Battery power packs for camping

[CapnLoki]

Keep in mind that in order to get to around 12 volts you need to use 3 of those cells.

Now you have a battery pack of 3 cells with a nominal voltage of 3.7 * 3 = 11.1 volts and a capacity of 42 Ah.

How does that compare to a 12 volt lead acid battery?

This isn't the right way to look at it.

I couldn't find any info on the cell organization of this pack. Since lithium packs almost always have serious electronics to control the charging, and to provide various voltages out, its possible that there could be any organization of the cells. Neither 5 nor 12 volts is "natural" for Lithium Polymer, so we can't tell anything from that. I might guess its 4 cells in series ("4S") because the charger is 15 volts, but it could be anything.

This is why I always like LiFePO4 batteries (Lithium Iron Phosphate) for this purpose because the 3.2V cells in a 4S configuration are 12.8 V, which is then a drop in replacement for Lead Acid. They are also much safer, but they never quite caught on and are still rather expensive.

[amenite]

...
They are also much safer, but they never quite caught on and are still rather expensive.

If the numbers are to be believed the higher cost is somewhat or completely offset by the duty cycle being something like 4-5 times greater than other Lithium chemistries. Also the extremely low self discharge is a nice side benefit. I really wish the price on these would come down to earth.

[CapnLoki]

...
Actually my more immediate need is for a one night solution for a flight overseas. I've got sticker shock from the BPS and Medistrom batteries and moreso from the ResMed solution. I'm scouring ebay and craigslist, but would welcome suggestions. I'm currently hoping to catch an agreeable flight crew who will let me plug into the onboard plugs, despite it being contrary to Delta/ Air France published policy. Going forward, I guess I'll just avoid redeye flights.

I have a bit of trouble advising for this because the market is moving so quickly. You should be able to find a 100 Watt-hour pack legal for airline travel for around $100 to $150. That should be about 28000 milliamp-hours in the Lithium pack vernacular. If I could find a quality one with an easy 12V socket I would be tempted for the occasional one night trip. I didn't bring a battery for an overnight boat delivery last year and regretted it!

[CapnLoki]

...
Actually my more immediate need is for a one night solution for a flight overseas. I've got sticker shock from the BPS and Medistrom batteries and moreso from the ResMed solution. I'm scouring ebay and craigslist, but would welcome suggestions. I'm currently hoping to catch an agreeable flight crew who will let me plug into the onboard plugs, despite it being contrary to Delta/ Air France published policy. Going forward, I guess I'll just avoid redeye flights.

I have a bit of trouble advising for this because the market is moving so quickly. You should be able to find a 100 Watt-hour pack legal for airline travel for around $100 to $150. That should be about 28000 milliamp-hours in the Lithium pack vernacular. If I could find a quality one with an easy 12V socket I would be tempted for the occasional one night trip. I didn't bring a battery for an overnight boat delivery last year and regretted it!

[HoseCrusher]

Let me re-frame the question...

If I have a 40 Ah lead acid battery, I have 6 cells that have 2.1 volts per cell and a capacity of 40 Ah. I want to compare that to a LiPo pack that has 4 cells of 3.7 nominal voltages with a minimum voltage per cell of around 3.0 volts and a maximum voltage per cell of 4.2 volts.

Will a xPAP machine run the same amount of time on both batteries?

I understand that the claimed capacities may be optimistic and we don't know if the LiPo battery is made up of 3 or 4 cells.

On the surface it would appear that the two battery packs would perform about the same.

[amenite]

Let me re-frame the question...

If I have a 40 Ah lead acid battery, I have 6 cells that have 2.1 volts per cell and a capacity of 40 Ah. I want to compare that to a LiPo pack that has 4 cells of 3.7 nominal voltages with a minimum voltage per cell of around 3.0 volts and a maximum voltage per cell of 4.2 volts.

Will a xPAP machine run the same amount of time on both batteries?

I understand that the claimed capacities may be optimistic and we don't know if the LiPo battery is made up of 3 or 4 cells.

On the surface it would appear that the two battery packs would perform about the same.

Your 40 AH lead acid AGM battery will be degraded if you attempt to wring out that last 8 AH of power (discharge below 20% capacity). Worse if it's a flooded lead acid type rather than AGM. Conversely I've read that your LiPo pack will be happier if keep it charged up to about 80% and not all the way up to 100%, but it would not be as dramatic an effect as draining a lead acid battery to 0%. I go by rated watt hours. If there's no watt hour rating I tend to move along to the next one.

[palerider]

Let me re-frame the question...

