DIY CPAP UPS - LiFePO4 Battery - Dead Simple DC Power For CPAP

[nifty-stuff.com]

I felt like I was hijacking / monopolizing this awesome CPAP discussion: How to power a cpap

... so I thought our of respect for the guys in that thread, I'd start my own thread.

Short Details:

The SF Bay Area has had a lot of blackouts and my girlfriend's elderly Grandfather wants a battery backup for his ResMed AirSense 10 Elite CPAP machine.

1. He wants it to basically act like a UPS where he's got it plugged in all the time
2. He'd like it to last 2-3 days with the humidity / heater on (if I'm reading the threads correctly, this uses about 200Wh per night)
3. This unit uses 24v
4. Computer UPS systems (under $500) don't seem like they are made to run for multiple days in power outages
5. All the "power stations" I've seen aren't made for this 24/7/365 kinda use. I even emailed EcoFlow and they discouraged me from using their system as a long-term DC UPS.

So, I decided to channel my inner @Will Prowse and DIY!

... with these components:

1. Cheap AC wall adapter set at a voltage that keeps the battery (LFP, Lead-Acid, whatever) at a safe "topped-off" level. For example, maybe somewhere between 13.2 - 13.6v for LIFEPO4 (as discussed with @Will Prowse in his interview with BattleBorn here. - $25
2. Battery - LifePo4. A 50 Ah battery should easily last 2 nights with humidity and heater on the CPAP. - $180
3. CPAP DC Converter- Gpa's Resmed Airsense needs 24 volts with a "special" adapter. This adapter can safely take anything from DC: 12V-24V (9–32V DC MAX) and put out the needed 24V DC: - $32
4. Plug: Something like the following that will make it easy for the CPAP adapter to plug into, while also displaying the voltage, and providing USB ports (bonus)! - $17

.... and basically looks like the setup below and costs only about $254. (If I want to go bit for 100Ah, it's only about $375... and would run it for like 5 days!)

So, what's wrong with this idea? PLEASE poke holes at it! Tell me what could be missing / wrong with it?

Thanks in advance!!!

 

[sunshine_eggo]

Looks mostly fine.

If the goal is to have the battery mostly charged, it should be floated at 13.6V.

 

[Rednecktek]

Spend the extra and get the 100ah battery. Many BMS's will cut you off at 20% and that's another day's worth of power. Plus, if you plan for a 2 day outage, it'll last 3 days.

You'll also want an inline fuse between the battery and the socket.

Other than that it looks like a good simple solution. In the future a cheapie PWM controller and used panel can suppliment if you really want, but not needed at this point.

 

[sunshine_eggo]

Spend the extra and get the 100ah battery. Many BMS's will cut you off at 20% and that's another day's worth of power.

I've never personally seen this except on the rack mounts in communication with inverters. Most "canned" battery BMS have LVP a little below 2.5V.

You'll also want an inline fuse between the battery and the socket.

Truth.

 

[nifty-stuff.com]

Brilliant, thanks for the quick replies guys!!!

If the goal is to have the battery mostly charged, it should be floated at 13.6V.

Since I'm (probably overly) worried about cell longevity, I thought keeping the LFP battery at around 3.3V (which *should* be about 90% full) would be a great way to go... and if I'm using a 100Ah battery, then 90% is PLENTY for Grandpa's needs... even if he also wants to charge phones, etc.

Spend the extra and get the 100ah battery. Many BMS's will cut you off at 20% and that's another day's worth of power. Plus, if you plan for a 2 day outage, it'll last 3 days.

Ya, that's the direction I'm leaning. For just $100 more, I could double capacity/energy and have lots of extra headroom, etc.

You'll also want an inline fuse between the battery and the socket.

Ah, great idea / point! I was starting to wonder about that.

This socket says it comes with a 15A fuse, so hopefully that's sufficient?

https://www.amazon.com/dp/B0B1HWVVDL/

 

[Rednecktek]

I've never personally seen this except on the rack mounts in communication with inverters. Most "canned" battery BMS have LVP a little below 2.5V.



Truth.

My JBD and my Ampertime won't let me go lower than 20%. Even when I talk to the JBD and set the lower limit it just adjusts the upper capacity higher than what the battery actually is.

Just something to consider. Look up reviews of which ever battery you go for and make sure the capacity tests allow full capacity.

 

[sunshine_eggo]

Brilliant, thanks for the quick replies guys!!!


Since I'm (probably overly) worried about cell longevity, I thought keeping the LFP battery at around 3.3V (which *should* be about 90% full) would be a great way to go... and if I'm using a 100Ah battery, then 90% is PLENTY for Grandpa's needs... even if he also wants to charge phones, etc.

It won't be. It will be low. consider that most new cells that come in from China are sitting at 3.29V and are at 30-50% SoC.

The absolute minimum that I would consider is 13.5V.

