How Long can a Car Battery Power a CPAP?

Have you ever found yourself in a situation without conventional power and wondered how long your car battery could power your CPAP in a pinch.

A 12-volt car battery can certainly power a CPAP machine.  The duration, efficiency, and long-term damage to your car battery leaves much to be desired, however.

How long can a car battery power a CPAP machine?

A 50 amp-hour equivalent car battery should be able to power a 2.5-amp CPAP for 6 hours and a 5-amp CPAP for 2.63 hours with an inverter. With a direct-current connection (no inverter), a car battery can power a 2.5-amp CPAP for 7.34 hours or a 5-amp CPAP for 3.2 hours and still be able to start the car.

There are so many variables that go into answering this question and everyone’s results may vary depending on their makes, models, and settings.

The goal of this article is to explain the shortcomings of a car battery in powering a CPAP, why powering a CPAP with a battery is more efficient when using DC power rather than an inverter and AC power, and the simple formulas you need to use when sizing up any battery’s capacity that you might decide to use.

Let’s begin!


How Much Power Does a CPAP Require?

On average, a CPAP will require 2.5 to 5 amps to function. The amount could be more or less, and will be dependent upon the personal settings that you choose such as the pressure, heat, and humidifier. Newer models with sleep tracking features generally require more power.

It’s a pretty accurate assumption though to rate a CPAP without the humidifier or heat at about 30 watts (or 2.5 amps = 30 watts / 12 volt battery).  

A CPAP using the humidifier or heat will be around 60 watts, or 5 amps, since 60 watts / 12 volts = 5 amps.

Some will be higher (especially newer models with advanced features for helping users track sleep patterns), some will be lower, but we’re going to stick with the numbers above for the examples below.


How much Energy can my Car Battery give to Power my CPAP?

Amp Hours vs. CCA

Car batteries are “starter batteries” and are rated in CA or CCA (cranking amps, or cold cranking amps).  These are not the same as AH (Amp Hours). The engineering behind a car battery is such that it is not designed for “deep cycling”.

It is designed to quickly release a high amount of amperage in order to start your vehicle and then to be immediately recharged by your car’s alternator.

Using the car battery while disconnected from the car in order to power something for hours on end is a sure way to reduce its lifespan.

With that being said, it is a pretty commonly agreed upon that many car batteries have around a 50AH equivalency.  Some have more, some have less. We will be using 50AH for our examples in this article and assuming it is a healthy battery.

Inefficiencies

Inefficiencies need to also be taken into account with either method, but especially if we are an inverter.  The inverter would be necessary if you didn’t have the DC (direct current) plugs, cords, and sockets to attach directly to the battery terminals.

If all you have is a normal looking plug that would fit into a wall outlet, then you will need an inverter or you will need to buy the necessary DC accessories to make your CPAP more efficient.

When using an inverter for AC power (alternating current), the battery will lose about 15% of its efficiency.

Furthermore, your AC cord has a PSU (power supply unit) attached to it and it converts the AC power from the inverter into DC power for your CPAP.

You’ll lose about another 10% efficiency from the PSU attached to your AC power cord.

If using a DC connection only (no inverter), we’re still going to factor in a 10% inefficiency due to the DC cord modifying and regulating the strength of the current to match your CPAP’s requirements.

In effect, you’re taking DC power from the battery, converting it to AC power for your CPAP cord, which then converts it back to DC power in the PSU for final use in your CPAP!

A 400-watt inverter would be more than sufficient to power your CPAP, but regardless of which wattage you choose, you’ll need to make sure that it is a pure sine wave inverter — especially if you plan on using the heating or humidity features.  

The choppy electricity provided by a modified sine wave inverter (cheaper in price) can cause the heating elements to remain on when they’re supposed to be off and you can burn them out prematurely.  

This inverter is an example of a pure sine wave model and has excellent reviews from Amazon.  It is more than adequate to power your CPAP machine.  You can always scale up to a larger wattage model or a model with more outlets if you plan on powering more than just your CPAP.

Peukert’s Law

Finally, we have to consider Peukert’s Law when it comes to the discharge of our battery.  Peukert’s Law essentially says that the more amps you pull from a battery (especially a lead-acid battery) with regards to its total capacity, the fewer true AH (amp hours) you’ll get out of the battery due to inefficiencies in transferring all of that energy so fast.  

For example, a 50AH battery will likely give you 50 hours of use if you only draw 1 amp per hour.  However, if you want to draw 25 amps per hour, you won’t get 2 hours. You’ll likely only get just over an hour.  

You’ve lost nearly 50% of the amp hours hours available by using 50% of the total amp hour capacity. If you use 5% or less in amps when compared to the total AH rating, your AH should stay true or even increase since the battery was rated at a higher draw than you’re actually using.

The Peukert Number that we’ll be using in our examples below will be 1.2, which is common for lead-acid batteries like the one used by your car.


The Math: How Long can a Car Battery Power a CPAP Machine?

Recap for the numbers:

We’ve already established above that a CPAP with basic settings will run at 2.5 amps, and one with a humidifier will generally be around 5 amps.

To factor in the inefficiencies when using the inverter, we need to back up (go in reverse) the chain from the final amps required by the CPAP.

