Backup Battery Basics Help

[edy]

Hi folks,

I have a CPAP machine (Resmed S9 Elite with H5) I'd like to run in the house on backup battery in case of a power failure. I put together a little setup for such an emergency but would like help figuring out what capacity is available and the expected running time.

Here is the device link:

http://www.resmed.com/us/products/s9_elite/s9-elite.html?nc=patients

Here is the attached humidifier:

http://www.resmed.com/us/products/h5i/h5i.html?nc=patients

Here is the battery guide:

http://www.resmed.com/us/assets/documents/service_support/battery_guide/198103_battery-guide_glo_eng.pdf

I've attached some pictures of what I bought from the local Wal-mart. I already see a problem in that the inverter I got is rated for max 140W and based on the PDF above they want 150W minimum. The battery I got was the Maxx-51R because that is the same as what fits in my Honda CR-V and so in the future I could always use it in my car (so I don't waste a perfectly good battery).

The charger is supposed to be able to handle car batteries, according to the box. So I assume I should just charge it up and leave it? How long does it take to drain just sitting around? Is there any way to tell how much "juice" is left in it, or do I just use it and watch my machine eventually not turn on and that means it's too low? Is there a voltage drop curve I should be monitoring?

Sorry for all the newbie questions but just want to know if I'm doing the right thing. Thanks in advance for your help!


Sincerely,
Edy

[SeanB]

Your CPAP unit does have a 12V input option, so you can connect it directly to the battery. It draws around 7A, so a 40AH deep cycle battery will run it for 1 day or so before needing charging. Rather than using the inverter buy the 12V converter and use that.

A friend of mine also uses a simpler unit, and it will work happily on 12V from the car, we just made a long lead to plug into the socket on the unit.

[edy]

Thanks for the advice. The 12 V power adapter converter costs about $150 from the ResMed rep, versus a 400 W inverter at Wal-mart which is about $60. Also, the battery I have looks like a standard car battery.... not deep cycle type. The other advantage of getting the inverter is that I can also use it to charge up cell phones or run other equipment if needed? Otherwise if I buy the 12 V adapter meant especially for this CPAP machine I can't use it for anything else.

I don't know how I'm supposed to figure out what kind of discharge level the battery is at. Simply checking the voltage drop is probably not reliable, or is it? The PDF says not to let them discharge more than a certain amount or I can ruin the battery. So how do I know how much it has discharged? If I plug into the charger I assume it will go to 100% and safely trickle or stop charging when it hits top (so it doesn't overcharge or ruin the battery). However, how do I know the capacity remaining?

I guess with practical use, since this is an emergency backup only, I can do the following (please let me know if this makes sense):

1. charge it up fully now (maybe set it to 4A on the charger?)
    - 2A is meant for trickle charge, not enough, and 6A is for rapid charge but I can wait

2. every week charge it up using 2A setting (trickle) just to keep the battery topped up
    - will this ruin the battery or be ok? I want to make sure it doesn't lose charge on me, so it is ready for an emergency.

3. in emergency situation....
   - use battery with $60 inverter (or $150 converter)
   - use as long as I need it, and then as soon as power comes back up, charge the unit
   
So I won't exactly know how much the battery drained during #3, but it doesn't matter since in an emergency I have to use it so I have no other choice. Presuming that it lasts maybe a few hours only, hopefully the battery will not have drained too much, and I can just charge it fully up (start back at step #1) and I will have it ready for the next emergency.

Does this strategy make sense? Should I still go for the inverter (I can get a 400W one instead of the 140W option). Here is what I can get:

http://www.walmart.ca/en/ip/410-watt-digital-power-inverter/10045041?trail=SRCH%3Ainverter&fromPLP=true&ancestorID=alldept&searchString=inverter&moduleName=S0011&startSearch=yes&fromSearchBox=true&addFacet=SRCH%3Ainverter

I assume it is modified sine wave, but not sure. Thanks for your help, any thoughts/comments appreciated.

