Question for battery experts: proper inverter?

[ziggytosh]

So I plugged in the inverter and attached it to the battery last night and got nothing except a lot of heat in the wires, so I disconnected asap. Didn't even get around to plugging in the PR power brick. Erratically flashing LED on the inverter and the heat made it clear something wasn't right. I tested it on my jump starter and in the car itself and still nothing except the flashing LED. This inverter was a serious piece of crap, mind you, the kind you get off ebay for $20. I hadn't used it in over a year, since I installed a 1000 w inverter in the car permanently, and I think it probably just was no longer working.

I did find a Respironics supplier in South Africa who can get me the cigarette lighter adapter. So I'll go pick that up when I have a chance and then test out my setup for a night or two using straight DC-to-xPAP.

One thing I noticed (and I'm guessing this has to do with the bum inverter) was that my alligator-clip-cigarette-lighter-socket thingy heated up quite a bit when I attempted to use it. Do you think I should be worried about that and maybe get a new one of those too before using the cigarette lighter adapter, or just give it a try first?

[123.Shawn T.W.]

Hook up the alligator clamps to the battery ... With nothing else hooked up does it get hot?

If no, then ... Plug something other than the invertor into it ... Does it work? Get hot?

If not ... It should be fine.

[khauser]

I'm not sure if that was in reply to my message, but 50 watts@120 volts is .42 amps, and would be about 5 amps at 12 volts ...

Yeah it was ... Your math is a bit off ... Re-read this:

I am going to "third" the battery cable. Here's why: An inverter has it's own losses ... if you can run directly from the battery you'll get much more out of it.

My jump starter has a built in inverter. If your's does, you can get an IDEA (they're not super-accurate) of how many watts your machine is drawing. That's at 120V, so divide watts by 120 to get amps. Let's say it shows 120 Watts (that is probably a little high but it is an easy number to work with). 120 (watts) divided by 120 (volts) is 1, so 1 amp. On 12V (DC doesn't matter much to this), that 1 amp turns in to 10. (120 / 12 = 10.) So a 10 amp draw from a 50Ah battery should last 5 hours. (All of this is theoretical in that it assumes perfect efficiency, which is never the case).

If memory serves, my aPAP at around 16cm draws around 50-90 watts (it jumps up and down as I breathe ... very hard to get a fixed number). My jump starter lasts almost 8 hours. The battery in the jump starter is a 22ah battery. I used it overnight and it lasted just about 6 hours. So you're 50Ah battery should do the job for a night, possibly more depending on current draw.

Just out of curiosity I ran it with the humidifier to see what the extra draw was. That was over 400 Watts (!!), so 3+ amps at 120V, or over 30 amps at 12V (!!!!)

So humidifier and battery means BIG battery.

One last note: The jump starter takes 3 days to recharge, with a trickle charger as you are planning. That means you're not going to get a full recharge unless you use a more aggressive charger. In fact, I would prioritize a better charger over the DC cable.
-Kevin

You will be hard pressed to find a PRS1 that draws 30 amp of 12vdc! Mine has been running with a 5 amp fuse for over 6 months ... 10.5 cm H2O and humidifier set at 5 (highest) never blown the fuse ...

PR tech told me to use a 300w power inverter for my 550 model ...

Hence the reason I don't believe the power usage on the jump starter.

At any rate, I don't think my math is off ... but I agree 30 amps seems unlikely (which is why the exclamation points). I didn't make that clear, though...

[ziggytosh]

It works!

Thanks all for the helpful tips and interesting discussion.

I ended up with two solutions. First, I found a 5.5mm x 2.5mm tip in a closet, then spliced, soldered and heat-shrunk it to some trailer harness wire with alligator clips attached to the other end (running through a 10 amp fuse right next to the positive alligator clip). Tested this for a couple of minutes to verify it works. It did, but even though I can't see anything obviously wrong with the setup, I figure discretion is the better part of valor when it comes to electricity, so I stuck it in my camping supplies as a just-in-case option and resolved to get something less jerry-rigged before my trip next weekend.

As luck would have it, while cleaning up after the above project, I found I already had another pre-made cigarette lighter adapter with the correct sized tip. It only has a 2 amp fuse in it, but I gave it a try and it powered my xPAP all night last night (and well into the morning). I was surprised 2 amps was sufficient, but apparently it was. I'm not sure how much juice is left in the 50aH battery, but at less than 2aH draw, it is clearly enough to get me through a couple of nights. So I'll charge the thing back up and I should be fine.

Thanks again for all the helpful advice.