Help with math for battery and solar panels

I have a PR system 1 auto bipap pro. I have a battery pack for it but it isn't big enough to run the humidifier or use more than one night without recharging.
I recently purchased a small solar panel array with three 15 watt panels for a total of 45 watts with a controller. The battery I have is a 35 amp hour battery.

According to the plug on my cpap is uses 125v ac @ 2.1 amps and below that says 12 volt 5 amp. It has been a while since I have done these calculations.
Can anyone tell me how long this battery will take to charge with this array of solar panels and how long my cpap will run on that bettery?

Thanks

Gerry

Gerryk wrote:I have a PR system 1 auto bipap pro. I have a battery pack for it but it isn't big enough to run the humidifier or use more than one night without recharging.
I recently purchased a small solar panel array with three 15 watt panels for a total of 45 watts with a controller. The battery I have is a 35 amp hour battery.

According to the plug on my cpap is uses 125v ac @ 2.1 amps and below that says 12 volt 5 amp. It has been a while since I have done these calculations.
Can anyone tell me how long this battery will take to charge with this array of solar panels and how long my cpap will run on that bettery?

Thanks

Gerry

Gerry I believe you will find it useful to actually measure the AC current into the box with the box in actual operation. How much energy it uses will depend upon how much pressure you use as you use it.

Also, look into the batteries that people provide for these machines.

Your system may well be sufficient as is.

Also with solar it will depend upon the installation, time of year, and weather. You will need a comfort bump to make it consistently work.

I have a battery pack made for this machine and it works fine for one night without humidifier use.

I am looking for the figures for the solar array working at rated output as it claims even on cloudy days it will have a high output just not 100% of the rated.
If anyone knows how to do the math it would be appreciated.

Gerry, I'm not sure what you are trying to calculate, but watts = amps times volts (W=IE).
At a 5 amp draw, theoretically your battery should battery should provide 7 hours use (7 hours times 5 amps = 35 amp-hours).

Jay Aitchsee wrote:Gerry, I'm not sure what you are trying to calculate, but watts = amps times volts (W=IE).

I am trying to figure out two things
1. How long will my machine run off a 35 amp hour battery
2 how long will a 35 amp hour battery take to charge using a 45 watt solar panel.

Sorry, I can usually figure this out but the brain isn't working today.

Sorry, I was editing while you posted. I'm not sure there is enough info to determine how long it will take to charge. It would seem if the draw was 5 amps at 12 volts for 7 hours, that would be 420 watt hours of energy used. And, if the out put of the controller is 45 watts, then it would take nearly 10 hours to charge the battery. But this is a very simplistic approach.

Gerryk wrote:

Jay Aitchsee wrote:Gerry, I'm not sure what you are trying to calculate, but watts = amps times volts (W=IE).

I am trying to figure out two things
1. How long will my machine run off a 35 amp hour battery
2 how long will a 35 amp hour battery take to charge using a 45 watt solar panel.

Sorry, I can usually figure this out but the brain isn't working today.

As mentioned #1 will vary depending on pressure (and humidifier settings).

Both will depend on the condition of the battery.

Your best bet is to try both and let us know.

Guest wrote:

Gerryk wrote:

Jay Aitchsee wrote:Gerry, I'm not sure what you are trying to calculate, but watts = amps times volts (W=IE).

I am trying to figure out two things
1. How long will my machine run off a 35 amp hour battery
2 how long will a 35 amp hour battery take to charge using a 45 watt solar panel.

Sorry, I can usually figure this out but the brain isn't working today.

As mentioned #1 will vary depending on pressure (and humidifier settings).

Both will depend on the condition of the battery.

Your best bet is to try both and let us know.

I agree the time to charge will depend on the condition of the battery before charging and the amount of power used will take into account the setting on the machine.
However the machine has a maximum draw of 5 amps at 12 volt dc max and consider the battery fully charged.

As far as charging the battery, consider it discharged from use.

My calculations are nearly 20 hours run time and about ten hours to charge.

I bet that 12v at 5A rating is an absolute peak when it starts up. When powered by a battery ( no humidifier ) my cpap pulls
about 2.4 A for the first second or two when I start it then it pulls about 1A. Pulls a little more if I run it without the mask on
and it pulls about .1 A when the blower is off. I am running it at 14 inches of pressure with a full face mask less pressure
would drop my power drain from my battery.

