Problem is that the original poster (Crog
) has an S8 AutoSet Vantage
From this ResMed Battery Guide
[PDF], that unit uses .98 amps for the unit itself, but this does not include the amperage draw from the H3i humidifier
that attaches to that unit...
If you consider the CPAP machine PLUS the H3i humidifier, if you scroll down further in the document, where it lists the requirements for the S8 AutoSet Vantage +
HumidAire 3i, the listed amperage draw is 4.12 amps at 10 cmH20 of pressure, which is even higher that my estimated amperage draw of 3.0 in the above example. But keep in mind, the 3.0 may be closer (depending upon how high you set the heat setting). ResMed is probably using the heat on maximum level to get that 4.12 amp draw listing. If it's half that, who knows, it could be closer to 3.0 as my original post estimated.
So, this solution of an 18 amp-hour battery is even worse than I thought. I'm surprised that he got over 5 hours of use before it went ka-put. Must not have had the heated humidifier turned all the way up, or the 95% pressure is pretty low.
Also, please keep in mind that you cannot discharge a 12 volt lead acid battery below 60% of capacity, unless you want to destroy the battery
. So the so-called "amp-hour rating" is not a complete picture. You must use the formula I listed in the above post to get the accurate number of hours you can use the battery without doing permanent damage to it.
So, getting a new 18 amp-hour battery will do nothing but waste money and (although it will run the CPAP and humidifier for up to 4-5 hours max before a complete discharge causes the malfunction Crog described
) if you use it past 3 or so hours (the safe level), you'll destroy the battery. You may get lucky and doing this once or twice will not permanently destroy it, but overall capacity will definitely be negatively affected and do this more than 1-2 times and you'll definitely destroy the battery.
You need a higher amp-hour rating (minimum of 60 amp-hours), not a new low 18 amp-hour battery.
, not sure where you are getting your numbers. On your S9 Machine, according to the official Resmed Battery Guide, the "S9 AutoSet + H5i (EPR Setting 0, H5i setting 3)" has a total amp draw of anywhere between 2.57 - 4.10 amps, depending upon the pressure.
Here's the numbers for the S9 AutoSet with H5i Humidifier
8 cmH2O of pressure = 2.76 amp draw
10 cmH2O of pressure = 3.01 amp draw
12 cmH2O of pressure = 3.32 amp draw
16 cmH2O of pressure = 3.77 amp draw
20 cmH2O of pressure = 4.10 amp draw
And, since this doesn't include the amperage draw from the heated tubing, I'm sure that with that, you'll have an even greater amperage draw.
And yes, I'm talking about 12 volt DC amperage draw here. That's the best-case scenario with 12-volt. If you run it through an inverter that changes it from 12 DC to 110 volts AC, you'll have an even greater amperage draw (on the battery) than the above numbers, since the inverter itself uses power in the inversion process.