[SeanB]

You can charge it at the high current rate, and than use a time switch to just trickle charge it at the low rate for 15 minutes a day. This will keep it reasonably happy, and will give it a long life. Just pull the cap off the top and look at how you can fill the cells with water when needed, as otherwise it will boil off all the water in 2 years or so. If you can live without the humidifier on you will get a much longer runtime out of the battery if the power is down.

Time switch plug from WallyWorld, cheap one will do. Just set it to turn on the output for 15 minutes once a day ( probably the shortest time it will do in any case on the mechanical ones) and connect the battery via a 6A silicon diode ( only needed because the charger will draw some power from the battery when the charger is off, a cheaper one with only a transformer and bridge will not need anything) then it will do.

[edy]

Ok, so I picked up the 410W inverter and it is using a modified wave (looks more like a square than a sine). I am also starting to understand why not all of my appliances run on this and wanted to check my logic with the experts here on the forum!!! 

The CPAP machine is powered by 24V DC and it comes with either a regular transformer (120V AC to 24 V DC) for home use, or with a car-plug type converter for travel (12V DC to 24 V DC)  that goes into the cigarette-lighter. Either way, the plug on the BACK of the machine is a PROPRIETARY 3-prong type special plug that I cannot find anywhere.

So when they suggest you use a car-battery, the CPAP manufacturer just wants you to hook the car battery to their travel-converter 12 V DC to 24V DC. However, that travel-converter costs $150!!!!  So they suggest it may be cheaper to buy an INVERTER which then lets you plug the standard transformer which already comes with the CPAP machine for free. However, it must be 150Watt min. The brochure states it does not matter if it is modified-wave.

The reason it probably doesn't matter is because the modified AC is going through their 120V AC to 24V DC transformer anyways, which smooths it out and provides a continuous DC. Is this logic correct so far?

On the other hand, what happens when you try to plug in CPAP machines that have an AC motor inside. Are they designed in a way that utilizes the SINE curvature of the AC? There are plenty of AC-driven motors and I believe many of them rely on that smooth AC wave for proper timing of the coil charge build-up and dissipation, along with induction properties to get them rolling smoothly.

So my question is, what kinds of limitations should I expect with a car-battery hooked up to an AC inverter that only outputs modified wave (and not sine), will this shorten the life of my devices or overheat them, and WHY do they not make these inverters put out a sine? Is it too complex or inefficient a circuit, too costly, or just not necessary?

By the way I found this neat article which I am in the process of reading:

http://www.xantrex.com/documents/Tech-Doctor/Universal/Tech1-Universal.pdf

[edy]

Just to provide an update....

I bought a Schumacker brand charger (2A/4A/6A to 6V or 12V option) charger and successfully charging my 51R size car battery. So I returned the previous Schumacker inverter at 140W (which wasn't enough to power a suction motor) and bought instead a Schumacker 420W inverter with the LED display (from Wal-Mart).

Much to my disappointment, the thing was dead-on-arrival! DOA! The inverter powered on, the LED did some fast cycling through a bunch of digits and then showed "bAd" on the 3 digit display. I look up the manual and it says to call Schumacker because it is a faulty unit!

Come on, do they not test these things before they ship them off from China? They are in heat-sealed plastic packaging so I know nobody used it before me. As Dave would say, what a Bummer!!!

So I went to my local Canadian Tire and picked up one of their store branded "MotoMaster" 600W inverters, plugged it in and able to run everything no problem, and it also has an LED display to show me battery voltage, AC output voltage, and watt consumption on the load.

Interestingly, I may have figured out WHY the other inverter (140W) started my suction motor but wasn't able to sustain it... The motor which just fizzle to a stop a few seconds later, each time I turned it on. It wasn't because of using modified sine wave. When I look at the load on my new inverter I notice when I turn on the suction motor the watts briefly go up to about 160W and then drop down to about 60W sustained.  I bet if I make the motor work a bit (by plugging up the suction) the watts will increase too.