 

[sunshine_eggo]

My JBD and my Ampertime won't let me go lower than 20%. Even when I talk to the JBD and set the lower limit it just adjusts the upper capacity higher than what the battery actually is.

Just something to consider. Look up reviews of which ever battery you go for and make sure the capacity tests allow full capacity.

Weird. My JBD will let me drill it all the way to 0%. I don't even see how I can set it to limit to 20%. Mine only cuts off if a cell goes low (2.5V), or the whole pack goes low voltage (10V).

Do you have your cycle capacity set 20% below your total capacity? If so, that's why. Cycle capacity is the tested capacity.

 

[nifty-stuff.com]

It won't be. It will be low. consider that most new cells that come in from China are sitting at 3.29V and are at 30-50% SoC.

The absolute minimum that I would consider is 13.5V.

Hmm... valid points. I've wanted to be extra careful not to degrade the cells having them at a constant "charge" that is too high.

So, I'm now thinking about getting one of these that can be set to 13.5 V

The watts is SUPER small... it's only 3A, so it would probably be only 40 watts. Not a really fast-charge if the 100Ah battery is dead, but if it's plugged-in 24/7 then this little adapter should be able to charge the whole 100 Ah battery in like 1.5 days.

https://www.amazon.com/dp/B08FZLXT8M/

 

[sunshine_eggo]

Hmm... valid points. I've wanted to be extra careful not to degrade the cells having them at a constant "charge" that is too high.

It's really only a major concern in hot environments. If kept in a human-comfortable-climate-controlled environment, then high SoC storage isn't a big deal.

So, I'm now thinking about getting one of these that can be set to 13.5 V

The watts is SUPER small... it's only 3A, so it would probably be only 40 watts. Not a really fast-charge if the 100Ah battery is dead, but if it's plugged-in 24/7 then this little adapter should be able to charge the whole 100 Ah battery in like 1.5 days.

https://www.amazon.com/dp/B08FZLXT8M/

Looks good. 15V gives the option of an occasional run to true 100% - that will be needed from time to time. A battery sitting at 13.5V may see its cells go out of balance if left that way for weeks/months.

 

[justgary]

Hmm... valid points. I've wanted to be extra careful not to degrade the cells having them at a constant "charge" that is too high.

So, I'm now thinking about getting one of these that can be set to 13.5 V

The watts is SUPER small... it's only 3A, so it would probably be only 40 watts. Not a really fast-charge if the 100Ah battery is dead, but if it's plugged-in 24/7 then this little adapter should be able to charge the whole 100 Ah battery in like 1.5 days.

https://www.amazon.com/dp/B08FZLXT8M/

You say 200Wh per night at 24v, so that's maybe 8.3Ah per night, so that's about 1 Amp over eight hours....

Your 1 Amp at 24v is 2 Amps at 12v, leaving only 1 Amp for charging your battery while you are using the device. That now would be ~100 hours to charge from dead empty to full, so just over four days if you were using the device the whole time (which you aren't). Considering that your charge rate is 3 Amps for ~16 hours and 1 Amp for 8 hours, the full charge is longer but still under two days.

I'd probably go for a bigger (better) power supply to get it charged in one day no matter what, and also because you are dealing with something possibly considered close to a life support device. This one is adjustable and hard for someone to bump after you set it. You can buy or build an enclosure for it also to keep someone from shocking themselves on the terminals.

https://www.trcelectronics.com/View/Mean-Well/LRS-150-12.shtml

 

[nifty-stuff.com]

Looks good. 15V gives the option of an occasional run to true 100% - that will be needed from time to time. A battery sitting at 13.5V may see its cells go out of balance if left that way for weeks/months.

I figured I'd do a full-charge on my benchtop power-supply every 6 months or so... but I didn't think (with the BMS, etc.) that forcefully balancing would be needed more often than that, no?

I'd probably go for a bigger (better) power supply to get it charged in one day no matter what

Ya, I go back and forth about that. I might go with something more powerful / higher Amps.

also because you are dealing with something possibly considered close to a life support device.

I thought similar, but her Gpa didn't seem like it was as huge of an issue. In fact, when power in SF Bay Area went out for 3 days, he had the option to go to his kid's house to sleep with his machine and opted to just stay home?!?!

This one is adjustable and hard for someone to bump after you set it. You can buy or build an enclosure for it also to keep someone from shocking themselves on the terminals. https://www.trcelectronics.com/View/Mean-Well/LRS-150-12.shtml

Interesting! I thought those were all fixed-voltage, but then I saw "VOLTAGE ADJ. RANGE: 10.2 ~ 13.8V"

I'm surprised they don't clearly say the voltage is adjustable all over the main page (without diving into the data sheet!)
... what I can't see is HOW / WHERE the voltage is adjusted?!?!

I'll look into one of those. I can easily 3D print a nice enclosure that holds that and the other bits (like the plugs, etc.)

