Battery power packs for camping

[palerider]

In order to get a battery pack close to 12 volts you need to put 3 cells in series. Now you have higher voltage than the 3.7 volts stated. Higher voltage battery pack means higher Watt hours.

Plus, there's a misunderstanding of basic principles here, or at least a mis-stating.

One can't get higher watt hours by re-arranging batteries, you're trading off amp for volt by going from parallel to serial (and you, cap'n, well know). Watts remain the same.

[HoseCrusher]

In order to get a battery pack close to 12 volts you need to put 3 cells in series. Now you have higher voltage than the 3.7 volts stated. Higher voltage battery pack means higher Watt hours.

Plus, there's a misunderstanding of basic principles here, or at least a mis-stating.

One can't get higher watt hours by re-arranging batteries, you're trading off amp for volt by going from parallel to serial (and you, cap'n, well know). Watts remain the same.

Last time I checked Wh comes from Ah times V.

You can increase Wh by taking the same Ah and multiplying it by a multiple of V.

For example a 40 Ah 3.7 V batter has 148 Watt hours where a 40 Ah 11.1 V battery (3 cells in series) has 444 Watt hours.

I increased the Watt hours by adding additional cells to the battery.

[palerider]

In order to get a battery pack close to 12 volts you need to put 3 cells in series. Now you have higher voltage than the 3.7 volts stated. Higher voltage battery pack means higher Watt hours.

Plus, there's a misunderstanding of basic principles here, or at least a mis-stating.

One can't get higher watt hours by re-arranging batteries, you're trading off amp for volt by going from parallel to serial (and you, cap'n, well know). Watts remain the same.

Last time I checked Wh comes from Ah times V.
You can increase Wh by taking the same Ah and multiplying it by a multiple of V.
For example a 40 Ah 3.7 V batter has 148 Watt hours where a 40 Ah 11.1 V battery (3 cells in series) has 444 Watt hours.
I increased the Watt hours by adding additional cells to the battery.

Like I said, a mis-stating. "higher voltage" does not mean "higher watt hours". MORE CELLS means higher watt hours.

take your 40ah 3.7v CELLS, indeed, 148 wh each. three of them in parallel would give you a 3.7v 120AH battery, or 444wh.
if you put them in serial config instead, you'd have your 11.1v 40Ah battery, or... 444 wh.

There's no good reason to put three in series, you STILL need a boost converter to get it to a usable voltage for things that are designed for 12.6v nominal, you get better cell management just using them in parallel and boosting the output voltage to what's needed.

[HoseCrusher]

The best reason to put the cells in series is that buck circuitry is more efficient than boost circuitry.

Keep in mind that a 12 volt battery is made up on 6 cells of 2.1 volts connected in series.

[palerider]

The best reason to put the cells in series is that buck circuitry is more efficient than boost circuitry.

Keep in mind that a 12 volt battery is made up on 6 cells of 2.1 volts connected in series.

And if one of them gets a bit out of balance on charging/discharging, it doesn't burst into flame.

[HoseCrusher]

My involvement with this thread was an attempt to clarify this statement. 42000 mAh is the individual cell capacity.

I have no argument with this - its a well established and often discussed aspect of Lithium pack specs.

In order to get a battery pack close to 12 volts you need to put 3 cells in series. Now you have higher voltage than the 3.7 volts stated. Higher voltage battery pack means higher Watt hours.

If you want to use a lithium replacement battery in your car you should go for a 4 cell battery. That seems to handle the output from the charging system much better than a 3 cell battery would. The advantage of lithium is with momentary high discharge loads. I can start my car using an 8 Ah battery with power to spare.

An 8 Ah battery won't power your CPAP for very long, but it will start your car.

This where I take issue: while your points about 3S and 4S are correct, they simply do not apply to cpap battery packs. Perhaps if you have specific experience with powering a cpap with bare lithium-poly cells it would be appropriate to advise on how to do it, but as near as I can tell this is bad practice, and certainly not for folks who do not understand all of the issues. Every Lithium power pack that we've ever discussed here is packaged with battery maintenance hardware that controls the charging, keeps the cells balanced, and provides a synthesized, regulated output. I've only found occasional reference to the internal organization. Its quite possible that all the cells are wired in parallel and a regulator bumps it up to 12V, or there could be some other organization, but for us its all a black box.

You can certainly find much talk about these issues in the RC model racing and drone forums, and I would encourage anyone so inclined to explore these. And I'd love to see a report on a successful (or even unsuccessful) cpap application.

Wow...

I am amazed at the total lack of information concerning battery pack details.

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. This allows you to start the car with the dead battery in spite of a voltage mismatch. These packs incorporate circuitry to control the charging process but usually there is no provision for controlling discharge except for a low voltage cut off.

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.

[CapnLoki]

Wow...

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.

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.

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.