So the 140W probably had some protection circuitry that would trip over 140W but it said on the package that it could handle an initial burst of much higher... Not sure then why it didn't do the job. At the time I don't think my battery was fully charged so that could have contributed to it.

Nevertheless, the 600W option although more expensive is better just in case I need to run both CPAP and suction unit at the same time. They are safely within the limits so I know it will be able to handle things.

[plunger]

The inverter is not as efficient as the Resmed converter. I wouldn't run the humidifier on battery. It drains a lot of power.

I don't think a car battery is correct for this application. A marine battery would be better and cost around the same.

If you must use the humidifier, pour distilled water in it, but don't turn it on. Keep it in passover mode and don't turn on a climate line hose either.

The Resmed converter is only $80 in USA.

[Jon Chandler]

My situation is a bit different than yours, but here is my solution.

My CPAP pressure is pretty low, so the current draw isn't too high...a couple amps max as I recall, and I don't use a humidifier.  I got a small UPS on sale at one of the office supply stores for less than the cost of the batteries in it.  It's got an "anti-feature" for my use - when the AC supply is off and the inverter is running, it beeps every few seconds.

Rather than let the UPS make AC power which the CPAP's power supply converts back to 12 volts, I've wired the UPS batteries to a cable to plug into the 12v input jack on the CPAP.  In normal operation, the UPS keeps the battery charged and the CPAP is powered from the floating battery.  If the power goes off, the UPS beeps; since nothing is plugged into the AC outlets, it eventually turns off if I don't roll over and manually turn it off.  The battery will run the CPAP for 2 nights without charging.

Using the UPS this way gives the maximum efficiency and provides a nice self-contained package.

[kjpye]

My experience of running ResMed CPAP machines from batteries is with an older version (S8?) so it may not be applicable in this case.

ResMed recommend using their 12V converter when running from batteries even though the input is 12V. Unfortunately the converter (AUD99) is far too clever for its own good, and is essentially useless.

The problem is that it contains circuitry that shuts down the converter when the input voltage gets too low, and there is no control whatsoever over this "feature". This means that whenever you have anything else running from the battery, or if you have long leads to the CPAP machine, it is likely to shut down when there is still significant charge in the battery; possibly even if it is fully charged. This is exacerbated by the lack of any time delay on this feature, so the initial inrush current of the CPAP machine, coupled with resistance in the cable between the battery and the converter, will often cause it to shut down even when the battery is fully charged.

In my case I am always running the CPAP machine from a deep cycle battery, so I would rather run the battery almost flat and get a good night's sleep rather than having a still charged battery in the morning.

I solved the problem by running a cheap 150W modified sine-wave inverter near the battery and running an ordinary 240V extension lead to the CPAP machine. Works flawlessly, even when I needed a long extension lead a couple of days ago to reach from the car in the garage to my bedroom when the power was off for an hour or two.

[plunger]

Since we have a bunch of cpap'ers in one place, has anyone thought about wiring up a remote on/off cable to a cpap machine? No need to reach over to hit the on/off button when you're getting up 4x a night =)

A remote display would be even better.

[Rick Law]

...... The CPAP machine is powered by 24V DC and it comes with either a regular transformer (120V AC to 24 V DC) for home use ......

Edy,

I take it you go through all these trouble because of an allergy?   If that is the case, reliability and endurance is important.  Why not double up the battery - two 12v batteries in series give you 24V.  That would be most efficient, and allow you to breath until both batteries are dried and it could last you two full days instead of just one.

You said you still need to run some kind of a suction thing.  If that thing can run on 24v or takes 12-24v input, two batteries and you are home free.

If you do run 2 batteries in series to get 24v, you will either have to charge them separate (one at a time) or get a different charger.  Single 12v charger cannot charge both while they are connected in series, nor should you connect two 12v battery chargers to them while they are connected because the chargers may share a common ground once plugged into the wall socket.  You will need a different charger that runs at about 30v.  If you charge them separate, time-based auto-top-up will be difficult.  You have to make it a point to topping up the batteries every 2-3 months or so, your batteries will be always be almost full.