 

[nifty-stuff.com]

Update: I just watched videos on the Mean Well power supplies. I see some have adjustment pots for the output voltage... very nice!

One concern / question: Are the fans always on with these supplies? I HATE that with my 3D printer and the power-supply is always on. I don't want to do that to Gpa and worry the fan will be on 24/7... which is another reason I was thinking a slow dummy wall adapter would be ideal.

 

[Rednecktek]

Do you have your cycle capacity set 20% below your total capacity? If so, that's why. Cycle capacity is the tested capacity.

Yeah. The last version of the app I had wouldn't let me set the cycle capacity at all. I updated the app and now not only do I not see any cell voltages, but my cycle capacity is ALWAYS 20% of the total capacity. If I change the Total, the cycle auto-sets at 80%, but if I change the Cycle capacity, it auto changes my Total for 120% of what the cells can do. There is always an 80% difference.

 

[justgary]

Update: I just watched videos on the Mean Well power supplies. I see some have adjustment pots for the output voltage... very nice!

One concern / question: Are the fans always on with these supplies? I HATE that with my 3D printer and the power-supply is always on. I don't want to do that to Gpa and worry the fan will be on 24/7... which is another reason I was thinking a slow dummy wall adapter would be ideal.

The one I linked is fanless (convective cooling). If you put it in an enclosure you might want to leave plenty of places for air to move through, especially since it will basically run at maximum current when the battery is charging. Adding a silent fan to your enclosure would probably be a good idea.

I swapped my 3D printer's hot end fan to a Noctua silent fan. It is pretty quiet, but in that case I had to deal with running a 12v fan on the 24v supply. You can just hook yours up to the supply and rock on.

 

[sunshine_eggo]

I figured I'd do a full-charge on my benchtop power-supply every 6 months or so... but I didn't think (with the BMS, etc.) that forcefully balancing would be needed more often than that, no?

That's pretty optimistic for cheap LFP batteries.

 

[nifty-stuff.com]

That's pretty optimistic for cheap LFP batteries.

Bummer since I'm not going to be able to manually go in and rebalance / change the charging profile of the battery all the time.

Do most LFP battery manufactures require rebalancing / top-charging to maintain the health of the battery (and the warranty)?

Other than manually full-charging the battery on a regular frequent basis, what are my other options for simplicity and cost?

Is there a way to "best-guess quantify" the potential issues or damage if I set this up as initially designed in my first post, and only "fully charge" to 14.6, say... once a year?

 

[sunshine_eggo]

Do most LFP battery manufactures require rebalancing / top-charging to maintain the health of the battery (and the warranty)?

Yes. A Battleborn user lost 50% of their capacity because they never charged it into the balancing range.

Other than manually full-charging the battery on a regular frequent basis, what are my other options for simplicity and cost?

Automating that process.

Is there a way to "best-guess quantify" the potential issues or damage if I set this up as initially designed in my first post, and only "fully charge" to 14.6, say... once a year?

Once a quarter and again following every event.

For a "standby" application like this AGM and GEL batteries may be a better value. They love to be held fully charged and can do so for 10+ years with infrequent discharges.

LFP primary benefit is $/cycle. You're not cycling these batteries, so you're paying a premium for a battery that's not ideal in a standby application.

 

[nifty-stuff.com]

Thanks for all that!

Yes. A Battleborn user lost 50% of their capacity because they never charged it into the balancing range.

Was that a permanent loss, or just until the cells were re-balanced?

For a "standby" application like this AGM and GEL batteries may be a better value. They love to be held fully charged and can do so for 10+ years with infrequent discharges.

I was wondering about this when I started the project and was hoping that LFP would be better "all around"

Now I'm curious... which is "worse" or "better" and to what degree?

1. LFP maintained at 14.4?
2. LFP maintained at something else between 13.5 and 14.4?

People talk about "degradation" etc., but it's hard to really quantify the risk/return. Are we talking: "Battery will perm. lose 50% of capacity in 1 year if done this way!" or "Battery will lose 20% capacity if done this way for 5+ years"

Reminds me of when an arborist came by to warn me not to trim an oak tree a specific way. He said: "It will shorten it's life! Don't do it!!"

I asked: "Ok, but to what degree? Can you quantify that for me?" His reply: "Well, it would usually live for about 400 years, but if you do that, it might only live 200!!!"

Ya, I trimmed the tree

 

[nifty-stuff.com]

Hmm... after reading your comments @sunshine_eggo and watching this video, I'm wondering if this Renogy Deep Cycle AGM Battery 12 Volt 100Ah battery would be a better fit? (On sale at Walmart for $189).

I guess even if only discharged down to 50%, it *should* still give Grandpa 3 solid-nights of CPAP.

...and this would 100% get me around the issues with balancing, maintenance, etc?

I'd just have to figure out the float-voltage on the battery... which the listing states: "Float Charging Voltage: 13.5V~13.8V "