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

[nifty-stuff.com]

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.

Seems like the info on this is mixed. It sure would be nice if balancing wasn't an issue / concern with LFP.

 

[Rednecktek]

For the most part in off-the-shelf batteries it isn't. If it does start to drift it should rebalance itself when the power comes back on. Don't be too paranoid about the 1 or 2 percent capacity from drifting cells in a new factory battery.

in other words, for your project, don't worry about it.

 

[nifty-stuff.com]

I appreciate all the help and feedback!

I ended up just going with the Renogy 100Ah AGM.

Here's my setup so far (it's a mess, but still just testing everything before I fully commit).

I've got the power-supply unplugged to just test the 12v outlet.

I've got a 12v plug (a 3D printed a cigarette plug since I can't find any at my house!?!?) that I stuck in the outlet.

NOTE: I'm intentionally using small-gauge wires from the outlet to test if/how warm they get

That's going to my homemade volt/ammeter.

Which then goes to a 500 watt inverter

Then into a killowatt type meter

... which is then powering:

• Box Fan on high
• Soldering Iron
• Dremel on high

You can see I'm pushing a lot of juice... 12 amps, which is dropping the voltage close to the battery to about 12.2, and then a pretty big voltage drop at the homemade meter (in part due to the long thin wires).

Grandpa's CPAP shouldn't ever draw close to this much power... but glad to know it can handle any big spikes if ever needed.

I ran it like this for about 10 minutes and I could feel the tiny wires getting a little warm, so I turned off the dremel and power dropped to about 8 amps, which it seemed perfectly happy running continuously.

I'm going to drain the battery down a bit and then plug in the power supply and see how that does.

One thing I'm considering changing is bypassing the 12v car adapter / plug and cutting the 12-24v ResMed adapter and wiring it right to the battery.




Oh and you'll see the 3D printed pentagon bolt covers I 3D printed for both safety and the ability to screw the bolts by hand :D

So, for those curious 3D printed parts in this pic:

1. Bolt heads
2. Homemade meter box cover
3. Cigarette plug adapter
4. Car Fuse holder (from power supply to battery)

:D

Looking forward to your thoughts, feedback, criticisms, etc.

 

[Bud Martin]

I can't see the picture.

 

[sunshine_eggo]

I can't see the picture.

Same

 

[nifty-stuff.com]

OH! weird... thanks for pointing that out. is it showing now?

 

[nifty-stuff.com]

Update: I drained the battery by about 30% and then plugged in the power supply. I should have been, but wasn't measuring the DC from the PS, but going into the PS was 225 watts via the AC killawatt.

... and the power supply was getting TOASTY hot. Not "melting plastic" hot, but also too hot to keep my hand on it more than a second or two.

The PS is "attached" loosely to the battery top with some foam double-sided tape. Just enough to keep it from slipping around, but also to give a little air between the PS and battery.

I'm not super comfortable with it being so hot, even though I believe it should be rare that it's pumping that much power for long periods of time.

I wanted to avoid a fan, but I'm wondering now if it will be necessary

Wondering if a smaller (slower / lower amp... like maybe 2-3) charger would be a better option?

Curious what you guys think?

 

[justgary]

The rule of thumb with temperature is that the device is below 120*F if you can keep your thumb on it indefinitely. Above that temperature, you can't keep your thumb on it. The quicker the thumb removal, the hotter it is. I would use a small fan, and add some ventilation holes that allow the fan to move air across the power supply efficiently.

30% is a very deep discharge for lead acid, and will greatly affect the service life of the battery. It is best to not discharge below 50%. Recharging as quickly as possible after a discharge is very important also. A smaller charger will not provide a fast enough charge to help save the battery from sulfation.

How do you have your charger set? I suppose it is best to set it to the charge voltage to 13.65v per the Renogy data sheet. Set the voltage on the charger before you attach a load or the battery. With 225 Watts in, you were probably getting around 14 Amps at 13.6 VDC out (assuming 85% efficiency).

 

[nifty-stuff.com]

The rule of thumb with temperature is that the device is below 120*F if you can keep your thumb on it indefinitely. Above that temperature, you can't keep your thumb on it.

That's helpful, thanks!

30% is a very deep discharge for lead acid, and will greatly affect the service life of the battery.

Sorry, I wasn't very clear. I should have said "discharged to 70%".

How do you have your charger set? I suppose it is best to set it to the charge voltage to 13.65v per the Renogy data sheet. Set the voltage on the charger before you attach a load or the battery. With 225 Watts in, you were probably getting around 14 Amps at 13.6 VDC out (assuming 85% efficiency).

I believe it's just a 10 Amp dummy power supply set at 13.6v output. There is no way to control the current as far as I know. I thought it would just deliver 10A continuously and safely (even without a fan) until the voltage of the battery got closer to the set 13.6 v. and the current would drop down to almost zero... which it more/less eventually did, and the last hour or so of charging, the PS was only just warm as it wasn't pushing as much current.

 

[Bud Martin]

BTW, Do you have full spec of the power supply?
What kind of OL/OC protection does it have?

From MeanWell:
Q69 What are the protection forms of overload/overcurrent?
When current drawn exceeds the rating of the PSU, the protection circuit will be triggered to protect the unit against overload/overcurrent.
Protections of overload/overcurrent can be divided into several forms:
(1) FOLDBACK CURRENT LIMITING
Output current decreases about 20% of rated current, shown as curve (a) in the figure below.

(2) CONSTANT CURRENT LIMITING
Output current remains at a constant level and within the specified range while the output
voltage drops to a lower level, shown as curve (b) in the figure below.

(3) OVER POWER LIMITING
Output power remains constant. As output load increases, output voltage decreases in
proportion, shown as curve (c) in the figure below.

(4) HICCUP CURRENT LIMITING
Output voltage and current keep pulsing ON and OFF repeatedly when protection is activated.
The unit automatically recovers when faulty condition is removed.

(5) SHUT OFF
Output voltage and current are cut off when output load reaches protection range.
NOTE: Protection mode of some of the products combines with different types of the forms
mentioned, such as constant current limiting + shut down.

Recover method:
(1)Auto Recovery: PSU recovers automatically after faulty condition is removed.

(2)Re-power on: PSU restarts by manual AC re-power on after faulty condition is removed.
Note：Please do not operate PSU in overcurrent or short-circuit condition for a long period of
time to prevent a shorten lifespan or damaging the PSU.

 

[nifty-stuff.com]

BTW, Do you have full spec of the power supply?
What kind of OL/OC protection does it have?

Unsurprisingly, this cheap one from China has zero specs and I'm guessing no OL/OC protection

I guess to cover my bases I should upgrade to a MeanWell with built-in fan?

 

[justgary]

Unsurprisingly, this cheap one from China has zero specs and I'm guessing no OL/OC protection

I guess to cover my bases I should upgrade to a MeanWell with built-in fan?

Or since you have an AGM battery, just get a NOCO Genius charger to throw in the box. Remove the clips and crimp ring terminals on. A 5A will be a bit slow recharging, but overall will probably keep your battery in better shape than a constant float will. Presumably you will get it back after Grandpa is done with it.

 

[nifty-stuff.com]

Thanks. I didn't think I needed a "smart" / fancy charger, but I guess that's the best/safest way to go?

 

[Camp20144]

@nifty-stuff.com I have a NOCO genius 10. They are great! You set it and forget it. It monitors the battery and acts accordingly. In the NOCO instructions, it says you can leave the charger connected indefinitely. You might want to check them out.