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

[sunshine_eggo]

Thanks for all that!

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

It took a week holding the battery at 14.2V. I don't remember the final results, but at that point, it was running 93Ah.

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?

Bad. Will damage the cells.

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

13.8V is a possibility. That's an average of 3.45V/cell, and that's usually where a BMS will start balancing. It's not great for them, but it's way way way better than 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"

10-20% in hot conditions per year is absolutely possible. In a temp controlled comfortable room, maybe 1-3%/year. Down at 50°F, expect it's hard to measure.

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

LOL

As mentioned before, it's tied more to temperature.

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.

IMHO, I would not hesitate to go to 20% on an AGM. You'll get 300+ cycles like that. Time will kill it before capacity loss.

...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 "

IMHO, 13.8V is needed to truly get the battery back to 100% in a reasonable time frame (24 hours after 13.8V is hit).

 

[nifty-stuff.com]

Great stuff, thanks for all that!

So, I guess I need to really forget al the LFP hype (at least for this application) and go with AGM sitting at a float around 13.8?

 

[sunshine_eggo]

Great stuff, thanks for all that!

So, I guess I need to really forget al the LFP hype (at least for this application) and go with AGM sitting at a float around 13.8?

LFP is truly wonderful, but sometimes a different tool is called for. If you need lightweight and/or long cycle life, LFP wins hands down. Until their price is in parity with AGM, there's just no compelling reason for their use in a standby situation.

 

[Rednecktek]

LFP is truly wonderful, but sometimes a different tool is called for. If you need lightweight and/or long cycle life, LFP wins hands down. Until their price is in parity with AGM, there's just no compelling reason for their use in a standby situation.

Well, if you go by usable amp hours, LFP and AGM are about even. If you want to save money, the WallyWorld 29DC batteries are about $100 out the door and 60ah usable.

 

[sunshine_eggo]

Well, if you go by usable amp hours, LFP and AGM are about even. If you want to save money, the WallyWorld 29DC batteries are about $100 out the door and 60ah usable.

For standby operations, there's no reason to limit AGM/GEL to 50%. One might only see a few dozen cycles a year - a couple hundred deep cycles down to 20% might last a decade. AGM still wins here.

https://www.renogy.com/content/RNG-BATT-AGM12-100/AGM100-Datasheet.pdf

7 years float life @ 25°C


100% discharge = 200 cycles. - probably around 300 for 80% depth.

How many outages a week?
How long do they last?
How many occur during CPAP use periods?

etc.

 

[nifty-stuff.com]

For standby operations, there's no reason to limit AGM/GEL to 50%. One might only see a few dozen cycles a year - a couple hundred deep cycles down to 20% might last a decade. AGM still wins here.

Thanks for finding / pointing out those graphs!

100% discharge = 200 cycles. - probably around 300 for 80% depth.

How many outages a week?
How long do they last?
How many occur during CPAP use periods?

Yup, and this is why you've got me about 91.25432% convinced that I should shift-gears to AGM.

My girlfriend (again, this is a project she gave me for her Grandpa) is expecting maybe 2-5 outages a YEAR... and of those, most being only about 1-2 days (which keeps the battery above 50%)... and maybe once every year or two that he needs to use it down to 100% discharge. 98% of the time, it will just be "floating".

 

[nifty-stuff.com]

Ok, I'm pretty sure I'm going with the Renogy 12 Volt 100Ah battery. Now I just need to decide the best-value way to keep it topped-up.

I don't need an expensive super-high-amp charger. I'd like to think that one that does 2-5 amps would be plenty.

Should I go with one that I can set the voltage myself (at a fixed voltage around 13.5 - 13.8), or get one of the "multi-stage" chargers made specific for AGM / SLA batteries?

 

[sunshine_eggo]

Ok, I'm pretty sure I'm going with the Renogy 12 Volt 100Ah battery. Now I just need to decide the best-value way to keep it topped-up.

I don't need an expensive super-high-amp charger. I'd like to think that one that does 2-5 amps would be plenty.

Should I go with one that I can set the voltage myself (at a fixed voltage around 13.5 - 13.8), or get one of the "multi-stage" chargers made specific for AGM / SLA batteries?

Since it's a standby application, 13.5-13.8V constant voltage supply. Multistage is only needed if you want to recharge it as quickly as possible.

 

[nifty-stuff.com]

For charging I think I'm going to get one of those cheap 12V 10A DC Power Supply Universal Regulated Switching AC to DC Converters:

... and hope I can adjust it as high as 13.8V.

If not, maybe I'll just stick with one of the simple adjustable wall-wart AC adapters that is around 2-3 amps.

 

[nifty-stuff.com]

The cheap 12v 10amp "Regulated Switching Power Supply" arrived today and I can fine-tune it all the way up to 14.4 volts... so hitting 13.x for AGM will be no problem.

The other bits are (hopefully) coming tomorrow... but as I've been testing the power supply some questions came up:

1. I wonder how long the power supply will last being plugged-in "on" and pushing some (small) power to the battery pretty much 24/7?
2. Do I need to worry about keeping the power supply cool, vented, etc? This one has "passive cooling" but I don't have a thermal camera and have no idea if I need to add a fan or not. My intent was to avoid a fan entirely if possible to keep the backup battery system quiet for Grandpa.

Sheesh, this small simple project is getting bigger and more complex all the time

I wonder what else I'm missing / not considering?

 

[sunshine_eggo]

Is it a meanwell?

 

[justgary]

1. It will easily outlast grandpa, especially if it rarely has to provide high current.
2. You should put it in a vented enclosure to keep dust, fingers, spilled coffee, etc. out of it. Consider adding a quiet fan and proper cord grommets to the enclosure. It will be quieter than his CPAP, which these days is pretty quiet.

