http://forum.woodenboat.com/showthread. ... -batteries

For that much money, time and effort, I wonder if it wouldn't be better to just bite the bullet, buy the deep cycle marine battery with 115 amp hours, a marine battery charger and be happy that you can go almost a week without recharging.

FlightEngineer wrote: figure I can get nearly two nights usage per charge

If that battery is a sealed lead acid type (which IIRC, it is) then you shouldn't discharge it below ~50% of rated capacity...... so count on 1 night before recharging.

Cheers,

Bill

Using a car to recharge a extra (car) battery isn't useful ot economic unless you are going to be driving all day, it also can overload the alt.

The safest way to recharge would be to wire your car with a 110v a/c converter large enough to power a 12v d/c battery charger.

The problem using a car to recharge a low battery is many times the alt control voltage is too low for the battery to charge quickly, by using the 110 volt charger you can better control the rate. Jim

Be sure it's a lead acid battery, not some type of lithium battery. A lithium battery may be damage, burn, or explode if charged from anything other than the correct battery charger recommended by the manufacturer. This is not the usual manufacturer's warning, "use only our stuff, or we won't be responsible." Lithium batteries actually do get damaged or burn with the wrong charger.

DreamDiver wrote:These are things I wonder about:
1. Will a standard alternator be capable of charging a deep cycle battery and your regular car battery at the same time without putting undue stress on the alternator and/or the car battery?
2. Assuming you have proper control circuitry to prevent car battery or deep-cycle battery drainage/damage, how long would you have to leave the car running?

Here's some other stuff I found:
http://www.cloudynights.com/ubbthreads/ ... er/4704161
http://forum.woodenboat.com/showthread. ... -batteries

For that much money, time and effort, I wonder if it wouldn't be better to just bite the bullet, buy the deep cycle marine battery with 115 amp hours, a marine battery charger and be happy that you can go almost a week without recharging.

Your car alternator will suffer no strain from being hooked to two batteries as long as the one in your car is already charged. It probably won't even harm itself if both are dead.

Sometimes if you hook up two batteries in parallel, only one gets fully charged.

If you're not going to be moving it around, and if you can put it somewhere that you won't knock the battery over, the deep cycle marine battery is a good choice. Check my signature line for some details on how to set one up.