My battery backup setup

[Marthajoy7]

Ok, if anyone else uses mobility chairs or scooters, they use a XML connector for charging the batteries. Pin 1 on the connector is battery positive, Pin 2 is Negative and Pin 3 seems to be Floating just a few volts above the negative line and varies in level. I cut a microphone extension cable and found Positive to be the shield connection, White to be the negative connection and red to be the floating line. In a charger the short together Pins 2 and 3 for negative. There are no diodes between pins 1 and 2 but maybe on 3. So when using to support PAP therapy Use pin 1 for positive and 2 for negative. Open circuit the battery voltage can read slightly over 30vdc but as soon as current starts being drawn it will quickly drop under 30v. They are charged with a 24vdc charger, but like car batteries the are charged with a higher voltage like 13.5 to 16vdc. So that would be fully charged voltage but will drop to 24vdc quickly. Mentioning it just to make sure electronics can handle the voltage. For a couple of dollars I ordered a 6-50vdc to 5vdc 2 amp usb converter. I plan to tap the 5v line to feed a 3.3v regulator 800ma to a 2.7k ohm resistor. This way voltage variances on the 24vdc battery will not affect the center pins voltage. The price for both converters adds up to less than $5.

[Marthajoy7]

I just found a couple of boost converters I bought years ago, planning on using them to charge mobility batteries from our car. I never got around wiring it, and forgot where I put them. Anyway they are 12vdc in and adjustable up to 35vdc output. Said rated to 150 watts but reduces it to 90 watts without additional cooling. So I am going to wire one to see how well it works. Should be similar to Daves except I will have more connectors for versatility. So system is broken into sections, and can be mixed based on needs.
The converter has + and - terminals for both input amd output and a 10 turn pot for adjusting output voltage desired.
The 2nd photo shows the xlr connector which is the same connector type as on all my mobility devices. So cables I make from mobility aids to next breakout which will be split offs to power Pap machines. So 3 sections to what I want. Maybe overkill but lots of versatility
The gray connector is an Anderson SB50 which is the connector I have put on most of my batteries, for quick ans easy removal and swap. Also allows for charging while still using scooter.
Ok, here us the boost converter I am using

[Maskup N. Sleepwell]

0.75 amp Battery Tender Jr.($27.69 on Amazon)

Be careful about running your machine while the tender is connected. I've read some posts that indicate that is a no-no as it can damage your machine.

Also ...

I would not ditch the humidifier while on battery, but I would power it off or set it to zero. Leave the humidifier physically attached and filled with water. Although it is not heating the water, the air will still pick up some moisture as it passes through. A little humidity is better than none at all ;-)

Another added benefit of leaving the humidifier physically attached is it, at least on the machines I've owned, acts nicely as muffler. Makes for a quieter night.

[Marthajoy7]

Our machines are both Resmed's. Spouses is Airsense 10 in auto mode, and mine is an AirCurve 10 VPAP. Both have integrated reservoir's. I wish I had known that Medicare pays full price up front for that portion, maybe I could have saved big bucks for Medicare. I had to change machines from APAP - BIPAP and mt spouse had to change machines because insurance changed and old DME was no longer approved. So twice Medicare as paid multiple times for reservoirs that we had to return. Almost bet they doubled billed and gave it to someone else.

What are you talking about tenders, These are allowing use of our machines in case of power outages. At least now we know pretty much what Resmed has done to make it difficult to power the machines during times of emergency

Charging of the batteries will take place during daytime from a gasoline generator. I have 12v and lots of 24vdc chargers.

Oh, and cost of Boost converter is $2.68 with free shipping on EBAY

[ToroOne]

I have an Optima Blue deep cycle and get about 8 hours with max humidifier settings. I also installed a second deep cycle battery in my 4Runner for camping. Car needs to run for about 20-25 min. to recarge for next night.

[Marthajoy7]

I just wired up my first setup, I can run fron one or two batteries, Several options on first section, One has an boost converter set to about 28vdc so it can compensate for voltage loss is using long cables, One plugs into XLR connector to get power from mobility devices directly.
Middle section takes 24vdc and on the far end I have a usb converter to be able to charge things like phones or oximeter. I take a 5vdc line a feed a 3.3vdc converter.
From there I feed two more XLR connectors. The 3.3vdc output goes to two 2.7k ohm resistors, one for each connector.
Final sections goes from XLR to 7,4mm connectors that plug into respective machines. I have not run any current drains yet or amount of time batteries can be run with each setup.
But certainly made progress today. Only test was o verify that machine would power up and work at my pressure. Still want to get containers for circuit boards. Thinking of epoxy a dowel onto 10 turn pot and puting a voltmeter in container for adjusting outpit voltage as needed. But have not made up my mind yet.

[Marthajoy7]

OK. I have noticed a problem. When powering PAP machine from 24 vdc, I find the boot screen keeps resetting rather than going to dark screen. As I was wondering is the 2.7K ohms, resister too low. Based on resistances found to work with higher voltages, I would expect something closer to 4K ohms, since 6.8k was a good value for 5vdc. It concerned me because the 2.7K value was read in circuit resistance.

[justMongo]

Read up on Thévenin's theorem.
https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem

It will permit resistor-divider and Voltage transformation that are equivalent circuits when plotted in the I-V plane.

[DSchabel]

Cleaning it up...

I really don't think you mean to use a 90W resistor.
The most you could ever dissipate with a 3.3V output (let's say it's on the high side at 3.5V) and a 2.7kΩ resistor is 3.5^2/2700 = 0.005W (unless the resistor happens to be connected to the 24V input, in which case you have other problems).

[surferdude2]

That wasn't my schematic. I just cleaned up the lines some and didn't presume to edit it for clarity. I presume what the original author meant was as shown below: