change over circuit for cpath machine

[MrDEB]

We had another power outage last night and decided to look into a change over circuit.
cpath machine has an input of 90w 24v 3.75a
thinking of using a ryobi inverter power source.
I could use a 24v li-ion battery pack I had on my e-bike?
found this circuit https://www.circuitstoday.com/automatic-changeover-circuit
but max load is 2.5a
maybe put two in parallel? or even use a relay?
any thoughts on this project?

 

[Visitor]

I think you mean CPAP.

Your answer is in the very first paragraph of your link:

"....LTC4412 controls an external P-channel MOSFET to create a near ideal diode function for power switch over and load sharing. This makes the LT4412 an ideal replacement for power supply ORing diodes. A wide range of MOSFETs can be driven using the IC and this gives much flexibility in terms of load current...."

 

[Visitor]

Of course, all you really need is a SPDT 120VAC (North America) coil relay that can handle >4amp @24VDC.

Relay coil is connected to 120VAC supply voltage, and the 24VDC output of the power supply connected to the NO (Normally Open) relay contact. Battery voltage is connected to the NC (Normally Closed) relay contact. The CPAP is connected to the relay wiper terminal.

 

[MrDEB]

yes I saw that AFTER I read the data sheet. Yes a relay could be used but a relay has bigger losses than a mosfet.
Going to study the data sheet in more detail as it mentions a battery charge as well.
Looking at using a LIPO4 battery pack instead of a LI-ION for safety etc.
Design to utilize a LIPO4 battery that will run for 24 hours, include a charger. A larger Mosfet.

 

[MrDEB]

MOSFET selection?
I think I found a mosfet to fit the bill. 90w, 24v, 3.75a

https://www.mouser.com/datasheet/2/308/1/FQPF9P25_D-2313718.pdf

 

[MrDEB]

after more research I found a product that has the battery etc already manufactured.

https://www.thecpapshop.com/portable-outlet-rechargeable-cpap-battery-1?utm_source=sf&utm_medium=affiliate&utm_campaign=best_cpap_battery&utm_content=portable_outlet_rechargeable_cpap_battery

so the end of this thread

 

[tumbleweed]

but a relay has bigger losses than a mosfet.

Not any relay I've ever seen, unless it's a real POS.

It's usually pretty hard to beat the resistance of two pieces of metal connected together!

 

[Nigel Goodwin]

Not any relay I've ever seen, unless it's a real POS.

It's usually pretty hard to beat the resistance of two pieces of metal connected together!

I suspect he means the losses in the coil when activated? - however, in this case it obviously doesn't matter, because the coil would be fed from the mains anyway, and simply drops out for battery. No issue powering a low current mains relay coil from the mains.

 

[MrDEB]

got the bright idea to change battery chemistry.
the battery will just sit next to the bed so easy to move/carry is not necessary
Will look at both options for relay vers the LTC4412

 

[MrDEB]

I think this is what Jon was referring to using a relay
24v dc from power supply and a SLA battery (2-12volt in series)
needs refining but can just solder to a bread board.
the relay has 24v coil 8A contacts

 

[Visitor]

You have the relay contacts connected backwards. The line power supply output should go to the NO contact. The battery output should go to the NC contact.

When the relay is energized, the power supply should be connected to the load.

 

[MrDEB]

My mistake thanks
now the issue of a battery charger. If input voltage from power supply is 24v then how is that going to fully charge 2-12v batteries in series?
unless another battery charger is used and an additional relay.

 

[tumbleweed]

If input voltage from power supply is 24v then how is that going to fully charge 2-12v batteries in series?

I don't think it's normally recommended to charge a battery by connecting it directly to a power supply output...

 

[MrDEB]

having an auto-cut off when fully charged.
Looking at several circuit designs, several using LM317

 

[Visitor]

having an auto-cut off when fully charged.
Looking at several circuit designs, several using LM317

STOP. Do not pass go

If you are using LiIon batteries, you absolutely MUST use a proper proper charger designed for the purpose. Otherwise, there is an extremely high probability of burning down your house.

If you are using lead acid car batteries, they outgas during charging, creating both fire and respiratory hazards if used in the bedroom.

Sealed lead acid (SLA) would be safe, but getting the capacity you desire would require large and/or multiple batteries.

 

[MrDEB]

here is what I am thinking of for battery.
I realize the hazards of lithium batteries as well as lead acid batteries.
Thanks
looking at what it will cost to build.

 

[MrDEB]

maybe take a different approach by using a UPS power supply.

https://www.amazon.com/APC-Battery-Protector-Back-UPS-BE600M1/dp/B085JJZDFK?ref_=ast_sto_dp&th=1

need to do some converting. The CPAP power pack says 90w 24v 3.75amps but this is the output of power pack. Need to research power needed to run machine.
The UPS uses a SLA battery.

 

[MrDEB]

research my cpap machine specs = 53W typical 104w peak consumption.
now to figure out how many hours the machine will run using the battery UPS (450W) and compare

 

[MrDEB]

looking over the specs the https://www.thecpapshop.com/portabl...ent=portable_outlet_rechargeable_cpap_battery
is 159W and cpap machine should run 8.5 hours so based on that but using SLA battery( 450W) instead of LI-ion (159W) the maching SHOULD RUN about 3 times longer.
yes buyer beware?

 

[Visitor]

CPAP power draw is a function of pressure – you need to determine the current requirements at whatever your pressure settings are. You can't design for "average" drain and have any confidence it will last the night. You certainly can't design around "average" specs for an "average" CPAP and expect good results.