First steps

[Zizzer_Zazzer_Zuz]

Went to the local batterie and Bulbs store today and the guy there wasn’t very clueful or helpful so I’m hoping you folks might be of more kind.

I’m starting my conversion to 12v.  First I need 3 nights of back up battery for my CPAP.  This will be for emergencies but it will also move on to the future camper.  Once I have this I will be able to venture out in ways I have not been able to in years.  This will later remain an emergency backup for the CPAP if my house batteries are drained or if I’m separated from my vehicle for a few nights.

I’ve been doing some research as best I can and I’ve come up with a bit of a plan.  I’m not exactly certain how many AMPs my CPAP uses so I don’t know exactly how many nights the battery listed below will give me.  It is 30 pounds and that’s about as much as I want to have to carry. 

This should be a pretty simple build if I understand correctly and I think I do aside from a few details which I hope I can hit here.

A box for it all.  I wanted something with a handle so i can carry it.  

https://www.amazon.com/gp/product/B001PTHKMG/ref=crt_ewc_title_huc_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

A battery.  40 ampers.  I’m hoping 7 hours a night for 3 nights without charge.   It weighs 29 pounds so I’m not going to want to carry it far but I’ll want to be able to get it wherever I need it.

https://www.amazon.com/gp/product/B..._title_dp_3?ie=UTF8&psc=1&smid=A1T6NV7CED1KI5

From an Apnea board and a similar machine.  It looks like I'll use about one AMP on average and like 3.5 if I use the humidifier.

http://www.apneaboard.com/forums/attachment.php?aid=249 

Battery charger:  I’m wondering if this is overkill.  Any suggestions?

https://www.amazon.com/gp/product/B..._title_huc_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

Sockets and meter: 1 12v and 2 USB sockets plus voltage meter.  I wonder if this will be more precise than the one on the box.

https://www.amazon.com/gp/product/B..._title_dp_2?ie=UTF8&psc=1&smid=A261H57PVQ5RPO

DC adapter for CPAP.  I’ve ordered this and it’s being shipped.

http://www.bestcpapprice.com/Respir...qt5Iej5heOA3J4Yc2rNPVlhLc5N2BHe3nIRoCwmXw_wcB

Do I need to worry about fuses for anything yet?  I’m thinking I would like them to the power outlets.  Any suggestions?

How do you charge a battery like this from a running care?  I’m guessing direct jumper cables is a bad idea.

I may never even run an inverter off of this little battery.  I will be getting a house battery for my shed and converting my power tools to battery so I can take them with me on the road.  I’ve been eyeing some basic solar set ups to get light and some power to my shed so that’s next and would then transfer to the bus/van if nothing else as backup.

All of this is sitting in my Amazon shopping cart now.  Is there anything else I need?

Small steps toward big dreams.

 

[highdesertranger]

ok a 40 amp hour battery (ah) battery has 20 useable ah's. so if your CPAP uses 1 amp you will get about two, 8 hour uses out of it. if it uses 3.5 amps you would be lucky to get 5 hours. the discrepancies lie in losses. it sounds to me like you need a full size battery bank and enough solar to recharge everyday. highdesertranger

 

[ccbreder]

40 AH means they set a steady draw of 2 amps. The battery was completely drained in 20 hours. If they didn't lie about the testing. That drained battery is now junk. So a 40 AH battery at 2 amps draw will last 10 hours till 50% down and then need immediate charging. More amp draw will drain faster due to Peukert's law.

 

[Zizzer_Zazzer_Zuz]

ok a 40 amp hour battery (ah) battery has 20 useable ah's.  so if your CPAP uses 1 amp you will get about two,  8 hour uses out of it.  if it uses 3.5 amps you would be lucky to get 5 hours.  the discrepancies lie in losses.  it sounds to me like you need a full size battery bank and enough solar to recharge everyday.   highdesertranger

For right now, this is only for emergencies when my home loses power, for weekend camping and later as a spare in my permanent rig.

This first part of this project has two main requirements, it must be movable and it must provide power to the CPAP without the use of the humidifier.  If I have to charge the thing daily with a car I can do that too. I know I can't just jump it directly from the car battery while the car is running but I'm not certain how to charge it via the car otherwise.  Solar is coming but will be another small project once this is running.

My calculations at 1 ah and 7 hours per night would be 21 hours of use at 1 ah which would drop to just under 50%.   Is that unrealistic? I would not be running the 3.5 amp humidifier.

A battery more than 30 pounds will get to be cumbersome very quickly.  Is there a lighter storage available than the AGM's?

 

[Zizzer_Zazzer_Zuz]

> 40 AH means they set a steady draw of 2 amps. 

But it looks like it's more like 1 ah not 2.  I won't be using the additional 2.5 amps for the humifier.

> The battery was completely drained in 20 hours.

At 1ah x3 would be just under 50% after 3 nights without charge.  How much would that damage the battery?

>If they didn't lie about the testing. That drained battery is now junk. So a 40 AH battery at 2 amps draw will last 10 hours till 50% down and then need immediate charging. More amp draw will drain faster due to Peukert's law.

So, If I read this correctly and I'm using maybe just over 1 ah on average and I don't want to go under the dreaded 50% I could get 2 nights at 8 hours without charge.  Is that about right?

This then leads to several follow up questions.

What sort of battery can I use for more ah storage at that same weight limit of about 30 pounds?
Is there a simple shutoff that can be rigged so that if the battery gets to 50% it shuts off?
If a battery like this is at 50% and there is no draw will that damage the battery?
If at 50% how quickly must the battery be back on charge before damage happens?
How can I charge the battery other than the AC charger listed?  I know I can't just direct jump this but I would think that I should be able to charge this from the car somehow.

Again, house batteries, solar, etc is coming as part of larger projects.  This is to get me started and for emergencies.  

I'll try to go do some reading about  Peukert's law. But if you feel like giving an overview that would be cool too.

Thanks!

 

[highdesertranger]

the lithium battery folks will have to speak up. they are much lighter. you are doing straight calculations you are forgetting the loses nothing is 100%. also after the first 7 hour stint you must recharge the battery you can't let it sit all day and then discharge again at night. you are going to drastically shorten your battery life. yes you can just connect it to your car when it's running. you need to do it safely but it's not hard. highdesertranger

 

[Trebor English]

Amazon:
Basstop T241 16800mAh 800A Peak Car Jump Starter  $76.96   Weight 3 pounds
The 16 Ah is likely to provide 1 Amp for 8 hours.  It still needs to be recharged.

 

[blars]

The small battery packs tend to list the Ah at 3.6v, then have a dc-dc converter to up the voltage (and drop the Ah). So for 12v, you get about 1/4 the rated Ah if you are lucky.