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stevenal wrote: ↑

Thu Jan 13, 2022 2:05 pm

So you took 18Ah/16 cells =1.1Ah per cell. Trouble is, we have the same number of hours (8) and the same number of amperes (2.2) in each cell. Cells in series increase voltage, not amps. If your starting numbers are correct, each D cell will need to supply the full 18 Ah. If you want 1.1 Ah per cell, you will need 16 parallel strings of 16 cells in series, or 16^2 cells total.

Your math is oversimplified and thus confusing.

You can't look at Ah separately, you *HAVE* to factor in the voltage, Just talking about "18ah" in isolation is meaningless.

and your 'requirement' for 256 D batteries is ludicrous.

That is the point, voltage is useful. 18 Ah was not my number, I just took it to it’s logical conclusion. See my other post where Wh is used. Again I take no responsibility for the inputs that were supplied by others here.

palerider wrote: ↑

Thu Jan 13, 2022 4:51 pm

stevenal wrote: ↑

Thu Jan 13, 2022 2:05 pm

So you took 18Ah/16 cells =1.1Ah per cell. Trouble is, we have the same number of hours (8) and the same number of amperes (2.2) in each cell. Cells in series increase voltage, not amps. If your starting numbers are correct, each D cell will need to supply the full 18 Ah. If you want 1.1 Ah per cell, you will need 16 parallel strings of 16 cells in series, or 16^2 cells total.

Your math is oversimplified and thus confusing.

You can't look at Ah separately, you *HAVE* to factor in the voltage, Just talking about "18ah" in isolation is meaningless.

and your 'requirement' for 256 D batteries is ludicrous.

Actually he's right. My dividing the charge among 16 batteries was specious. Each battery is required to output and average of 2.2A, so over 8 hours that's almost 18Ah from each battery. Not only can an LR20 not sustain 2.2A but it's pretty optimistic to expect that much charge over that kind of battery's lifetime. I guess 2 packs of 16 batteries could give me the charge I need, but then there's still the voltage depression from drawing 1.1A from each battery, which would occur in a matter of minutes. So clearly, I couldn't make it through the night unless I had 100s of these batteries. This idea is dead.

Do you really need that much capacity? See my example above using Loggerhead's numbers.

As another example, my Z1 has a 30 watt power supply, which need to work with 20 cm H2O without humidification. Your 18 Ah at 24 V and 8 hrs would be 54 W, so I think you may be able to do with less.

I'm with Logger, I'd like to see you test it out.

Hi, Regular alkaline D-Cells will not work in powering a CPAP machine. Years ago I actually tried this with an 8 cell (12 volt) battery holder. The Duracell D-Cells lasted all of roughly 30 minutes before the voltage dropped too much and the CPAP shut off. The 0.5 - 0.6 amp my then CPAP drew was too heavy. The issue is that alkaline batteries are not designed for extended heavy drain. They are designed for intermittent use or a very light extended drain. Also during use they exhibit a significant voltage decline starting at about 1.65 volts per cell and then considered fully used at around 1.3 volts. You can see this with old-style non-led flashlights where the beam gets progressively dimmer over time. Electronic devices that run on alkalines are designed to run at a voltage range that covers the useful life of an alkaline battery. You can pull up D-Cell manufacturer spec sheets that illustrate how the voltage declines over time.

After this failed experiment I did a lot of research and build myself a modular system based on 20 amp hour (Half U1 size) lead acid agm batteries that power my CPAPs for two nights (no humidifier). Built a couple of these and later added a 30 watt solar panel for use when more than 4 days of CPAP was required. I've shared details on these via a number of early posts if anyone is interested. While lead acid was the only real cost effective option when I started building my modular system 10 years ago anyone starting from scratch today should also consider lithium based batteries that, while still more expensive than agm for the same capacity, are less costly than in the past. Have considered replacing my agm batteries with lithium but, given my intermittent use pattern and concerns about the quality of the made in china built-in lithium battery management systems, have held off for the time being.

I the voltage drop issue is why I would recommend using a DC to DC convertor such as the one Resmed sells. Make sure it you are always within the input range, and your output will be constant. 13 cells in series seemed like a fair bet for Resmed in my example above. If the 1.3 V is accurate, you are still well above the 12 V needed. Not exactly a standard battery holder, so you might want to try 12 or 14 instead.

And 0.6 A * 8 h * 12 V= 57.6 Wh. According to Wikipededia a D cell is good for 10 Ah (15 Wh) at a half amp, so one string should more than suffice for a night.

Wilbur Says: Put me out of my Misery, I've been beaten more than needed, put the "D" Batteries in your Cattle Prod, and send me to that Great Barn in the Sky. Instructions for Making a Silk Purse out of a Sows Ear can be found on U Tube. I think about 300 "D" Cells wired in series will be enough, but you will need to wait for the smoke to clear away to be sure. WARNING do NOT do this at home, the voltage would be 450 volts!

Back in days long gone they made Radios that didn't run on A/C current, they ran on two batteries one about the size of a 24 can case of Coke, it powered the vacuum tubes heaters, and a smaller battery to power the radio. Then aliens gave us the knowledge to make transistors, I bet they were sorry about that. Jim

stevenal wrote: ↑

Fri Jan 14, 2022 9:06 am

I the voltage drop issue is why I would recommend using a DC to DC convertor such as the one Resmed sells. Make sure it you are always within the input range, and your output will be constant. 13 cells in series seemed like a fair bet for Resmed in my example above. If the 1.3 V is accurate, you are still well above the 12 V needed. Not exactly a standard battery holder, so you might want to try 12 or 14 instead.

And 0.6 A * 8 h * 12 V= 57.6 Wh. According to Wikipededia a D cell is good for 10 Ah (15 Wh) at a half amp, so one string should more than suffice for a night.

Considering how things have gone while down this rabbit hole, I might be resorting to a DC-to-DC converter, but the thing about having a lower voltage at the input is that it will require a higher current. We've already established that 18 batteries would have to sustain 2.2A, which it would only be able to do for a few minutes before the voltage goes through the floor, i.e. you can't get those 10Ah in as little as one night. Whatever current this converter requires, I won't be able to supply it w/D cells.

Never underestimate the power of D cells.
The earliest Prius models ran on them--LOTS of them.