Question...How long will a 1.5 amp draw last on a 12v battery with 6.9 Ah rating????

[Antigrav]

I'm new and it seems there are very knowledgeable people on this site who could answer this question...

Ok here is the scenario...

I have a customer with a CPAP machine... it helps people who snore and they sleep with it at night...

They want to be able to go out camping and carry a lightweight battery, Lifepo 4 and use the battey to run the CPAP machine at night. I was wondering how long a 12v 6.9 Amp Hour battery would run the machine for.... assuming that the machine draws 1.5 amps and runs on 12v... and what is the mathmatical equation you would use to figure that out?

Thanks for any help

 

[Reloadron]

About 4 hours. On paper (and only on paper) a 100 AH battery would deliver 100 amps to a load for one hour. It would deliver 50 amps for two hours and 25 amps for 4 hours. I am sure you get the idea. However, looks good on paper but things don't work that way. In theory your 6.9 AH battery would deliver 6.9 amps to a load for an hour. So if the load is 1.5 amps we could take 6.9 / 1.5 = 4.6 hours. However, the greater the load the more the rule changes. I would feel comfortable wit 4 hours assuming the battery is fresh with a full charge.

Ron

 

[blueroomelectronics]

I'd say 4 hours is generous, depending on temperature and when the battery was last topped off, the condition of the battery plus the fairly high current draw the OP would be very lucky to get 4hrs.

 

[Reloadron]

I'd say 4 hours is generous, depending on temperature and when the battery was last topped off, the condition of the battery plus the fairly high current draw the OP would be very lucky to get 4hrs.

Yeah, I may have been a little generous on that. This is one of those we really don't know the exact battery condition. Snoring is one thing and something like safety of life is another. Revised edition maybe 3.5 to 4.0 hours on a good day (night) with a healthy battery.

Oh yeah, temperature also figures in there. I guess I just assume everyone camps out on lovely 65 degree F. evenings.

Ron

 

[Antigrav]

thanks for the responses, that is what I needed to know... I have to do some checking into this, but the 4 hour example is good... I assume then that if I doubled the Amp Hours of the battery I could get safely (of course assuming perfect conditions) about 7 hours of run time... That would be a pretty big battery pack.

 

[Reloadron]

thanks for the responses, that is what I needed to know... I have to do some checking into this, but the 4 hour example is good... I assume then that if I doubled the Amp Hours of the battery I could get safely (of course assuming perfect conditions) about 7 hours of run time... That would be a pretty big battery pack.

That would be about right with good batteries. I use plenty of 12 volt 7.5 AH batteries for my UPS back up power, seems a popular size. I would venture maybe 5 Lbs each. However, much as I mentioned in my previous post, this is well and fine for things that do not involve safety of life. When you get into medical devices that involve safety of life it is a whole different world of reliability.

Ron

 

[xpi0t0s]

1.5A seems a lot, I'm sure mine doesn't draw that much. But I can't find my power meter to check (although I do remember working out that it costs less than 10p/week to run, so from that you can estimate the max power draw). If you plug in one of those power meters that go between the wall socket and a plug, and leave it for an overnight run, you'll get the instantaneous current draw plus various stats for the whole night, from which you'll be able to determine what the battery requirements are. But the requirements are pretty high and the resulting batteries are heavy and/or expensive.

CPAP isn't a ventilator, you don't kick the bucket when it stops working cos your brain wakes you up and you breathe normally after that. So you're deprived of sleep, but that's better than being deprived of life...

 

[Gary B]

Antigrav, you need to get in touch with the battery manufacturer you are going to use and get his datasheet for the specific battery you are going to recommend. Most batteries are rated for an 8 hour discharge time (except for Gel Cell who uses a 12 hour rate) at 20 degrees C. If you discharge faster, you don’t get as many AH as specified. If you discharge over a longer, you get more than stated. Also, don’t forget to include a derating factor for aging of the battery before use. In your case, you are discharging faster than the specified rate and I estimate that you will be doing well if you get it to last 3 hours, 27 minutes at 20 C. So until you get the derating curves for the specific battery, I would not promise more than 3 hours. On the other hand, a 12 AH battery will last 8 hours.

By getting into bed with a specific manufacturer, you can ensure that your device will outperform what you state because you can use the worst case numbers from the datasheet and if the customer uses a different brand and is unhappy with the results, you can always say “I told you so” then sell him what you know will work.

 

[audioguru]

The datasheet for a battery shows its performance when it is brand spanking new.
But a battery (especially a lithium battery) loses performance with each discharge-charge cycle especially if it has been discharged too low.

