Battery Recomendations

[CapnLoki]

So I went ahead and looked up some stuff and have pretty much answered my own questions. The PR DC system is about $300 for a lead acid type with a charger. It is rated at 15 ampere hours and weights 13 pounds. Here is a solution using RC airplane hobby grade Lipo batteries. It is both lighter and less expensive. 2.6 pounds for the batteries and costs $158 dollars (plus some shipping) for the batteries, cable, and slow charger (4 hours) or $235 with a faster charger (1.5 hours). Interested? Then read on.

This battery is 5 Ah and costs about $29 dollars
http://www.hobbyking.com/hobbyking/stor ... duct=16768
You need three in parallel to get get 15 Ah so that is $87 dollars for the batteries. The weight of three combined is only 2.6 pounds. You could easily get as many as 6 connected if you needed more power or more time.

You need a cable to connect the batteries together for both operation and charging. Here is a cable that can connect up to six of those batteries and costs $13
http://www.progressiverc.com/parallel-6 ... cable.html

However, you need a special charger for this kind of battery and the price depends on how fast you want to charge. Here is the smallest charger I have. It costs $58 and will run off AC or your car battery. It will charge the three batteries in about 4 hours.
http://www.amazon.com/Thunder-Balance-C ... nder+AC680

This a medium power charging I have. The cost is $135 and will charge 15 Ah is 1.5 hours.
http://www.progressiverc.com/the-starte ... combo.html

I realize this might be a little too DIY for a lot of the folks here but I already have the chargers so for me it would be a nobrainer when I need DC. I am happy to help anyone that wants to go this route. People might say that LiPo batteries are dangerous. They are a fire hazard and will ignite by themselves if you rupture them open somehow. But if you are reasonable careful, they are no more hazardous then a lot of the other things we deal with everyday.

It will be nice to have someone with real experience with Lithium batteries in the forum. I do see a few issues with your solution - first, the 3S1P battery is only 11.1 Volts which may not be enough to startup a 12V cpap. Many users have reported issues that appear to be undervoltage related. Have you tried this with a PRS1 560? Second, the thought of running several of these in parallel will be daunting to many users. Just the concept of connecting a wire to battery leaves a large number of readers here out, so it would be nice if there was a larger power pack. And although the battery is surprisingly inexpensive, the charger raises the entry price considerably.

The "competition" for lightweight lithium battery discussed here is the Poweradd 32000 which, IIRC, is about 8 AH at 12 volts. Cost with charger is $129 on Amazon. Personally I've been curious about LiFePO4 batteries. Although a bit heavier, they are a good match for 12 volt appliances and chargers, not to mention somewhat safer than LiPO. How about something like: http://www.batteryspace.com/lifepo4-pri ... d-dgr.aspx
Since I don't need a lightweight pack, I use a AGM U1 battery (35AH, $60, 24 lbs) for backup at home, and a flooded golf cart battery bank on my boat.

[rhodesengr]

I do see a few issues with your solution - first, the 3S1P battery is only 11.1

Not to worry. The 11.1V number is a "nominal" voltage based on 3.7V/cell. That is actually the voltage at 50% discharge. LiPo cells are 4.2V when fully charged and decrease more or less linearly as you discharge them. So the (3S) battery I posted, any 3S LiPo really, is 12.6V when fully charged. We tend to use an 80% discharge guideline. That is we try to not use more than 80% of the rated capacity. So that is something like 3.75V/cell or 11.25.

Compare this to a 12V car battery. Its not really 12V. A new fresh 12V car battery is as much as 13V. I think you'll find that most of the problems have been due to too low a C rating. The batteries have internal resistance (IR) and when you draw current, the voltage drops instantly across the internal resistance. High C batteries have very low IR so they apply more voltage to the load when you draw current.

You could also go to 4S which is 16.8 fully charge or 14.8V nominal but that is probably too much for a 12V system.

Sorry if this is too techy for some of you but its better to explain things as much as possible.

Just the concept of connecting a wire to battery leaves a large number of readers here out, so it would be nice if there was a larger power pack

I fully understand. I said upfront this might be too DIY for a lot of the folks here. On the other hand, it might be a nice little cottage industry for someone that wanted to put this sort of thing together and resell it.

[CapnLoki]

I do see a few issues with your solution - first, the 3S1P battery is only 11.1

Not to worry. The 11.1V number is a "nominal" voltage based on 3.7V/cell. That is actually the voltage at 50% discharge. LiPo cells are 4.2V when fully charged and decrease more or less linearly as you discharge them. So the (3S) battery I posted, any 3S LiPo really, is 12.6V when fully charged. We tend to use an 80% discharge guideline. That is we try to not use more than 80% of the rated capacity. So that is something like 3.75V/cell or 11.25.
Compare this to a 12V car battery. Its not really 12V. A new fresh 12V car battery is as much as 13V. I think you'll find that most of the problems have been due to too low a C rating. The batteries have internal resistance (IR) and when you draw current, the voltage drops instantly across the internal resistance. High C batteries have very low IR so they apply more voltage to the load when you draw current.