[edy]

Thanks for the suggestions. ResMed does sell the 12V adapter for car-cigarette-lighter plug, but it costs something like $150 here from the supplier/rep who works through the hospital. And it has some proprietary plug so I can't just use an off-the-shelf adapter plug...  The only way would be to buy another AC cord and chop off the end just so I could get the proprietary plug!!! If I could get the plug, I would be able to buy an OEM 12V to 24V converter somewhere and power it off 1 battery or forget the converter altogether and just use 2 batteries in series as someone suggested above.

Even with the ResMed adapter, I would still need to buy a 12V battery and charger to run it at home. So for a $80 inverter although maybe not as effiient, I can plug the ResMed 120V AC cord into it (which comes with the unit anyways), plus I have a solution for the suction machine and perhaps other emergency needs (like cellphone charging, etc) which is also useful.

As far as the battery goes, I did want to consider a marine deep cycle battery. However, I ended up buying the same battery as found in my car just for practical purposes. I do not anticipate having critical power outages but I needed some protection, at the same time if I had a marine battery I would be stuck with a battery I can not use for any other application. Therefore by getting the same battery as my car, I have a spare in case I need one in the future or find my battery drained one morning, I can easily swap batteries.

I do believe the inverter has a low voltage cutoff so it will not completely cycle down the battery, protecting it so I can charge it again.

Right now I trickle charge the battery (which is kept at room temperature in my house) about once a week to top it up. It is never used, just there for emergencies, and after about 20-30 min the charger stops and says it is charged.

I figure the backup battery should last at least as long as my car battery which is being drawn on almost every day and goes through temperature extremes all year here in Canada. If I trickle it up maybe 50-60 times a year for less than an hour and it is stays in a constant environment, will it be ready for my car when my car battery eventually dies (car is only 2 years old)?

The whole Resmed and suction setup is actually for one of my children (baby) who needs a bit of help but they are outgrowing the condition, in fact we haven't had to even use the CPAP for a month. So originally at the hospital they wanted to sell us the $150 RedMed 12V supply, or to buy the ResMed backup battery pack for about $500....which would have done the same job (or better) than my inverter/charger/battery combination which cost me $250 and weighs much better and more portable.

However, as you can see if I went with the Resmed options it would have never even used it, and I would be stuck with a bunch of proprietary stuff with plugs that fit nothing else. At least this way I got a universal option for many possible future uses and also a battery I can always put to use in my car one day too.

That was my logic at least.

[Rick Law]

Edy,

I have some allergy myself and I can't handle too much candles.  So I need emergency lights to avoid wife needing too much candles.  Humidity will hit my breathing but I can deal with that better than candles.  I can see where you come from.  Let me share some experience with you - particularly my "open hood rapid charging" method.

If you have not already obtained your battery, there is some significant pluses to car battery over marine/deep-discharge. Particularly those "Portable Jump-Start Battery."  They don't hold as much charge (mine is 22AH verses typical small car batteries in the 25-30AH range).  The "portable car jumper/starter" are the kind with built in car jumper-cables and some has CLA (cigarette lighter adapter) built-in.  Some even have inverter build-in for 110v.  Almost all have battery-level indicator light or gauge.  They are design for starting cars, so they are well prepared for high Amp discharge --  which means they likely can take on high-Amp rapid-charging easier and more conveniently.  A regular car-battery "on the side" can do that too, but you can't walk it around with the same ease and none has battery gauge built in (that I know of).    I use external CLA-plug inverter so I can drag the portable jumper-battery where ever I need it easily and switch between my two jumper-batteries.

When you connect the jumper-cable to a car with engine already running, the car's alternator will charge the jumper battery - rapidly because the battery is design with more plate area for rapid discharge.  I connected my jumper-battery to the car's battery (while the engine is running).  With the car's alternator pumping out (I think) 60+ Amps , my 22AH jumper-battery was fully charged in well under 30 minutes with engine at idle.