When a discharged battery charges normally in 4 hours but now it charges in 6 minutes then its discharge time is also reduced to 3 minutes.
I have many Li-Po batteries that charge in 10 seconds then fly my electric model airplane for only 5 seconds. A new battery charges in half an hour and flies my airplanes for 15 minutes.

 

[Ubergeek63]

About 4 hours. On paper (and only on paper) a 100 AH battery would deliver 100 amps to a load for one hour. It would deliver 50 amps for two hours and 25 amps for 4 hours. I am sure you get the idea. However, looks good on paper but things don't work that way. In theory your 6.9 AH battery would deliver 6.9 amps to a load for an hour. So if the load is 1.5 amps we could take 6.9 / 1.5 = 4.6 hours. However, the greater the load the more the rule changes. I would feel comfortable wit 4 hours assuming the battery is fresh with a full charge.

Ron

ummm no. 6.9 amp hour is likely to be a lead acid and they are liars. they are speced at the 20Hr rate, under normal reasonable drain rates they are half the 20Hr rate

 

[crutschow]

The lightest weight battery for a given energy storage, although expensive, would be a lithium ion type, such as used in a laptop.

 

[Ubergeek63]

The lightest weight battery for a given energy storage, although expensive, would be a lithium ion type, such as used in a laptop.

A 3AHR LiFePO4 for $45: LiFePO4 18650 Battery: 12.8V 3000 mAh (Side by Side, 38.4Wh, 7A rate )

would be the safest, most reliable battery for his application. SLAs a high drain rates only supply a little over half their rated capacity and are generally limited to around 300 cycles. LiFePO4s run around 80-90% rated capacity for 2000+ cycles and there is no dangerous failure mode that you find in normal lithiums.

 

[Antigrav]

Thanks for the input guys....

Actually I'm the maker of the battery packs...I use Lifepo4 batteries and I'm trying to figure the size pack I would need to power one of these CPAP machines for an 8-9 hour period. The lifepo4 pack someone advertised below is sort of what we do, but our packs are much cleaner and we use usually a layouts in the 4sxP and we only use A123 Systems batteries while the other companies use a copy of the A123 that are not nearly as stable or reliable.

My situation is this.... A customer knows I make battery packs that are super lightweight and wants to know if I could make a battery he could use for his CPAP machine when he goes camping over night...The battery will charge off his motorcycle while he is riding out in the boonies... He has told me his machine has the 1.25 amp draw, but I've been trying to contact CPAP machine companies to get the actual draw... or even just get one so I can test it with a 16 cell 9.2 Ah, or 11.5 Ah 20 cell pack .

 

[Ubergeek63]

well the A123s are wonderful batteries, if a bit pricey. i have found them to be very helpful, why did you not call them instead of asking what purports to be a bunch of hobbyists? ( even though myself and a couple of others actually are EEs )

 

[Antigrav]

well the A123s are wonderful batteries, if a bit pricey. i have found them to be very helpful, why did you not call them instead of asking what purports to be a bunch of hobbyists? ( even though myself and a couple of others actually are EEs )

Actually only the Gods can talk to anyone at A123... they do not respond to email, or phone calls... they only deal with GM, or other big companies.They have a spec sheet but that means nothing to me... real world testing is the only method I can go by, though though the spec sheet can give me an idea. I was coming to the hobbyist because they have usually done a lot of real world testing on their own home grown projects so have some data that can apply to a project like this... But I also wanted the equation to figure out what AH figure I was looking at or the 1.25 amp draw that the customer thought was the draw from his CPAP unit... Most my questions have been answered here... and I think I just need to test a real unit with some of my packs and monitor the usage.

 

[Ubergeek63]

Actually only the Gods can talk to anyone at A123... they do not respond to email, or phone calls... they only deal with GM, or other big companies.They have a spec sheet but that means nothing to me... real world testing is the only method I can go by, though though the spec sheet can give me an idea. I was coming to the hobbyist because they have usually done a lot of real world testing on their own home grown projects so have some data that can apply to a project like this... But I also wanted the equation to figure out what AH figure I was looking at or the 1.25 amp draw that the customer thought was the draw from his CPAP unit... Most my questions have been answered here... and I think I just need to test a real unit with some of my packs and monitor the usage.

perhaps things have changed ... generally in high drain situations A123s will give you 70% or more their rated capacity though you do lose some across the internal resistance...

mind you high drain to an A123 is 50+ amps instead of 10A for a comparable AHr rating.

 

[Gary B]

I used to spec batteries for telephone backup systems. Post the spec sheet and I will make a recommendation that I am sure my colleagues will dispute.