Gerryk wrote:I am trying to figure out two things
1. How long will my machine run off a 35 amp hour battery
2 how long will a 35 amp hour battery take to charge using a 45 watt solar panel.

1. As Jay said. Simplistically your machine draws 5 amp/hours for every hour it runs, so it will run 7 hours (5AH x 7RH (run hours) = 35AH) However, nothing is that simple, so it may run more or less than that.

2. Jay also said that it would take about 10 hours for the recharge and that's pretty close if the cells are producing 100% output. The 45 watts at 13.5 volts will produce about 3.33 amps at 100% output. At 3.33 amps per hour it will take 10.5 hours to recharge 35AH. Again nothing is that simple. The cells will very rarely be at 100% production and any (did I say any?) clouds or shade will drastically reduce the output of the cells.

Having said all that, it's just pie in the sky numbers as we don't really know how much power the machine really uses, and we don't really know how much of that 35AH battery is actually usable before you get to cutout voltage for the unit.

Gerryk wrote:

Jay Aitchsee wrote:Gerry, I'm not sure what you are trying to calculate, but watts = amps times volts (W=IE).

I am trying to figure out two things
1. How long will my machine run off a 35 amp hour battery
2 how long will a 35 amp hour battery take to charge using a 45 watt solar panel.

Sorry, I can usually figure this out but the brain isn't working today.

Your BiPAP will use roughly one Amp without humidification, or 8 Amp-Hours in a night. Humidity can range from zero Amps for passive only (water, but no heat) to 1 or 2 Amps for moderate humidity, to 3 or 4 Amps at full humidity and heat. The newer systems measure the ambient humidity so a lot will depend on local conditions. Only you can predict this load, but a moderate humidity setting plus the base load would give you a total of around 25 Amp-Hours per night, but you might assume a range of 15-35 Amp-Hours.

You don't say what technology your battery is; I would guess its lead-acid, possibly an AGM battery typical of power wheelchairs and UPS power supplies. They have many good properties, but are not forgiving of deep discharge. In other words, you might get several hundred discharge/charge cycles down to 50%, but only a few dozen cycles if you take it down to 20% or lower. It really depends on whether this is to be used for the once a year storm, or for extended off-grid camping. Also, while the battery will recharge in a few hours to 70%, getting it to 100% might take another 10 hours or more. This means that for one time use you might get 30 Amp-hours out of the battery without serious damage, but for daily off-grid cycling you would want to discharge to 50% and the recharge to 85% (this is the common protocol for boaters).

Now for the solar panels: Vendors love to show high Wattage ratings but in practice panels never come close. I have 3 50 Watt panels which on paper could deliver 150/12 or 12.5 Amps. However, I rarely see them producing more than 6 Amps. A light overcast reduces that at least 25%, thicker cloud cover will cut it in half or more. And of course they really only put out in the midday sun. I usually figure I can get about 50 Amp-hours from the panels in a day.

The way the solar industry estimates is to use a figure for the Average Sun Hours for a location for the summer or winter. In Boston and Chicago, for instance, the summer number is a bit over 4 hours of sun; you then assume the panels put out their full rating for that number of hours. In my case (Boston) that becomes 150 Watts X 4 sun-hours => 600 Watt-hours => 50 Amp-hours (at 12 Volts), which corresponds to my empirical evidence. The winter number is more variable. For instance, Boston has 3 sun-hours per day, but Chicago is only 1.5; presumably due to overcast. Clear desert conditions obviously help - Arizona gets over 7 or more sun-hours in the summer, and up to 6 even in the winter.

Of course, any shading from trees or buildings will essentially kill the power, and you can improve the numbers by constantly re-aligning the panels to the sun.

Assuming you're in Chicago, in the summer your panels will deliver: 14 Watts X 3 x 4 sun-hours => 168 Watt-hours. Since this is a charging situation, you have to divide by the 14 Volt charge voltage, and then apply a 90% charge efficiency factor: (168 Watt-hours/14 Volts)*90% => about 11 Amp-hours charge replaced in the battery. Bottom line is if you have good weather, your panels should be able to power the BiPAP, but not the humidifier.

BTW, my experience in these matters comes from living off-grid for several months each year.

If you are using the unit in the summer for a short duration, why not just forego the humidifier? It is an energy hog. I never use mine.