I do not recommend the alternatively of using a CLA-to-CLA adaptor cable connecting the car's CLA socket to the batteries' CLA socket and charge the portable battery that way.  My portable battery came with such a cable.  It can be done, but between the alternator and the car's CLA, too much is in between to cause a fairly high voltage drop by the time the juice gets to the car's CLA socket.  Also, the car's CLA socket has a lower current limit.  This CLA-to-CLA method was not working well for me so I put a DMM in between to see why.   I can see unless the engine is at fairly high RPM, my portable battery was actually discharging into the car.  The jumper-cable right at the car's battery terminal give it a almost-direct connection to the alternator.  It works like a charm and do so quickly.

With my two jumper-batteries, I can take them out to the car with ease and charge by car-jumper connection.  I switch battery every 5 minutes or so - to let the charge settle and to do a voltage check while the other battery is charging.  I do that for around 30 minutes and both my batteries are near 90% full.  The batteries will start to gas if overcharged.  Gassing voltage depending on temperature, you want to stop charge at about 14.2volt.  I use 14.5v as my hard-stop.

During Sandy (the storm), I got caught unprepared.  I was determined not to get caught again.  With my two jumper-batteries, one good and the other too old to start a car.  Both my batteries were on LOW at the start of Sandy and I didn't know exact what that means.   ***Know your battery***  After the storm, I did full charge and discharge tests with the batteries.  I used time-lapse photo rather than sit there and watch.  Now I know the voltage each battery-level light coming on, and how many more AH can it still put out.  When the 2nd to last battery level light is still on (or when first switch to the lowest light).  I have 50-60 watt-hours (@ 2-3A)left in good battery, and about 20 watt-hours on my older battery.  About 1/2 that in watt-hour if I push it to 4-5A discharge.  Note, means 1/4 the time.  1/2 the stored watt-hours, but twice the amps meaning 2x the draw rate giving you 1/4 the time.  I use voltmeter/dmm because the exact voltage give me more accurate estimation of watt-hour remains than the "hi mid-hi, mid..." indicator lights on the battery.

I also measured the current drain on my must-have's and my like to have's.  So I know with confidence "I can wait till next day to do an open-hood jumper or until the rain stops".   I have hard data based on battery voltage how many Watt-Hours I have left on the stuff I need/like to run.  I know unless both batteries are at bottom-level light, I have 8+ hours of what I consider "survival minimum" with a 10% margin, and I plan to do another discharge test again in a year since batteries do age.

1. If you already have regular car battery (batteries), think about getting battery carrying-straps or those travellers' luggage "pull cart" like the ones airline employees use (to take the batteries to the car), and get car jumper cables.  Now you know you can easily charge as long as you have some gas in the car.

1a. If you already purchased a car battery, do consider adding a portable jumper battery.  That could be your "last line of defense" and it is darn portable.  My small one was $30 (8-10yrs ago)  with 7AH at walmart.  Not good for car-starting anymore since it is rated at 300CCA only, but great as emergency battery.

2. With portable car jumper batteries, you can charge it so easy and self-discharge is so low, I think you really don't need trickling.  A top-up every 3 months would do the job nicely.  It will lengthen battery life.  I rather use up the battery life during emergency than trickling it away.

3. Get those car-volt meter that plugs into the CLA and/or a DMM.  Don't leave the meter plug in but keep it handy (my CLA-voltmeter drains at 30ma).  You need at least 2% accuracy +- 1 decimal is good.  1/2 volt difference on the SLA may mean a dead battery verses one that can run your stuff for yet another few hours. 

3. Get some RV/camper LED lights with CLA and a long cable - they are efficient and very useful when you need to hook-up for "open-hood rapid charge" at night.  I made a few of them, just G4 socket with 8 feet wire into a CLA adaptor.  Those are my primary emergency lights.  I can connect that to the hood and do my "open-hood rapid charge" in the night.