If I have a 40 Ah lead acid battery, I have 6 cells that have 2.1 volts per cell and a capacity of 40 Ah. I want to compare that to a LiPo pack that has 4 cells of 3.7 nominal voltages with a minimum voltage per cell of around 3.0 volts and a maximum voltage per cell of 4.2 volts.

Will a xPAP machine run the same amount of time on both batteries?

I understand that the claimed capacities may be optimistic and we don't know if the LiPo battery is made up of 3 or 4 cells.

On the surface it would appear that the two battery packs would perform about the same.

Well, to use lithium parlance, that's 40000 mah at 12v.

if you figure 4v for the lithium cells, where they like to specify things, that's 120,000mah equiv.

so, no. besides, you didn't mention your mah ratings on your hypothetical lipo cells.

Besides, as mentioned above, rating things in AH (or worse, mAH) without specifying the *voltage* is meaningless. But, hey, the wankers in marketing love meaningless big numbers.

If you don't convert things to watt hours, you can't meaningfully compare.

40AH lead acid battery: 480 watthours.

Also, you have to take into account whatever buck/boost converters (and their efficiency) are in your lithium pack to get that nominal 12v....

[CapnLoki]

Let me re-frame the question...

If I have a 40 Ah lead acid battery, I have 6 cells that have 2.1 volts per cell and a capacity of 40 Ah. I want to compare that to a LiPo pack that has 4 cells of 3.7 nominal voltages with a minimum voltage per cell of around 3.0 volts and a maximum voltage per cell of 4.2 volts.

Will a xPAP machine run the same amount of time on both batteries?

I understand that the claimed capacities may be optimistic and we don't know if the LiPo battery is made up of 3 or 4 cells.

On the surface it would appear that the two battery packs would perform about the same.

You're overthinking this. Since 12V is not a "natural" voltage for a collection of lithium cells, some "silicon magic" must be performed to create the 12V output. Although we can guess at the performance of the cells (if we knew the type it would help) but they almost never post the specs of the various synthesized outputs. All we can do is take the total watt-hours and apply some safety margin. The anecdotal evidence is that these packs are fairly efficient; it isn't too hard to be 90%+ for this. Since lithium is more forgiving of deep discharge than AGM, I've just assumed its fair in both cases to take 80% as the maximum juice that can be extracted. So if you have a 150 watt-hour lithium pack, that will be roughly the same as a 150 watt-hour AGM pack, which is 12.5 Amp-hours at 12V. Remember, these are all approximations because the actual voltage is changing - is it 12.6 or 12.2 or 11.8? Yes!

Actually, I'd guess the lithium does a bit better in since it can recover better from deep discharge, but the AGM wins points on being cheap enough so you don't worry if you kill it prematurely.

...Also, you have to take into account whatever buck/boost converters (and their efficiency) are in your lithium pack to get that nominal 12v....

exactly - and for us this is a black box.

[HoseCrusher]

palerider, let me clear things up...

LiPo cells charge to 4.2 volts, deliver most of their capacity at about 3.7 volts, and then are fully discharged at somewhere between 3.0 and 3.2 volts.

If we look at a full charge, a 3 cell battery would have 3 X 4.2 volts = 12.6 volts and if it is made up of 40 Ah cells it would end up with 12.6 * 40 = 504 watt hours.

If we look at "nominal voltage" of 3.7 volts, that same battery would have 11.1 * 40 = 444 watt hours.

If the battery pack is made up of 4 cells we would have 672 watt hours at full charge and 592 watt hours at "nominal voltage."

Keep in mind that when you put cells in series you add the voltage and the capacity remains the same.

Looking at the "nominal voltage" of a 3 cell pack we come up with 3.7 * 3 = 11.1 volts. If your application will run at 11.1 volts no additional circuitry is needed. With a 4 cell pack that "nominal voltage" is 14.8 volts.

The issue with a 4 cell battery is that its maximum voltage will be 16.8 volts and that may be a little high for some applications. The 3 cell battery comes in at 12.6 volts and that seems to be more in line with 12 volt applications.

When looking at possible run times I think it is better to look at "nominal voltage" and using that times cell capacity to determine watt hours.

[palerider]

palerider, let me clear things up...

LiPo cells charge to 4.2 volts, deliver most of their capacity at about 3.7 volts, and then are fully discharged at somewhere between 3.0 and 3.2 volts.

If we look at a full charge, a 3 cell battery would have 3 X 4.2 volts = 12.6 volts and if it is made up of 40 Ah cells it would end up with 12.6 * 40 = 504 watt hours.

If we look at "nominal voltage" of 3.7 volts, that same battery would have 11.1 * 40 = 444 watt hours.

If the battery pack is made up of 4 cells we would have 672 watt hours at full charge and 592 watt hours at "nominal voltage."

