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I have a 12V 5A CPAP device that I'm looking to use while camping for 2 nights. Therefore, I'm trying to determine a deep cycle battery to meet my needs without burning a hole in my wallet. Based on calculations I've seen while browsing the net, I need a battery that can deliver 200AH. Calculation is based on C'' found here: http://www.powerstream.com/battery-capacity-calculations.htm

However, searching various car batteries at different stores, they are not listed as deep cycle, but rather CCA (Cold-cranking amps). Searching for a 200AH Deep Cycle battery results in batteries that are over $300.

Am I calculating this incorrectly? I was hoping to spend far less considering I won't use the battery very often.

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  • \$\begingroup\$ Batteries don't deliver Amp hours, they deliver Watts. \$\endgroup\$ – user86234 Sep 12 '16 at 16:04
  • \$\begingroup\$ You say you're camping. I assume you aren't going to be carrying the battery about? Because if you are, you can find something much lighter than lead acid. \$\endgroup\$ – Jack B Sep 12 '16 at 16:04
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    \$\begingroup\$ @tuskiomi Whoah.. 'current loss from being on the ground'? Where did that come from? \$\endgroup\$ – Spehro Pefhany Sep 12 '16 at 16:21
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    \$\begingroup\$ Sorry, @tuskiomi, that hasn't been true for a long time. Check any of the results from this Google search: store car battery on concrete. \$\endgroup\$ – bitsmack Sep 12 '16 at 16:46
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    \$\begingroup\$ Today I have learned. Maybe i'm just old fashioned... :-) \$\endgroup\$ – user86234 Sep 12 '16 at 16:48
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To calculate the capacity you need, you just multiply the number of amps by the number of hours. So 5A for 40 hours is 200Ah, simple as that. You should build in some safety margin, as the capacity will drop as the battery gets older.

Batteries which are not advertised as "deep cycle" or "deep discharge" will age faster as you use them, but if you're not using it often, it won't really matter too much. The car batteries you've been looking at should really have a capacity marked on them somewhere (it's required by law over here, maybe not in the US), but they are probably in the 40-80Ah range. 300USD seems not unreasonable for a 200Ah version. Because the car batteries are mass produced, it may work out cheaper to get several car batteries and put them in parallel, a quick look at my usual suppliers suggests that could be done for £180 (about 240USD).

Whatever you do, check your equipment can run OK from a car battery. The actual voltage of a car battery at 5A will drop a bit below 12V, and when charging from a car may go as high as 14.5V.

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  • \$\begingroup\$ I came up with 200Ah based on the safety calculation, but I only need it for 16 hours (80Ah) at most, realistically 12 hours (60Ah). I would most likely keep it in a plastic battery box and set on the ground in my tent. \$\endgroup\$ – SoupRDude Sep 12 '16 at 17:02
  • \$\begingroup\$ I don't know how bad it would be to run out of battery, so I can't comment on the safety margin you should allow. But at 100Ah or less, you're getting into the sizes that are mass produced for vans and trucks. I can find 100Ah batteries for as little as £55 (70USD), though these are cheap makes and a decent quality one is £75 (100USD). \$\endgroup\$ – Jack B Sep 12 '16 at 17:11
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It is good practice to limit lead-acid type battery discharge to no more than 50%, otherwise you significantly reduce the battery life (charge / discharge cycles). This means that you want a battery with an amp-hour rating of twice your expected usage. Lithium batteries can generally be discharged 80%, and they are lighter, so these may be a better portable solution. Unfortunately, Lithium batteries are more expensive than lead-acid. For portable use, with lead-acid batteries you want gel-cell or AGM (Absorbed Glass Mat) batteries as these don't spill and can be operated in any orientation

There is a newer lead-acid battery design that allows for much deeper discharge -- this is called "carbon foam". Firefly makes these batteries.

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5A load for 40 h requires 200 Ah according to your assumptions for 2 nights and daytime hours.

You are reasonable in your cost calculations. Maybe consider a small gas generator.

I just read CPAP is a Continuous positive airway pressure ( power assisted breathing machine )

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  • \$\begingroup\$ I thought CPAPs were for sleep apnea? \$\endgroup\$ – user86234 Sep 12 '16 at 16:28
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    \$\begingroup\$ I might consider if just for apnea that 14h *5A or 60Ah is just enough \$\endgroup\$ – Sunnyskyguy EE75 Sep 12 '16 at 17:28
  • \$\begingroup\$ If I camped a lot, Depending on trail, consider a rickshaw with bike tires to power the wheels and 500W generator to charge battery and carry all supplies. DIY only \$\endgroup\$ – Sunnyskyguy EE75 Sep 12 '16 at 17:35
  • \$\begingroup\$ Yes, this is for a sleep apnea device. I only added all the safety calculations in order to not completely drain the battery as I would like it to last for future use. I plan to use the battery 3-4 times a year, and not more than about 16 hours between charging. \$\endgroup\$ – SoupRDude Sep 12 '16 at 17:35
  • \$\begingroup\$ You might consider 12 LiPo cells each rated for 10Wh to get 120Wh using 3.5V avg @2800mAh typical @$7/cell (18650) by shopping around. Panasonic are better than most. Arranged in 3P4S array with inexpensive cell balancer online for charging. Total cost<$100 and much lighter. \$\endgroup\$ – Sunnyskyguy EE75 Sep 12 '16 at 17:47

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