change over circuit for cpath machine

[MrDEB]

thanks will take under advisement.
seeing how I need to breadboard this I will look at using a relay.
I based my calculations on other cpap battery supplies but Jon has made a good point
correction the battery charger is 2.8 amps and my cpap shows only 3.75amp (3.75ah). but yes should plan for worst case

 

[Visitor]

20 Ahr battery / 3.75A = 5.33 hours

15 AHr battery / 3.75A = 4 hours

 

[MrDEB]

wonder how the cpap batteries say 8 - 10 hours?
will look at my cpap settings etc.

 

[ChrisP58]

The voltage of your 24 Volt battery will vary from about 26 Volts (or higher when on charge) down to whatever Volts it's at when you turn the cpap machine off in the morning. Your cpap machine is expecting to see the regulated 24 Volts from the AC power supply.

You'll will probably be OK, but this should be considered.

An alternative would be to use a 12 Volt battery (of twice the ampacity) and a boost converter to make a regulated 24 Volts.

 

[Visitor]

Here's one example of a CPAP battery pack – it's rated at 80AHr. Four times your large battery option with about four times the operating time. Imagine that.

The energy density of LiIon batteries is much, much greater than that of SLA batteries.

Looks like most of the CPAP battery specs show run times for lower-current-draw portable/travel CPAPs. Also note – if you use a humidifier, that cuts down battery life considerably.

 

[MrDEB]

curious where did you get 80AHr?
the unit says 297WHr
amp hours= watt hours / volts
WH = amp hours * volts
maybe I missed something?

 

[Visitor]

They do make it tough to figure out.....

297Wh/82.5Ah = 3.6 volts, the battery voltage of a lithium cell.

My sleepy brain says my last post was incorrect. Sorry about that. So this battery supplies (nominally) 300 WHr, and your CPAP draws 3.5A @ 24V = 84WHr. That provides 3.6 hours of run time.

Sorry, I can't find the chart I saw last night, listing power consumption for several different CPAP models vs pressure. The screen shot below provides some info.

If your current measurement(?) (Guess?, random number?) is accurate, the calculations don't lie. Did you check it with humidifier and heated hose off?

 

[MrDEB]

With the info presented I guess I will break down and purchase a ready made unit.
Thanks for all the info

 

[MrDEB]

I do have an extra 48v Li-po4 battery maybe?
use for electric trike etc.

 

[MrDEB]

the Lipo4 battery is rated at 20ah so 460wh.
I might just go this route but need to check if the charger is a smart charger.

 

[Visitor]

[Biting tongue. No, don't say it!]

 

[MrDEB]

say what?
I am wondering if chopping the 48v down to 24 affects the wh?

 

[tumbleweed]

I am wondering if chopping the 48v down to 24 affects the wh?

Any conversion, no matter if it's up or down, is never 100% efficient so you always lose energy in the process.

 

[Pommie]

I do have an extra 48v Li-po4 battery maybe?

the Lipo4 battery is rated at 20ah so 460wh.

48x20≠460.

Mike.

 

[MrDEB]

mis-typed 48 x 20 = 960
with stormy weather coming I just might go this route with the 48v Lipo4 battery pack.
us a voltage regulator to get the 24 volts and still contemplating using either the relay or the LTC4412
leaning towards the relay but looking at different relays and getting concerned about mounting on circuit board and life span.
researching voltage regulators and such first. going from 48v to 24v there is a lot of wasted energy requiring heat sinks.
Maybe just look at getting a ready made unit but what fun is that.

 

[MrDEB]

another option I am looking at is using a Ryobi 18v battery pack with inverter unit but can't run the humidifier.
these are rated 150w

 

[Visitor]

us a voltage regulator to get the 24 volts.....

researching voltage regulators and such first. going from 48v to 24v there is a lot of wasted energy requiring heat sinks.

You can't use a linear regulator, unless you're trying to heat your house too! A linear regulator converting 48v --> 24v will convert more than half of the battery energy to heat. Geez oh grief. You should understand this by now.

The cavalier "us(e) a voltage regulator to get the 24 volts" says it all. You'll need a switch-mode regulator that can handle 6 amps+ at 48 volts. This is not something you'll find at Radio Shack. And yes, it will need to handle a lot more than the drain you have calculated because it has to handle the motor starting current. A switching regulator will go "nope, I'm done – too much current for me" when faced with the motor starting current if it's too large. The result will be a loop of

motor starts – switcher overloads and shuts down – motor tries again the process repeats.

another option I am looking at is using a Ryobi 18v battery pack with inverter unit but can't run the humidifier.
these are rated 150w

How many times have I stated for your various commercial battery options:

"These runtimes are based on not having a humidifier." or words to this effect? Please go back and count.

I dare say most CPAP users understand that a humidifier is a large current draw for a battery, and forego using one to, you know, be able to run a CPAP on batteries when needed and to breathe all night.

[Note to self – why on earth did you get involved in this fruitless time sync again?!?!?]

 

[shortbus=]

I dare say most CPAP users understand that a humidifier is a large current draw

I don't. Mine is just a reservoir of water that the CPAP machine is connected to then to the mask. No electrical connection at all.

 

[Visitor]

I don't. Mine is just a reservoir of water that the CPAP machine is connected to then to the mask. No electrical connection at all.

Some work as you say. Many are heated, as is the one in this discussion (apparently).

 

[MrDEB]

research as I am aware that going from 48v to 24 would create LOTS of heat