Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
CPAP gets 18650ed.
Not much progress, but I did get the battery all dressed up and labeled.
-Mike G
I think having the BMS sitting on multiple cells without any padding/isolation is a potential hazard. This is going on road trips / camping trips and get shaken/bumped a lot along the way, right? Pointy parts on the back could rub through the cell wrappers and eventually cause a short.
Korishan likes this post
Modular PowerShelf using 3D printed packs.  60kWh and growing.
It's there, just not pictured.  Smile

Also, I actually made a little progress with the box.

Unfortunately, the charging hardware doesn't fit. But all the stuff necessary for camping does. I still need to fit the power meter and switch in such a way that it doesn't flop around during travel. The battery fits pretty snug, but I'd also like to make sure it doesn't have space to move around after the box is shut.
ajw22 likes this post
-Mike G

I finally took an afternoon to build the little power box.

The Watt meter will keep track of the total energy used, and there is a low Voltage alarm that I can set myself that should go off before the BMS low Voltage cutoff does. I think I'll keep the little battery checker in the box too. It gives some peace of mind knowing the series are all balanced.

I'm hoping to get around 32 hours of sleep between charges. I'll start using it tonight and see what happens. Also, someone asked how much the whole thing weighs. It is 21 lbs.
Korishan and ajw22 like this post
-Mike G
I have some early results from my first night on the CPAP battery. It looks like I may have overestimated the power requirements with the heat and humidifier off. According to the power meter, I used 42 Wh, and the CPAP itself counted up 7 hours 49 minutes. When I woke up, the meter was reading 0.3A instead of the 0.8A I was expecting.

So 42 Wh / 7.82 h = 5.37 Wh/h, approximately.
The battery should have about 632 Wh, so 632 Wh / 5.37 Wh/h = 117.6 hours of use.
That's 14.7 nights if I sleep 8 hours per night. Around 2 weeks! I was hoping for 4 days.

I'll keep using the battery until it's discharged, and see what the numbers are then. Hopefully my math's not wrong!

Also, for anyone who's curious, I ended up building this whole thing for about $180. I don't normally keep track of the cost of my projects. Takes some of the fun out of them. But in this case, I wanted to compare what I could do with what is available store-bought. The cheapest battery I could find would have cost around $250 per 8 hour night. And there wouldn't be a nice foam padded, water proof box either.
Korishan and jdeadman like this post
-Mike G
Your're right, with just the fan, the CPAP hardly makes a dent in the capacity! I tried running the humidifier/heated tubing too (in airplane mode). I can get 3 nights with the additional load, and the pack is in the 5% SOC range. My pack is a similar Wh capacity as your's, albeit a different voltage (663Wh, 7s8p).

Fan only: 5.4W draw
Bells and whistles: 27.6W draw
I ran the CPAP off the battery for 12 days, I think. It shut off some time at night, but I was able to get some numbers.

It looks like one of my series discharged faster than the others. When it reached 2.5V the BMS did its job and shut the whole thing down. So I didn't get the full 632 Wh I was expecting. And there appears to be a fault somewhere in the battery.

Here's what I got though. The CPAP itself recorded 69.97 hours of total use. The energy meter recorded 412 Wh total. An average of 6.34W. I also noticed that the regulator uses about 1W even when the CPAP is unplugged. So that seems consistent with Lucan's observations.

My first thought about the battery is that one or more fuses might be blown or broken. That would reduce the capacity of a series, and those glass axial fuses are pretty fragile. After visually inspecting each one, and checking continuity between one cell and all the others, It seems the fuses are all intact. I'll have to open the other side of the battery and check the solder joints.

If I can't find anything physically wrong, I'll test each series individually for capacity. Depending on what I find, the whole battery may have to come apart. All these cells are the same make/model and all tested with similar capacities. I packed them with repacker, so it seems unlikely that the cells would be at fault. More likely I did something wrong. But I can't have a battery that's going imbalanced after a single discharge. I'll just have to test everything until I find the culprit.

But on the bright side, I got 12 nights out of this battery when I was initially expecting only 4. That means if I need to rebuild the battery, I can make it smaller and still have plenty of capacity.
Korishan likes this post
-Mike G
On those LG cells the cutoff is 2.75 volts. When you read 2.5 volts, did it bounce back up with no load. I aim for 3 volt cutoff on my packs.

I would only take the pack apart if one of the cells is severely out of balance with the other cells and multiple charging discharging doesn't get it to balance. If you can't get a full charge even with a balance charger, then you might have a cell that is self discharging. All it takes is one bad cell to prevent the pack from being in balance.

Instead of the cellmeter8 I recommend the tenergy lipo checker cellmeter-6 (about 12 dollars on amazon), it's smaller and will give a loud alarm if the cell voltage gets too low (programmable) of if the cell voltage goes above 4.22 volts. If you do have a bad cell, you want something to alarm if the pack voltage goes too low. I had a pack that I didnt use for a few weeks, when I decided to use it , found it had drained completely, the cells were at 1 volt. If the pack had an alarm I could have saved the pack. 

Yes, the low series eventually recovered to about 3V. If they had all discharged evenly, I would have heard an alarm at 16.8V for the batterty, or 2.8V per cell. I haven't determined yet why the one series discharged so much quicker than the rest. The battery was balanced when I started. The cell checker is just to give me a quick view of the individual series. I don't have it plugged in all the time. Fortunately, the BMS did its job and cut the battery off when it sensed one of the series was too low. Better 2.5V than 1V eh? Smile
-Mike G
I got all 6 series tested on the CPAP battery, and here's the results:
1: 23.2 Ah
2: 27.1 Ah
3: 26.1 Ah
4: 15.2 Ah
5: 26.6 Ah
6: 18.5 Ah

Series 4 was definitely the one that caused the BMS to shut off the whole battery. But series 6 wouldn't have been far behind. I'm not sure why there is such a large variation in capacities. I'm definitely going to pull the battery apart and retest each cell individually.

When I rebuild it though, I'll be making it smaller. Probably 5p instead of 11p. That'll still give me about 5 nights sleep, reduce weight, and take up less space in the box.
-Mike G

Forum Jump:

Users browsing this thread: 1 Guest(s)