Ok.
I've used the battery back for two nights now with complete success.
After charging the battery (115 amp hour battery) to full capacity and leaving it on the maintain mode in the charger, I disconnected the battery and let it rest for a few hours to get rid of the surface charge.
I took a voltage reading at 9:57pm: 12.84 volts. Turned it on and went to bed.
In the morning I turned it off and took another reading at 6:00am: 12.61 volts. 8 hours sleep.
Night number two I took the voltage readings again: 10:20pm, 12.68 volts.
In the morning---8:00am, 12.59 volts. Almost 10 hours sleep.
Does anyone know how to figure the the actual amps being drawn from my cpap machine given the information I've listed above? I'm going to give it a go in the paragraph below....here it goes...
According to some information I've found on the internet, the voltage readings show that the battery is at about 90% capacity right now after two nights of use. That means I've used 10% (5% per night) of the battery. The battery is rated at 115 amp hours. 5% of 115ah is 5.75 ah. That means (I think, and please correct me if I'm wrong) that my CPAP is drawing an average of .64 amps per hour based on an average of 9 hours sleep per night (8 hours the first night and 10 hours the second night).
Is that about right? Did I figure that out correctly?
If I did do the math right, that means I could go camping for 10 nights and only draw the battery down to 50% of its rated capacity. Thus prolonging the battery life for hopefully years to come. WoooooHooooo. That rocks!
Thanks,
Jackie
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Another question about camping with CPAP
Jackie,
That's great, however I want to warn you about drawing too many conclusions about the state of a lead-acid battery from voltage measurements. It is generally accepted that this gives at best only a very rough estimate of how charged or discharged a battery is.
1) Measurements should only be made after a battery has sat with no current in or out for an hour or so.
2) A fresh fully charged battery will show a voltage of about 13.3 volts when taken off the charger. The voltage will fall to about 12.6 volts after a few hours. From there the voltage will fall slowly.
The best way to measure the state of a wet-cell battery is to measure the specific gravity of the acid using a hydrometer, that can be purchased at an auto parts store.
Here's a link to a site that will tell you more about batteries than you ever wanted to know:
William Darden's battery info site
I don't have a multimeter that will let me accurately measure the current drawn by my apap, but it does seem to be less than 1 amp. That is why I suggested 8 amp-hours per night in my earlier post. So you are in the ball-park with your 10 nights, but I might suggest dropping your estimate to 7-8 nights until we get a real measurement. I'll see if I can borrow a meter tomorrow.
As an avid cruising sailboat enthusiast I do battle with 12v batteries and systems every weekend. I do my best to maintain my batteries in tip-top condition, but I have had several premature failures.
That's great, however I want to warn you about drawing too many conclusions about the state of a lead-acid battery from voltage measurements. It is generally accepted that this gives at best only a very rough estimate of how charged or discharged a battery is.
1) Measurements should only be made after a battery has sat with no current in or out for an hour or so.
2) A fresh fully charged battery will show a voltage of about 13.3 volts when taken off the charger. The voltage will fall to about 12.6 volts after a few hours. From there the voltage will fall slowly.
The best way to measure the state of a wet-cell battery is to measure the specific gravity of the acid using a hydrometer, that can be purchased at an auto parts store.
Here's a link to a site that will tell you more about batteries than you ever wanted to know:
William Darden's battery info site
I don't have a multimeter that will let me accurately measure the current drawn by my apap, but it does seem to be less than 1 amp. That is why I suggested 8 amp-hours per night in my earlier post. So you are in the ball-park with your 10 nights, but I might suggest dropping your estimate to 7-8 nights until we get a real measurement. I'll see if I can borrow a meter tomorrow.
As an avid cruising sailboat enthusiast I do battle with 12v batteries and systems every weekend. I do my best to maintain my batteries in tip-top condition, but I have had several premature failures.