And a friendly Wow back at 'cha!
I am amazed at the total lack of information concerning battery pack details.
Yes, its generally impossible to really figure out what's in a black box.
HoseCrusher wrote:My experience comes from working with jump start battery packs. They are usually 3 cells in series and utilize the ability of lithium ion chemistry to provide a large surge current for a short period of time....
I've been amazed at the ability of Lithium cells to deliver huge amounts of current - I never would have guessed that a small pack could start a car. That said, there are a few differences. First of all, the starter consumes about 1 kW, but only runs a few seconds, so thats about 1 Watt-hour or 0.08 Amp-hours per start! Secondly, cells that are chosen for lithium jump starters are designed for high output - they aren't the same you'll find in packs designed for slower discharge, and its not clear you would want them for cpap power. Also, car starters demand a lot of current but are rather forgiving of the voltage. Traditional starting batteries can't deliver current when partially depleted so we tend to think the starting motor needs 12 volts, but they will turnover with 11 or even 10 volts if enough amps are supplied. Electronics, however, are different and they will often shut down at some under-voltage, such as 11 volts. The fridge on my boat shuts down a bit under 11.5 volts, so I tend to consider that the low limit.
HoseCrusher wrote:Are you aware of any testing done that shows voltage under load as a function of time? It seems that would be a reasonable way to evaluate a battery pack.
Of course, since I live off grid on battery power for several months every year, and have done this for 20+ years, I'm quite familiar with voltage/discharge charts, and have posted them here on occasion. For example, a few days ago in this thread I posted this link in response to your post. I'm guessing you didn't scroll down to the middle where two versions of the 18650 cell are compared.
https://batterybro.com/blogs/18650-whol ... c-18650-be
Looking at the first chart, a few things of interest to this discussion are apparent: All of the tests are a single cell with a 2.5 Amp load. Not quite the same as a cpap but close enough to extrapolate. The cell is able to meet the load at its full range of voltage, starting at about 4 volts and ending at 2.5 volts, with a runtime of 74 minutes. Converting to a 3S configuration, this would be starting at 12V going down to 7.5V. It hits 11V at less than 25% discharged, and 10V at about 62% discharged (or 38% State of Charge). Another point: this was a comparison between two different versions of the same type of cell, one designed for 10 amp discharge, the other for 3 amps. They have very similar characteristics, but the high load cells take 40% longer to charge. Clearly some skill is needed to select the proper cell for an application.
So the question is, at what point will a particular cpap shut off from low voltage? I can't say I've tested this. I've looked for specs, but everything just says "12V." There have been anecdotal reports of some equipment shutting down at 11.5V, and I've definitely seen that with non-cpaps. I wouldn't be surprised if some gear works down to 11V or even below, but I wouldn't count on it, or even be comfortable relying on it even if it worked once! The ResMed 12/24 converter is a special case, since it already is a voltage regulator it could be designed to take any input. Their spec only says 12 or 24 volts. In other words, if we took 11 Volts as necessary for a cpap, then a 3S configuration would only allow us to see about a quarter of the battery power! Clearly research is needed before relying on this configuration.
On the other hand, a basic DC-DC converter/regulator can be had for a low cost. Why not simply add one and know that you have a good voltage? My point has been that all of the packs we've discussed recently, generally targeting campers needing to charge cell phones and tablets, will have this. Car starters, such as the Antigravity Micro-start line also tend to have 5V, 12 V and 19 V regulated outputs in addition to the high current car charger output, although their total capacity is a bit low. So it may be possible for the hobbyist to build a pack suitable for cpap backup power, it isn't just as simple as wiring up three cells. On the other hand, there are a growing number of battery packs available for the consumer that will suit the purpose.