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Dedicated 28V DC backup power


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Hello everyone, 

My dad sleeps with a CPAP machine. Load shedding really messes with his sleep (not to talk of health effects when the machine switches off).

Machine has 24V DC input, existing PSU rated at 90W.

Thinking (need a cheap and (not too) nasty solution):  

2 x 12V AGM batteries connected in series. 20AH, maybe slightly bigger (withstand good 2hrs loadshedding without draining the batteries beyond 60%).

24V 5A power supply.

Wiring: input 240V. Output - parallel 24V to battery and CPAP machine.


Will this work? Am I missing something?

Will that PSU overcharge the batteries? Or can I just set it to a certain max voltage which will keep the batteries safe?

Batteries will obviously need higher than 24V to charge correctly; should I install a regulator inline to the CPAP machine to keep its input at 24V?

In my mind this should work nicely, but obviously don't want to blow the CPAP machine 😉




Edited by markus_m2
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  • 3 weeks later...
On 2021/11/08 at 7:01 AM, markus_m2 said:

... to charge correctly...

You really need a proper battery charger to charge a battery and not have it overcharge etc. Then the voltage will vary a lot.

So the best solution would be to get a small UPS and use your existing 90W PSU. That's designed to run the CPAP, so you know it won't damage it, and if anything goes wrong, they can't blame you.

Alternatively, it's probably designed for two nominally 12 V lead acid battery modules in series; you could use a proper 24 V battery charger in parallel with the battery and CPAP machine. But you should first check that the CPAP machine will tolerate a lead acid battery on charge (perhaps 28.8 V or so). It should be in the instruction booklet or on a label somewhere on the machine.

You might be able to find a suitable LFP battery that is nominally 24 V. These will last much longer (both in years of service before replacement and hours of load runtime), but will be more expensive and awkward to set up. I'd still vote for the small UPS, and replace the battery modules inside it as required.

I don't think you'll need a 90 W unit; my wife's CPAP machine draws only about 1 A average from 12 V, so a unit capable of about 50 W peak should be fine. It should use less than about 20 W average, though I suppose that could vary considerably from model to model.

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