CapnLoki wrote:

Gerryk wrote:

Jay Aitchsee wrote:Gerry, I'm not sure what you are trying to calculate, but watts = amps times volts (W=IE).

I am trying to figure out two things
1. How long will my machine run off a 35 amp hour battery
2 how long will a 35 amp hour battery take to charge using a 45 watt solar panel.

Sorry, I can usually figure this out but the brain isn't working today.

Your BiPAP will use roughly one Amp without humidification, or 8 Amp-Hours in a night. Humidity can range from zero Amps for passive only (water, but no heat) to 1 or 2 Amps for moderate humidity, to 3 or 4 Amps at full humidity and heat. The newer systems measure the ambient humidity so a lot will depend on local conditions. Only you can predict this load, but a moderate humidity setting plus the base load would give you a total of around 25 Amp-Hours per night, but you might assume a range of 15-35 Amp-Hours.

You don't say what technology your battery is; I would guess its lead-acid, possibly an AGM battery typical of power wheelchairs and UPS power supplies. They have many good properties, but are not forgiving of deep discharge. In other words, you might get several hundred discharge/charge cycles down to 50%, but only a few dozen cycles if you take it down to 20% or lower. It really depends on whether this is to be used for the once a year storm, or for extended off-grid camping. Also, while the battery will recharge in a few hours to 70%, getting it to 100% might take another 10 hours or more. This means that for one time use you might get 30 Amp-hours out of the battery without serious damage, but for daily off-grid cycling you would want to discharge to 50% and the recharge to 85% (this is the common protocol for boaters).

Now for the solar panels: Vendors love to show high Wattage ratings but in practice panels never come close. I have 3 50 Watt panels which on paper could deliver 150/12 or 12.5 Amps. However, I rarely see them producing more than 6 Amps. A light overcast reduces that at least 25%, thicker cloud cover will cut it in half or more. And of course they really only put out in the midday sun. I usually figure I can get about 50 Amp-hours from the panels in a day.

The way the solar industry estimates is to use a figure for the Average Sun Hours for a location for the summer or winter. In Boston and Chicago, for instance, the summer number is a bit over 4 hours of sun; you then assume the panels put out their full rating for that number of hours. In my case (Boston) that becomes 150 Watts X 4 sun-hours => 600 Watt-hours => 50 Amp-hours (at 12 Volts), which corresponds to my empirical evidence. The winter number is more variable. For instance, Boston has 3 sun-hours per day, but Chicago is only 1.5; presumably due to overcast. Clear desert conditions obviously help - Arizona gets over 7 or more sun-hours in the summer, and up to 6 even in the winter.

Of course, any shading from trees or buildings will essentially kill the power, and you can improve the numbers by constantly re-aligning the panels to the sun.

Assuming you're in Chicago, in the summer your panels will deliver: 14 Watts X 3 x 4 sun-hours => 168 Watt-hours. Since this is a charging situation, you have to divide by the 14 Volt charge voltage, and then apply a 90% charge efficiency factor: (168 Watt-hours/14 Volts)*90% => about 11 Amp-hours charge replaced in the battery. Bottom line is if you have good weather, your panels should be able to power the BiPAP, but not the humidifier.

BTW, my experience in these matters comes from living off-grid for several months each year.

Thank you, that is what I was looking for. I have a battery pack made by the manufacturer of my cpap with a much smaller battery and it runs almost two nights no humidifier and not quite one night with it. I have a portable solar panel system I got from harbor freight and will probably use this to charge phones and other things that we will need. I will probably get a bigger battery and better panels to handle my BiPap as weight is not a problem. All of this will be transported in our trailer. I am looking for this info so I can take the scouts camping. If I am making it possible for these kids to go camping and complete their rank advancements, I will spend a few extra dollars for a nice solar system that will run my BiPap properly with humidifier operation included.

Thanks all.

I decided to use the solar kit I have for it's original purpose of charging the adults cell phones and some led lighting and going to use a kit I just got that is a two panel 200 watt kit with a bigger battery for my cpap.

Your mileage may vary, but you will probably find the CPAP draws less than this on average.

With humidifier 30 watts, or 2.5 amps at 12V.
Without humidifier 10 watts, or .83 amps at 12V.
With heated hose and humidifier, I don't know. Maybe 45W or 3.75 A at 12V. (As I said, just a guess.)

There's a link for ResMed CPAP machine battery info in my signature line.