Actually all of the apparent undervoltage issues have been with LiPo batteries, not lead acid. I've never seen a problem with reasonably charged lead acid, although I've always used larger batteries. The problem is that devices start cutting out at about 11.5 Volts, which is at about 3.83V per cell, not even half discharged for a LiPo cell. I appreciate what you're saying, but its well understood that lead acid should not be discharged too far (I stay above 50% for long life) so the voltage generally stays over 11.5.

You could also go to 4S which is 16.8 fully charge or 14.8V nominal but that is probably too much for a 12V system.

Hence my comments about LiFePO4, which functions well at 12.8.

Sorry if this is too techy for some of you but its better to explain things as much as possible.

Not too techy for me, but I still have the question, have you run your 560 from the battery yet? The theory is nice but practical experience is what really counts.

Just the concept of connecting a wire to battery leaves a large number of readers here out, so it would be nice if there was a larger power pack

I fully understand. I said upfront this might be too DIY for a lot of the folks here. On the other hand, it might be a nice little cottage industry for someone that wanted to put this sort of thing together and resell it.

I'm sure that good packs are out there - the problem that I find is that most lithium batteries are packaged for application other than CPAPs. And, most people don't want to pay triple for light weight when they really just need a backup at home. AGM is down in cost to where its not worth while getting flooded for home use; maybe in a few years LiFePO4 will be there.

[rhodesengr]

well I was going by the 15 Ah capacity for the PR unit but it would be nice to have some data on what people actual draw in a night.

As for LiFe, I have a few 2S 2.2 Ah low C LiFe batteries. We mostly use LiFe to run receiver electronics and servos and not for the high current motors. My 2S LiFe packs are 7.2V fully charged but they immediately drop to 6.5V as soon as you start drawing current. They stay at pretty much 6.5V until they are near the end of capacity. Then the voltage drops odd suddenly.

So I think for LiFe, you would want a 4S pack which would run at a solid 13V. Here is a high current 4S 8.4Ah LiFe battery. So you would need 2 to get 16.8Ah. But as you can see, they are about $82 each so you'd be at $164 dollars for the batteries alone.

http://www.hobbyking.com/hobbyking/stor ... ouse_.html

[CapnLoki]

well I was going by the 15 Ah capacity for the PR unit but it would be nice to have some data on what people actual draw in a night.

The basic cost for a 560 is 4 AH a night. Humidifier on a low setting can double that. Average humidity brings it up around 20 AH. Worst case is somewhere around 35.
viewtopic.php?t=102179

Since the battery has to run at least 8 hours, it is by definition, not a High C application.

As for LiFe, I have a few 2S 2.2 Ah low C LiFe batteries. We mostly use LiFe to run receiver electronics and servos and not for the high current motors. My 2S LiFe packs are 7.2V fully charged but they immediately drop to 6.5V as soon as you start drawing current. They stay at pretty much 6.5V until they are near the end of capacity. Then the voltage drops odd suddenly.

So I think for LiFe, you would want a 4S pack which would run at a solid 13V. Here is a high current 4S 8.4Ah LiFe battery. So you would need 2 to get 16.8Ah. But as you can see, they are about $82 each so you'd be at $164 dollars for the batteries alone.

http://www.hobbyking.com/hobbyking/stor ... ouse_.html

Yes, this pack is the equivalent of the LiPo packs marketed as tablet chargers. The fact that it can be charged directly by an automotive charger (BatteryTender, etc), small solar panel, or even just a running car alternator is a huge plus. There are a few issues: Is a pack like this certified for airline travel? A major reason for light weight is overnight flights (or traveling to a hiking site), and a pack like this would stick out in a TSA search. Also, does this come with any battery management? I understand that bare lithium cells will have a limited life time unless they are connected with balancing and protection circuits.

[rhodesengr]

Yes, this pack is the equivalent of the LiPo packs marketed as tablet chargers. The fact that it can be charged directly by an automotive charger (BatteryTender, etc), small solar panel, or even just a running car alternator is a huge plus.

well its LiFe and not LiPo. Not sure that you can charge it with a car charger or other 12V source. Maybe you can but it would be best to use one of the chargers I posted before. They have specific LiPo and LiFe charging programs with a constant current phase followed by a constant voltage phase. They are balancing chargers so they balance the cells as they charge. The batteries themselves are just batteries. They have a main, two-wire , high current plug and a separate smaller gauge balancing plug. Both plugs connect to the charger for balance charging.

