Electrical engineer question about charger

I have a CPAP machine and wanted to get a battery backup for it. The one I want comes in a nice kit and is capable of starting a car. I wanted to keep it in my motorcycle saddlebag for emergencies which are highly likely when the temperature drops.

It comes with adapters that fit my machine. Plugged it in and the machine said “fuck you”. Well actually it said it wasn’t compatible.

I checked the charger it comes with and it puts out 12.57 volts and is rated at 80W. The battery backup puts out 12.7 volts. Given it’s size it’s some kind of Lithium battery.

Could such a device be that picky about voltage or am I missing something? The batteries sold for these things cost 3 times as much.

You probably need to provide the actual make and model of both to really understand the issues. It is possible that the CPAP machine’s batteries include a charging controller, and the CPAP machine itself is looking for it to have a conversation. No controller, no compatible.

Any sort of medical device with approvals and licensing (not to mention massive liability insurance) is going to be designed to be very picky about how it is configured and powered. I would be very unsurprised that the machine is designed to only use the manufacturer anointed battery. For once I would support them in doing this. The problems with fake, substandard, and fraudulently re-badged old batteries makes laptop and phone battery replacement a minefield. For something like a CPAP machine, I would just say no, and pay the premium.

It doesn’t come with a battery. Just a 12V power supply. If I can find it I have a 300W inverter somewhere but that would cut down on the battery time available.

Shouldn’t be too bad - unless the inverter is very inefficient. But, yes, you will lose about 20% odd at least.

The CPAP machine might be looking at the off load voltage of the battery and deciding that it doesn’t look like the power supply it expects to see. It could be arbitrarily picky about on and off load voltages, and that would be a pain to match.

What kind of “battery backup” are we talking about here? Most battery packs are unregulated (i.e. there is no voltage regulator to keep the output voltage constant). The 12.7V may just be for nominal conditions, and actual voltage may vary depending on how much charge is remaining.

If the CPAP depends on the charger for battery voltage regulation, it may be incredibly picky.

while this makes sense most electronic devices working off DC voltage are generally tolerant of this variability. Virtually everything in a car is subject to this. Most anything in a house that runs on DC current (and a bridge circuit) has to deal with the variability of AC voltage which in my experience can vary quite a bit.

This is a CPAP machine, not the Hadron Collider. It’s a fan motor with a pressure regulator. No other line of similar products seem to be this picky.

I’ve since learned from poking around the net that even the expensive backup batteries need some kind of 12V power adapter specifically for my machine. I’m starting to think there is some kind of hand shake between the AC-to-DC power supply and the CPAP machine for no other reason than to require people to use their equipment.

FYI, I screwed up the thread header. I should have said “power supply” and not “charger” since the CPAP machine does not have a battery, just a power supply. I WANT a battery and my attempt to connect one is rejected by the CPAP machine.

Is the battery backup thing actually designed to be used as a constant 12v power source? Most of the backup battery gizmos out there these days are designed more to recharge another battery (be it a car battery or a cell phone battery or whatever) rather than provide a steady 12v to run something. You might get 12v at the plug with no load, but the backup will only supply steady current if it detects a depleted battery connected to it.

Sure the device can be that picky.
12.57v will be slightly in excess of its voltage needs, and it will then have a regulator.

It may have a simple transistor regulator, rather than a switch mode…

Now the thing with transistor regulators is that they can overheat, if you give it to much voltage, the regulator must waste the power , and that power is Current * excess voltage.
A simple way to drop voltage ? You can add a large diode into the cabling, and that will drop the voltage down to below 12.57… Be sure the diode is rated for air cooling at the required watts… eg 0.7 volts times six amps…you need a 5 watt diode… which means its rated for 5 watts without forced air cooling or heatsink.
YOu can add a heat sink…(surface area improver…) just cut the sides off a tin can, and attach it to the diode…The idea is that the metal conducts heat away faster than air.