Grid tied battery backup for CPAP design help

[Quest50]

I'm new to this forum, so please forgive me if this isn't the right place to ask this or if this is a frequently asked question here. I could really use some help in figuring out the best way to go about a project:

I want to build a backup system (grid tied UPS backup) for a CPAP that draws 0.8 (120v) amps at most (tested with a clamp meter - the actual draw is usually much lower), and I'd like to be able to run it for a minimum of 10 hours (that includes a small safety margin). This is NOT a life or death issue - It's okay if this thing goes down, it's just really annoying.

My current gameplan is a 100Ah 12v LiFePo4 battery connected to an inverter/charger/automatic transfer switch. I'm not currently planning on adding solar.

Is there a cheaper, but still reliable option other than a Victron Multiplus-II for the inverter/charger/transfer switch? Is this overkill? Because my wallet feels like this is overkill.....

Is there any advantage to mounting this system in my basement (running an additional circuit in my house to it's own dedicated outlet) over making this thing portable (fabricating a suitable container) and just plugging it into a 15amp outlet like a store bought UPS?

Is there a cheaper (but still reliable) way to do this?

Any advice you can offer would be greatly appreciated.

 

[sunshine_eggo]

My ResMed CPAP uses about .2-.3kWh/night with heater.

A MP-II sure feels like overkill. There is a multiplus 12/500 that should more than do the job. There are other cheaper inverter/chargers that would work, but you have to look at their idle consumption. Some of the cheap inverters will burn more battery than the CPAP.

Want cheap and simple?

Is your CPAP 12V or 24V DC powered?

Either way, what about DC-DC converter to deliver the needed steady 12V or 24V voltage to the CPAP and a float charger or suitable power supply on the 12V battery to keep it topped off.

 

[robbob2112]

Here is my solution for here at the house and portable enough to take car camping - 25lbs.

Seeking Guidance on Building a Custom UPS with LiFePO4 Batteries

I want to build my own UPS for my computer and appliances. Something that works just like the APC UPS that you can buy but I want to built the pack with LiFeP0 batteries. I want to be able to feed primary with my 110V power company feed and switch automatically to my pack in case the power fail...

diysolarforum.com

 

[Solar Guppy]

I want to build a backup system (grid tied UPS backup) for a CPAP that draws 0.8 (120v) amps at most (tested with a clamp meter - the actual draw is usually much lower), and I'd like to be able to run it for a minimum of 10 hours (that includes a small safety margin). This is NOT a life or death issue - It's okay if this thing goes down, it's just really annoying.

I would suggest an off the shelf UPS that support external batteries like this add a pair of 50ah Lifpe04 batteries of your choice in series and the required connector to the UPS to battery ( looks like this from photos ).

 

[efficientPV]

Just a little 14V 3A power supply running off 120V >>>> lithium 12V battery >>>>$30 amazon 12V Resmed converter >>> CPAP.
Just leave it plugged in all the time.

 

[Quest50]

My ResMed CPAP uses about .2-.3kWh/night with heater.

A MP-II sure feels like overkill. There is a multiplus 12/500 that should more than do the job. There are other cheaper inverter/chargers that would work, but you have to look at their idle consumption. Some of the cheap inverters will burn more battery than the CPAP.

Want cheap and simple?

Is your CPAP 12V or 24V DC powered?

Either way, what about DC-DC converter to deliver the needed steady 12V or 24V voltage to the CPAP and a float charger or suitable power supply on the 12V battery to keep it topped off.

It looks like it takes 24v. Resmed sells a DC converter that can be used with a 12v or 24v battery....

Wouldn't pulling from the battery while it's on a float charger be cycling it? Would that cause any issues? I guess even if I cycled it every day it would still be ten years before it hit 4000 cycles, wouldn't it?

Would that really work?

 

[Quest50]

Here is my solution for here at the house and portable enough to take car camping - 25lbs.

Seeking Guidance on Building a Custom UPS with LiFePO4 Batteries

I want to build my own UPS for my computer and appliances. Something that works just like the APC UPS that you can buy but I want to built the pack with LiFeP0 batteries. I want to be able to feed primary with my 110V power company feed and switch automatically to my pack in case the power fail...

diysolarforum.com

That is beautiful. Is there a downside to just constantly pulling off the battery instead of using a transfer switch?