Inverter: 2.5 Amps / .9 / .85 = 3.268 Amps (Or, in other words, 3.268 Amps * 0.85 for inverter * 0.9 for PSU = 2.5 Amps

Inverter: 5 Amps / .9 / .85 = 6.536 Amps (Or, in other words, 6.536 Amps * 0.85 * 0.9 = 5 Amps

No Inverter: 2.5 Amps / 0.9 (for the 10% inefficiency of the DC cord) = 2.778 Amps required by battery

No Inverter: 2.5 Amps / 0.9 (for the 10% inefficiency of the DC cord) = 5.556 Amps required by battery

So, the CPAP at 2.5 amps requires that 3.268 amps be drawn from the battery so that after the 85% inefficiency of the inverter and the 10% inefficiency of the PSU, the full 2.5 will be delivered properly to the CPAP.

The 5 amp setting on the CPAP will require that the battery discharge 6.536 amps to make it through both inefficiencies to arrive at the CPAP unit at the full 5 amps.

When not using the inverter,

Peukert’s Law to find out how much time you’ll have with your battery is as follows:

Estimated AH of the battery / (amps drawn by the CPAP ^ 1.2) = Total time for 100% discharge.  

If we take 50% of that total discharge time by multiplying it by .5, then we will have the halfway mark which is what you’ll want to keep if you want a chance at starting your car in the morning.  

You can use the formula above to estimate the total time you’ll have with any given flooded lead acid battery that you may find.  If you find a Lithium-Ion battery that you like, you could use 1.05 for Peukert’s constant for a good estimation. A sealed lead acid battery(Gel Cell), or an AGM battery should have around 1.1 or 1.15 for Peukert’s constant.  

The Results:

Inverter with basic settings: 50AH / (3.268 ^ 1.2) = 12.073 total hours @ 100% discharge ⇒ 12.073 * .5 = 6.037 hours @ 50% discharge

Inverter with humidifier: 50AH / (6.536 ^ 1.2) = 5.255 total hours @ 100% discharge ⇒ 5.255 * .5 = 2.628 hours @ 50% discharge

DC Connection (No inverter) basic settings: 50AH / (2.778 ^ 1.2) = 14.672 total hours @ 100% discharge ⇒ 14.672 * .5 = 7.336 Hours @ 50% discharge

DC Connection (No inverter) humidifier: 50AH / (5.556 ^ 1.2) = 6.386 total hours @ 100% discharge ⇒ 6.386 * .5 = 3.193 hours @ 50% discharge

Of course, these numbers are just a conservative estimate taking into account the inefficiencies of using an inverter to convert DC power into AC, and also taking into account Peukert’s Law.  

Your mileage may vary, however, and you might find yourself getting more time available than these estimates.  

A new battery might give you a lower Peukert Constant which will give you far more time, or your CPAP might use even a tenth of an amp less than these examples which could have major implications and add up to an hour or more by switching a tenth of a number.

The reverse is also true, as I have seen CPAP usage go up to 6.5 amps, and this will drastically cut down your time.


Is a Deep-Cycle Battery a Better Option than a Car Battery?

It is absolutely the case that a deep cycle battery is a better option for powering your CPAP than a car battery.

The deep-cycle battery is specifically engineered to discharge to low depths and be fully recharged again.  This type of regular use will kill a car battery quickly, whereas a deep-cycle battery can do it hundreds and hundreds of times — assuming you keep an eye on the electrolyte levels and specific gravity of each of the cells.  

A group 31 battery with 105AH would get you around just shy of 4 full nights of sleep before needing to be recharged when using only 2.5 amps and no inverter (based on 8 hour sleep cycles)!


How to use your CPAP with DC Power

Bypassing the inverter and AC power by hooking directly to the battery is the way to go if you want to conserve the battery’s power.  You’ll instantly have 15% more battery efficiency that can go towards powering your CPAP.

If your CPAP model only came with a standard plug that fits into a wall socket, you’ll need to contact your manufacturer or look online to see if there are adapter cables that you can purchase which will allow you to power your unit from 12-volt sources, 24-volt sources, or both.  

This is an example of what the cords would look like on Amazon. You’d want to research which one would actually fit your model.

You will need a cord with clamps on one end to connect to the battery terminals and a DC port (cigarette lighter socket) will be on the other end.  Then you’ll plug your CPAP’s DC cable into that DC port and the other end into the CPAP.

Now you’ll be using energy far more efficiently than if you were to use an inverter.


Other Options for Powering your CPAP

Let’s face it, the moving of the weight of a deep-cycle battery or even a car battery is a major downside when it comes to powering your unit.  Throwing out your back just trying to get a good night’s sleep isn’t ideal!

If you are someone who is on the low end of the power draw (2.5 amps or less), they have some pretty cool power banks with lithium ion batteries inside that weight only 10-12 pounds and will get you through a single night without a problem and maybe even a second night.  This is, of course, assuming that you would be powering your CPAP with DC cables and not using the wall plug that you use at home.

I personally like this model on Amazon that has a battery with 440-watt hours.  If you use a CPAP that requires 2.5 amps, you would follow these steps:

2.5amps * 12 volts = 30watts ⇒ 30 watts * 8 hours of sleep =  240 watt hours ⇒ 440WH / 240WH = 1.83 nights of sleep, or ⇒ 1.83 * 8 = 14.667 hours (14 hours, 40 minutes)

Or, to keep it very simple:

440WH / 30watts per hour for CPAP = 14.667 hours (14 hours, 40 minutes)

With this model, you can expect about an 11% reduction on efficiency (time) if you choose to power your device with the AC cord.


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Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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