I am going camping a few times in the near future and need some sort of power supply for my machine. I am not going anywhere remote for days on end or anything, but more or less for a night at a time. I have a spare lawnmower/motorcycle style battery that is near new, and plenty of electrical connectors and wiring laying around the shop. I can just throw the battery on the charger when I get back in town, so recharging is not an issue. I see manufacturers are making adapter kits etc. but would rather build one to better suit my needs. I also plan on running my machine and humidifier just as I would at home.
I would like to take the battery, directly wire from the battery to the barrel plug for the machine. I will make ring connectors or something to establish a good connection at the battery. From there, I would like to put an in-line fuse just as a safeguard to protect my machine. My power supply says at 12v dc, the amp rating is 5.0. What amp fuse would you recommend using? The next question is, should I worry about a voltage regulator or anything as another safeguard, or a 12v battery will be just fine? I would imagine that 22ga wire should suffice with no more than a 15' run of wire.
First, it would help greatly if you mentioned the model (or at least the make) of your pump, and the pressure used, especially if it is high. Also, many people find the humidity is not needed when camping if the humidity is high. Often the dew point stays constant while the temp drops at night, so the relative humidity shoots up to 90%. This might not be true in the high desert, but I never bother to bring the humidifier when travelling. This is significant because the humidifier can use 80% of the power, or even more.
The battery you have may be sufficient - but its a tossup. If its an old style flooded starting battery, it won't handle repeated cycles and could easily fail in a dozen or so outings. Many of the newer small batteries are AGM, and they may be rated as "deep cycle" or "dual purpose" which can handle repeated deep drains. The next problem is the these batteries are usually in the 4 to 8 Amp-hour range, which is marginal for even one night without humidity. My pump, a Respironics 560 is very efficient and uses 4 Amp-hours a night at a moderate setting of "9" with no humidity or heated hose. Since you don't want to discharge to far - going below 25% limits the lifetime severely - you would need at least 6 AH to get by.
If you need humidity, you'll want a battery in the 35 Amp-hour range, such as a U1 size (weighs 25 pounds) used in scooters. (Even that might not be enoughi f your settings are high!) As a rough rule of thumb, each Amp-hour weighs about .75 pounds, so if your battery weighs under 10 pounds, it won't make it through the night.
As for the connection, if its a Respironics pump a cable for it is under $25 and well worth it. Its heavy zip cord (12 or 14 gauge?) with two chokes and a fuse (8 amp?) in the tip. If you go without humidity the average draw will drop probably below an Amp but the startup draw (and the pump "starts" on every breath) is still up around 2-3 amps so don't go too small on the fuse or wire. 22 gauge wire carrying 5 amps for 15 feet will have a 20% voltage drop so you should increase that, especially if you use humidity. 16 gauge would have a 5% drop.