So! You are on a hot summer day, and a electricity just randomly shuts off? And you’re wondering can a car battery power an air conditioner? You are on the right place, because this was happened to me, and I wondered the same thing. It’s very stressful when power electricity goes down, not just in summer but at any time as well. But this question is very special, because what else could possibly make you cool? Cold drink perhaps? Yeah, right. It will makes you sweat a lot… I’ve been exploring this topic online and getting some conclusions, and I’d like to share this knowledge with you.
Let’s assuming that you have a 80Ah car battery, and all car batteries operate at 12V (Volts). And you have an air conditioner which works on 120V power outlet and draws 15A (Amps). Because A/C unit can’t work directly from battery alone you will need inverter too, in order to convert DC power from car battery to an AC power which air conditioner use. With all calculations of power consumption and power losses you will get at most 5 to 10 minutes of A/C unit to cool you down, all of that time from a single 80Ah car battery. So, answer is yes, but just for a short period of time, which is unfortunate.
For better answer on this question we must ask several important questions to get even more accurate answer:
- What is the size of the car battery, in particular Amp-Hours (Ah) or capacity?
- What is the size of A/C unit (power consumption and what is efficiency of one)?
- How big is the space you want to cool down, and how well is its insulation?
When considering car batteries, which are larger than most the other ones out there, they are just a energy storage devices in the first place, and second they are a power sources for an electric devices that needs power to run.
But still, car batteries are similar with all types of batteries from an energy storage perspective. And yet, they are not the biggest ones, because there are electric vehicles and UPS (uninterruptible power supplies) for large buildings batteries. This kind of batteries are much larger in physical size then car batteries.
After all I am an engineer, so we like to ask questions that ordinary person wouldn’t ask. When it comes to batteries and certain applications that needs to run out of the battery there are always some factors that must to be answered.
To get a more accurate answer on the original question I will give you some parameters. For example let’s say that you are using 80Ah 800CCA car battery, and a 120V 1800W air conditioner unit.
AC or DC, what is it in this case?
All batteries operates in DC mode, so the car batteries too. Our A/C unit operate in AC mode, so we need inverter to convert car battery DC electric power to AC electric power, to be able to power air conditioner on battery.
Required voltage for our application?
When you need higher voltage from any battery type, what you will get it’s a lower storage capacity. It is inevitably because that is how it works. When it comes to batteries usually is the best to work with lowest voltage possible.
If we connect batteries in series we will increase voltage output. And in our case we should connect 10 batteries in series to get 120V output for our air conditioner unit. This way we would waist lots of space. The best way is to use single battery and inverter to convert DC power from car battery to AC, to be able to power our A/C unit.
What is the current that our application require?
Everything that runs out of electric power requires current. In out example we use air conditioner which have some electrical moving parts that requires current. Those parts are evaporator fan, blower fan, compressor and control circuitry.
The initial current, or startup current, or even in-rush current is very high, and last just for a couple of seconds, after that A/C unit goes down and works stable on low, running current. Initial current can be as much as 6x of the running current.
The very crucial thing for us is the capacity of the current of which power supply can provide to an electric device. This is so important because our voltage sources are wall power outlets and batteries. This power sources deliver constant voltages.
That delivers enough amount of current just to overcome the resistance that passing through the terminals. In fact, if you apply direct short through the terminals you’ve be getting infinite amount of current. But in reality this is not quite true, and one of the reasons is the melting point of the materials that are used to pass high volume current.
Because our car battery has 800CCA or Cold Cranking Amps, and air conditioner has 15A of running current, of which if we multiply with 6, we get 75A of in-rush current, we have a steady system here from that viewpoint. As we already know, we’ll going to use inverter here, so we can’t use this numbers directly.
Inverter will increase the voltage from the car battery to the air conditioner, so the current will also be affected in this step-up process.
For example, if the voltage difference is 10x so the current will be also. Our A/C unit requires 15A to run and 75A to initially start from the output side of the inverter, that means that car battery needs to be able to supply 750A for startup and 150A to run. In this case our car battery will be just enough to run the air conditioner.
How much energy storage we actually need for this application?
Now, we can ask how much energy storage do we need actually, because we talked about requirements of the power supply in out system. In our case we need car battery so we can ask that question in the end.
And this is how it is, the more energy we need the more physical space must be. Batteries capacity are measured in Ampere Hours or Ah. Our smartphones and laptops uses batteries that are measured in milliapm hours or mAh.
For example 1000mAh = 1Ah. In order to tell the quantity of Ampere Hours for our system, we use the value from the steady current in Ah which is needed to run the system, not the starting current. And then simply multiply that number number of hours that we need to run out system.
Sadly, this is not that simple. We have to consider two things, one is rated capacity and second is useful capacity, this are two different things. We need to have in mind the discharge rate, because it can be affecting the useful capacity by much, in some cases by as 50%. So this tells us to think of scaling of the battery in capacity in any case.
We can use different approach to this and use Watt hours or Wh instead. This way we can avoid the inverter that will convert car battery 12V to an 120V of the A/C unit, and also current, 100A to 10A.
Our car battery has a capacity of 80Ah when distribute 150A, and converting in Watt hours will be slightly under 1kWh, to be precise 960Wh.
Lets see how long can a car battery run an air conditioner?
This question is so delicate that can’t be answer just straight up without any calculations and actually real practical test. In our case we only use one battery, so we are limited on what that battery can provide.
Yet, if we need to design entire battery system, the time we want our air conditioner to run should be our input in order to figuring out car battery system. In this case our A/C draws 1800W of electric power, and inverter is, let’s say 80% efficient, this tells us that car battery must supply 2250W of electric power when the A/C unit is running.
Now, let’s say that A/C unit works at a 50% duty cycle, it runs for 15 minutes and then it rests for 15 minutes. So, now we can actually calculate and get our answer. Out battery capacity is 800Ah, dividing by the 2250W of the power that is required and take in account 50% duty cycle, we get 0.18 hours which is 10.8 minutes.
And don’t forget of that 50% duty cycle, which it really means that A/C unit running 50% of those 10.8 minutes.
Conclusion is that a car battery can run air conditioner, but with the power inverter, and sadly it will only run for 5 minutes or so, which is not that practical to keep cool anything, except from your desire perhaps and expectations.