Is a pack like this certified for airline travel? A major reason for light weight is overnight flights (or traveling to a hiking site), and a pack like this would stick out in a TSA search.

As far as I know, no batteries are certified for airline travel but you can take such batteries on airlines. Batteries not inside equipment must be carried on and not checked. Batteries under 100W-hours there is no limit. batteries over 100 W-hours you can bring two. So that big LiFe pack I linked is 8.6Ah*13.2V=110.9 W-hours. It is over the 100 W-hour limit so according to the rules, you could bring two. If one picked a slightly smaller battery you could be under the 100 W-h limit and bring as many as you want. The FAA rules are here:
https://www.faa.gov/about/initiatives/h ... /?hazmat=7

Also, does this come with any battery management? I understand that bare lithium cells will have a limited life time unless they are connected with balancing and protection circuits.

No battery management. They don't need protection circuits and the charger takes care of balancing. There is a lot of lore going around about charge state and lifetime. LiPo batteries store best at 3.85V/cell and the chargers have a mode called "storage". Supposedly, if you leave them at full charge it reduces the lifetime. If you over discharge them (below 3V per cell) that will also supposedly damage them. Last weekend, I went through a box of old batteries (3 years) that I haven't used in while. Most were still ok. LiFe supposedly is ok to store at full charge.

I'll just say again, this approach is probably too DIY for most of the folks here but for people comfortable working with this stuff, it is an option. I will get around to setting something like this up and report back. I will need DC operation at some point. I'll have to think about LiPo vs LiFe and I appreciate your info about the machines crapping at at 11.5V.

[CapnLoki]

Yes, this pack is the equivalent of the LiPo packs marketed as tablet chargers. The fact that it can be charged directly by an automotive charger (BatteryTender, etc), small solar panel, or even just a running car alternator is a huge plus.

well its LiFe and not LiPo. Not sure that you can charge it with a car charger or other 12V source. Maybe you can but it would be best to use one of the chargers I posted before. They have specific LiPo and LiFe charging programs with a constant current phase followed by a constant voltage phase. They are balancing chargers so they balance the cells as they charge. The batteries themselves are just batteries. They have a main, two-wire , high current plug and a separate smaller gauge balancing plug. Both plugs connect to the charger for balance charging.

Most of the chargers I use are "multi stage" as you describe. The only problem with lead-acid oriented chargers is that the float voltage will be too low to keep the battery up at 100%. Even without the second stage it should charge OK though a bit slower. And it real hard to overcharge LiFePO4. I don't know how I feel about "balancing wires" as opposed to builtin balancing and protection circuits. I'm guessing that 10 years from now we'll be saying "remember when we had to balance cells?"

Is a pack like this certified for airline travel? A major reason for light weight is overnight flights (or traveling to a hiking site), and a pack like this would stick out in a TSA search.

As far as I know, no batteries are certified for airline travel but you can take such batteries on airlines. Batteries not inside equipment must be carried on and not checked. Batteries under 100W-hours there is no limit. batteries over 100 W-hours you can bring two. So that big LiFe pack I linked is 8.6Ah*13.2V=110.9 W-hours. It is over the 100 W-hour limit so according to the rules, you could bring two. If one picked a slightly smaller battery you could be under the 100 W-h limit and bring as many as you want. The FAA rules are here:
https://www.faa.gov/about/initiatives/h ... /?hazmat=7

I think you're wrong on this one - the page you reference links to the CFR which has "Each installed or spare lithium battery must be of a type proven to meet the requirements of each test in the UN Manual of Tests and Criteria, Part III, Sub-section 38.3..." The 38.3 certification is needed and is generally implied by mass market battery packs. However, the hobby packs often are not certified. I don't know how often these are rejected by TSA but it would be nasty to find out the hard way.

...
I'll just say again, this approach is probably too DIY for most of the folks here but for people comfortable working with this stuff, it is an option. I will get around to setting something like this up and report back. I will need DC operation at some point. I'll have to think about LiPo vs LiFe and I appreciate your info about the machines crapping at at 11.5V.

You're right that this is too DIY for most, and remember that its unnecessary for most home backup use, where an AGM will give more bang for the buck. The lightweight is needed for airline travel (where DIY might be a liability) and carry-in camping. I'm hoping you'll hook up some LiPo cells to your 560 to test my theory that a LiPo 3S won't work. If I'm wrong, I'm sure you'll find a few adopters here. The question comes up every few weeks and the best we can offer are packs that are a poor match for a cpap or very exspensive. Actually, I know of a few hikers here that would love to have rugged, cheap, lightweight pack.