Keep in mind that when you put cells in series you add the voltage and the capacity remains the same.

Looking at the "nominal voltage" of a 3 cell pack we come up with 3.7 * 3 = 11.1 volts. If your application will run at 11.1 volts no additional circuitry is needed. With a 4 cell pack that "nominal voltage" is 14.8 volts.

The issue with a 4 cell battery is that its maximum voltage will be 16.8 volts and that may be a little high for some applications. The 3 cell battery comes in at 12.6 volts and that seems to be more in line with 12 volt applications.

When looking at possible run times I think it is better to look at "nominal voltage" and using that times cell capacity to determine watt hours.

What does this have to do with anything?

btw, the biggest lipo cells I could find on a brief browse were 5ah.

[HoseCrusher]

You need to expand your search...

https://www.alibaba.com/product-detail/ ... 667f8ywKXC

[palerider]

You need to expand your search...

https://www.alibaba.com/product-detail/ ... 667f8ywKXC

well, pick up a thousand of 'em (minimum order) and let us know how it comes out.

[CapnLoki]

palerider, let me clear things up...

LiPo cells charge to 4.2 volts, deliver most of their capacity at about 3.7 volts, and then are fully discharged at somewhere between 3.0 and 3.2 volts.

If we look at a full charge, a 3 cell battery would have 3 X 4.2 volts = 12.6 volts and if it is made up of 40 Ah cells it would end up with 12.6 * 40 = 504 watt hours.

If we look at "nominal voltage" of 3.7 volts, that same battery would have 11.1 * 40 = 444 watt hours.

If the battery pack is made up of 4 cells we would have 672 watt hours at full charge and 592 watt hours at "nominal voltage."

Keep in mind that when you put cells in series you add the voltage and the capacity remains the same.

Looking at the "nominal voltage" of a 3 cell pack we come up with 3.7 * 3 = 11.1 volts. If your application will run at 11.1 volts no additional circuitry is needed. With a 4 cell pack that "nominal voltage" is 14.8 volts.

The issue with a 4 cell battery is that its maximum voltage will be 16.8 volts and that may be a little high for some applications. The 3 cell battery comes in at 12.6 volts and that seems to be more in line with 12 volt applications.

When looking at possible run times I think it is better to look at "nominal voltage" and using that times cell capacity to determine watt hours.

I'm not sure what the point of this is - the methodology of measuring cells is pretty well understood, though there isn't wide agreement on how the marketeers should publish. One thing is that Watt-hours is defined as nominal voltage times amp-hours. Amp-hours however vary with the application so the "rated capacity" is used. This is very conservative, which is why anecdotally new packs often outperform the specs. Off brand manufacturers suppliers (such as Ali) will of course fly by different rules from Panasonic. Here's an interesting blog on the type of Lithium cells used in the Tesla S and X:
https://batterybro.com/blogs/18650-whol ... c-18650-be

On the issue of 3S versus 4S configurations, neither works well for a general 12volt supply. 4S is clearly too high, and 3S might trigger undervoltage when its only partially discharged. I would assume that all such commercial packs have a variable output regulator, so it makes no sense to try to figure out how a 3S pack would behave with a cpap. I have wondered though, how the ResMed 12-24 volt converter would handle a 4S input - it claims to cover a wide range of voltage, but I'm not sure its all voltages and what the full range is.

[HoseCrusher]

Any opinions on this?

https://www.amazon.com/42000mAh-Portabl ... moso+42000

Combined with my ResMed DC converter cigarette lighter adapter, it looks like a good solution for several days use for my ResMed Autosense 10. $80 and under three pounds.

ResMed says the Autosense 10 pulls .93 amps at pressure of 10 without the humidifier.

https://www.resmed.com/us/dam/documents ... lo_eng.pdf

42000 mah, less 30% fudge factor would be 29400 mah. .93 amps would equal 930 mah

29400/930 = 31.6 hours of use.

Does this math look right? Thoughts?

There's a problem with the math. The rating of "42000mAh" is not measured at 12 Volts, which is the traditional standard. In our new world of Lithium cells this is actually measured at 3.6 or 3.7 Volts. The proper way to compare is to convert to Watt-hours as in: 42000 mAh X 3.7 volt => 155 Watt-hours, which would compare to about 13 Amp-hours at 12 volts. So this would probably not work for 2 nights.

I'm actually a bit surprised the load is that high - I thought the S9 was much less, and certainly the Respironics 560 auto is more like 0.5 amps. Also, the limit for airline travel is 100 watt-hours, so this exceeds that by a fair amount.

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

[CapnLoki]

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.