Search for "vented hobby enclosure" and "quiet pc fan." Noctua makes very good, quiet fans, but some other brands have come along recently. Post the dimensions of your power supply and we can help find an enclosure. IMHO, spending another $50 to make sure that grandpa doesn't shock himself or snatch the wires out of it picking it up is worth the peace and quiet you'll have when he doesn't call at 3 AM telling you how his machine quit working.

Heck, for that matter, just buy a storage box at Wally World and put the batteries, BMS, and power supply inside. Screw or strap everything to the inside of the box, add some ventilation holes on the sides, and add a quiet fan (blowing across the power supply). Use cable grommets to keep the wires safe. Now you are closer to $30, and the whole thing is in one box, ready to go camping.

I don't have a fan or power supply, but here is my box with a 4S 230 Ah battery, BMS, and Anderson connector. It is not really meant to be portable, but it easily could be. The point here is that enclosures can be cheap and work really well by adding a few holes in the right places.

 

[nifty-stuff.com]

Is it a meanwell?

Nope, a cheap one, so I don't have super high expectations of it and might go with a meanwell once I've tested everything.

1. It will easily outlast grandpa, especially if it rarely has to provide high current.

Thanks @justgary for your post! Yes, I think VERY VERY rarely

You should put it in a vented enclosure to keep dust, fingers, spilled coffee, etc. out of it. Consider adding a quiet fan and proper cord grommets to the enclosure. It will be quieter than his CPAP, which these days is pretty quiet.

Ah, good point that the CPAP isn't quiet. I think I'm just thinking about the morning / during the day when the cpap isn't running. Here is the box I purchased for it:

Search for "vented hobby enclosure" and "quiet pc fan." Noctua makes very good, quiet fans, but some other brands have come along recently.

I have a handful of these fans for my various 3D printing projects

I don't have a fan or power supply, but here is my box with a 4S 230 Ah battery, BMS, and Anderson connector. It is not really meant to be portable, but it easily could be. The point here is that enclosures can be cheap and work really well by adding a few holes in the right places.

That looks great, thanks for posting!

 

[BerndL]

Thanks OP and everyone who replied to this. I was trying to decide between that exact Renogy vs a Li-Time for a backup system, so this was incredibly helpful.
Is there really nothing else you have to do with AGMs beyond the constant float? Once it's recharging from a cycle, is there some minimum amperage required that the little float power supply wouldn't be capable of?

 

[justgary]

Thanks OP and everyone who replied to this. I was trying to decide between that exact Renogy vs a Li-Time for a backup system, so this was incredibly helpful.
Is there really nothing else you have to do with AGMs beyond the constant float? Once it's recharging from a cycle, is there some minimum amperage required that the little float power supply wouldn't be capable of?

Check with the specific battery maker of your AGM to see what float voltage they recommend. Normally a lead-acid battery wants to be recharged at about 0.15C, which would be 15 Amps for a 100 Ah battery. Lead-acid has no appreciation for being left uncharged or charged too slowly. Get it done, then let it float.

 

[nifty-stuff.com]

The Renogy AGM battery came today and it was sitting at a nice 12.7 volts.

I set the power supply at 13.4v and connected it with a volt and ammeter and (as expected) saw the voltage drop down a little and the current started "high" at about 5 amps, and quickly dropped down lower and lower and settled in around 1 amp. I then disconnected it because I didn't want it "too full" since I was sure I was going to want to do more testing

Thanks again everyone (especially @sunshine_eggo ) for all the help!

Now my biggest issues/questions:

1. How / will I be able to lug this thing around while it's in the case, or will I need to move the case (with all the electronics) separately?
2. To add a fan or not. I go back and forth since I don't imagine the power supply running very hard 97% of the time.
3. What do I not know that I don't know?

 

[BerndL]

Check with the specific battery maker of your AGM to see what float voltage they recommend. Normally a lead-acid battery wants to be recharged at about 0.15C, which would be 15 Amps for a 100 Ah battery. Lead-acid has no appreciation for being left uncharged or charged too slowly. Get it done, then let it float.

That's why I was curious about the 10A power supply nifty went with. I'd been looking at something similar, but thought AGM charging was more complicated than lithium, requiring a proper CC/CV charger if it was down to ~50% DoD.
(If you were to go with lithium for a project like this, would it be enough to bump your power supply up to 14.something Volts every few months for balancing?)

 

[nifty-stuff.com]

(If you were to go with lithium for a project like this, would it be enough to bump your power supply up to 14.something Volts every few months for balancing?)

That's basically what I understand would be required to keep the internal LFP cells in "balance"... basically going to FULL every few months. (something I'm not able/willing to do with Grandpa's system).

I'm honestly surprised that all these units don't have a fancy BMS that automatically balances the cells regardless of the voltage, but clearly this starts getting way over my head and above my pay-grade quickly!

 

[Rednecktek]

Even most aftermarket BMS's only balance while charging so it's a pretty moot point for your case. Don't worry about it and go for the 50ah or a lead acid.

 

[justgary]

That's basically what I understand would be required to keep the internal LFP cells in "balance"... basically going to FULL every few months. (something I'm not able/willing to do with Grandpa's system).

I'm honestly surprised that all these units don't have a fancy BMS that automatically balances the cells regardless of the voltage, but clearly this starts getting way over my head and above my pay-grade quickly!

For your usage case of standby use with only a few discharges per year, I would be surprised if you need to do a balance charge even once per year. If the imbalance is mild, you won't lose much capacity anyway. Balance when you can, but don't lose sleep over it.