Being able to go camping on hunting trips again would be amazing.

 

[sunshine_eggo]

It looks like it takes 24v. Resmed sells a DC converter that can be used with a 12v or 24v battery....

Wouldn't pulling from the battery while it's on a float charger be cycling it? Would that cause any issues? I guess even if I cycled it every day it would still be ten years before it hit 4000 cycles, wouldn't it?

No. The charger would actually supply the load until AC power is lost, then the battery would be cycled.

 

[robbob2112]

In my case I have it plugged into a wall timer - I run off the battery all night and draw it down around 270Wh, then at noon the next day the timer clicks on for 4 hours to let it recharge. In reality it takes about 1.25~1.5hours. The charger has a fan and is pretty loud so I wouldn't want to try and sleep through it.

For LFP it is bettery to cycle the battery than to just leave it float.

I know from accidentally leaving the charger switch off once that I get 4 days out of it and it is basically at 0% SOC. The cpap was still running but the battery monitor was beeping at me.

 

[efficientPV]

That is how I run mine 5 months of the year off a 50AH lithium battery. I also have mine on a timer to keep it from running all day. It draws about 3W even when off and there is no reason to have it on all day. It does make a rude alarm clock should I oversleep. You can just clip on a small 12V solar panel for camping, no charge controller is needed.

 

[robbob2112]

I have a victron 150/35 and 335w panel I am about to add to it here so I don't need the wall charger except on the off multiday string over cloudy/snowy days. And if we decide on a camping trip of more than 3 days I'll get a 100w panel to use with it. I think I figured it would take 3.5hrs to recharge a night of use.

 

[Quest50]

In my case I have it plugged into a wall timer - I run off the battery all night and draw it down around 270Wh, then at noon the next day the timer clicks on for 4 hours to let it recharge. In reality it takes about 1.25~1.5hours. The charger has a fan and is pretty loud so I wouldn't want to try and sleep through it.

For LFP it is bettery to cycle the battery than to just leave it float.

I know from accidentally leaving the charger switch off once that I get 4 days out of it and it is basically at 0% SOC. The cpap was still running but the battery monitor was beeping at me.

I think this is the route I'm going to go.

Thank you so much for your help - Seriously. You just saved me about $1k and a LOT of headache. I think now my biggest concern is maple, oak, or walnut for what I'm going to use to make the box this thing's gonna sit in!

One last question: Do you have a preference on a charger?

 

[robbob2112]

The litime 20amps charger. I tried a 10 amp fanless eco-worthy originally and the case had a melting plastic smell so I returned it. Been running off it for 8 months now.

Get your cables with lugs on it from windy nation or batterycableusa. Or buy the raw windy nation cable, selterm lugs, and Temco crimpers. It doesn't take many more cables than this project for pre-made cable cost to equal the th005.

TH0005 V2.0 Hydraulic Cable Lug Crimper (11 US TON) 10 AWG to 600 MCM Electrical Terminal Cable Wire Tool Kit with 32 Die Sets

New for 2020, TH0005 Version 2.0. Released September 1st 2020, all units now include the following improved features: TRUE-HEX Crimp Forming dies apply uniform pressure on all sides for even compression and high pull out resistance. 16 Standard size dies for American Wire Gauge 10 through...

temcoindustrial.com

If you plan to keep tinkering of diy a full house sized solar it makes sense. Don't fall for any of the $40 ones off amazon. The AMZCNC brand is metric marked as awg and not that great.

 

[robbob2112]

Make sure to read this one.

Seeking Guidance on Building a Custom UPS with LiFePO4 Batteries

I want to build my own UPS for my computer and appliances. Something that works just like the APC UPS that you can buy but I want to built the pack with LiFeP0 batteries. I want to be able to feed primary with my 110V power company feed and switch automatically to my pack in case the power fail...

diysolarforum.com

 

[DIYrich]

Expensive because they are medical grade:

Zopec Explore CPAP Battery Products

Established 2007 in Minneapolis USA, we strive to improve people's lives by providing quality, effective and safe medical products. Products include Medical Protective Equipment, Medical Power Banks (CPAP Battery), and Foot Neuropathy and Pain Electrotherapy devices. Many hospitals and schools...

www.zopec.com