4. Take time to know your battery (batteries) - how long to charge, and a voltage vs watt-hour-remain table.  Watt-hour will depend on current draw.  At least do one with your minimum need (lowest amp to survive).  You can do another one say with comfortable needs included.

5. CLA (for your must have) is more efficient than inverter.  If getting CLA for your must have's adds up to too much, don't oversize your inverter.  You may be wasting a lot of mAH running the inverter electronics if you oversize your need.

Hope this is useful info to you.

Rick
Stay healthy... good luck.

[Rick Law]

Three things to add since you seem new with this:

1. The CLA-to-CLA adaptor cable is useful if you have more than one battery.  It will allow the two batteries to "share the load" and avoid changing batteries in the middle of the night.  CLA-to-CLA is to have plus-to-plus connection and minus-to-minus connection between the two male CLA plugs.  You can join the two backup batteries together via the CLA's.  You will need a splitter since CLA-to-CLA between two batteries will block both sockets.  You need a 3-way splitter in one of them.  A three-way splitter gives you an extra socket for the CLA-voltmeter.  I leave my CLA-volt-meter in the 3rd socket loosely so it is in but not connected.  I push the voltmeter the way in to read the volts, and then pull it out a bit again to disconnect once done reading the combined two-battery-volt.  More convenient than disconnecting my equipment just to plug the volt-meter in.

2. Small car batteries are in the 50-60 RC (reserve capacity) range or approx 25-30AH range.  The CCA is not as important as AH for this application.  My large is 22AH and my small is 7AH.  Between the two, I have more juice than a small car battery 29AH verses my wife's car with 53RC which is just about 25-26AH.  Only my 22AH has enough CCA to start a car.

3. If you do "open hood charging", I said you should stop at 14.2v (ish) and I use 14.5 as hard-stop.  That 14.5 is the voltage when the battery is still connected to charge.  Once you disconnect it from the car, the voltage will drop within moments to 12v range.  12.6v is the theoretical max (if memory serves).  My charge "cut-off" is prior to connect back to car  (already 5-10 minutes settle time since I alternate the two batteries), I check for 12.3v.  For my batteries, it takes about 15 minutes to fully settle.  12.3-12.4 is the "darn-near-full" settled voltage.  5-10 minutes settle to 12.3 is good enough for me.  Your car's alternator circuitry will likely not go much beyond 14.5v charging since it is designed to avoid gassing a car battery to start with.  When connected to car, I stop at 14.5 non the less instead of relying on the car's circuitry to stop charge.

Rick

[plunger]

How do regular car batteries handle being discharged? You can buy marine batteries that are a hybrid of "deep cycle/starting".

[Rick Law]

How do regular car batteries handle being discharged? You can buy marine batteries that are a hybrid of "deep cycle/starting".

Not sure what you mean by "How do regular car batteries handle being discharge?"

They just discharge like any other battery.  So I am not sure that is your question.

[tom66]

Regular automotive batteries aren't rated for deep discharge, and will suffer sulphation if this occurs.

[Rick Law]

Regular automotive batteries aren't rated for deep discharge, and will suffer sulphation if this occurs.

Ah, now I understand the question.  When I researched into it, my understanding of what I read is that sulphation will occur if left in the discharged state for too long.  I did some research when I worked out my "no more candles" solution.  According to what I found on the web, a day or two of battery-at-empty  is no big deal.  (Correct me please if someone knows otherwise.)  If any damage is caused, it would similar to "forgetting to turn off the car head light" and left it on for a day or two.

Given that this is for emergency (power outage), that means deep discharged is only during an emergency - so one can be hopeful that it is no more than two or three times a year.  The battery damage for such infrequent full discharge should be limited.   Beside, this solution it is to avoid breathing problem due to allergic condition.  Under that premise, I would not worry about killing the battery too much.

If  I am wrong (about the limited damage), I sure would like to know.  So please leave a comment.