Before you purchase batteries it is wise to determine the maximum electrical load you need from the batteries to run the electrical items in your sleeper when your engine is not running.
Let’s assume:
- You do not have an APU or generator.
- You have a coffee pot, microwave, refrigerator/freezer, blender, TV, and a computer, all running on 120-volt AC power. Don’t forget any light bulbs that run on 120-volt AC power, for example if you have added a desk-style lamp.
- Your truck is equipped with a 3000-watt inverter.
Then determine which appliances you want to run at the same time:
- Will your computer be running while your TV is on?
- Will you be brewing a cup of coffee while microwaving a breakfast burrito? If your refrigerator/freezer is hardwired into the truck then it wouldn’t hurt to assume this unit is always running.
On the back or bottom of all 120-volt AC electrical appliances you will find a tag that tells you the electrical consumption of that appliance. It will state the designed voltage, designed hertz and how many watts the appliance will use when operating. A watt (W) is defined as the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V).Any truck that runs 120-volt appliances must have an inverter that changes 12-volt DC power from your batteries into 120-volt AC power for your appliances. The inverter is rated in watts and will not deliver more wattage than it’s designed to deliver. If you try to pull more watts than the inverter is designed to deliver, some appliances won’t work until there is enough capacity in the inverter to power those appliances, or if partial capacity is available in the inverter your appliance may run but without enough “power” to do the intended work.
You may think your appliance is broken or defective when it may just be a case of low power, in which case your steaming cup of coffee may end up being lukewarm and look like weak tea. Knowing the wattage that each appliance consumes will help you to not overload your inverter and you will be able to determine how many appliances the inverter will power simultaneously.
- W=Watts
- V=Volts
- A=Amps
- Watts (W) = Volts (V) x Amps (A)
- Amps (A) = Watts (W) / Volts(V)
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Here are a couple formulas to help when determining the electrical load used in your truck. Here is a flashback of the dreaded story problems from school. Your coffee pot is rated at 1500 watts (from the tag on the bottom) and is connected to 120 volts through your inverter. How much current will it draw? Since current is measured in amps we need the second equation: A = W/V. A = 1500W / 120V which equals 12.5A. Therefore running your coffee pot will pull 12.5 amps from your batteries.
From the definition section of my first article you will find Amp-Hour (AH). This is the unit of measure for a battery’s electrical storage capacity, which is obtained by multiplying the current in amps by the time in hours of discharge. For example, a battery delivering 10 amps for 30 hours = 10 amps = 300 amp-hours (AH).
If the battery powering your inverter is a 300 amp-hour battery and we divide our coffee pot amps into the amp-hours we get the following answer: 300 amp-hour / 12.5 amps = 24 hours. Theoretically if your battery is fully charged and you have a zero resistance electrical system (no electrical losses which is impossible) your battery will run your coffee pot for 24 hours. In reality it will run your coffee pot for a shorter time. If you follow this example and add together all the electrical appliances you use each night you will be able to get a fairly good approximation of the demand you are placing on your batteries which should help you in deciding which batteries to choose. Having this information should also give you an idea how long your batteries will last during your 36-hour re-start without having to run the engine to recharge the batteries.
If you are using 24-volt DC appliances like those found at most truck stop travel stores, I’ve found they will list their power consumption in both amps and watts. This is convenient since you simply need to add together the amps from the appliances you are using to determine their drain on the batteries.
As a side note, you have probably noticed that 24-volt DC appliances are much slower than 120-volt AC appliances. The reason a 12-volt DC coffee maker takes longer to brew is due to the limitations on the 12-volt lighter plug. Since it can only handle a limited amount of power, the manufacturer must reduce the power draw needed by the appliance through this plug. They are limited in that the amps drawn at 12 volts can’t exceed the vehicles 12-volt plug capacity. Hooking your 12-volt appliance directly to the battery will not make it work better because the appliance has been designed to draw a limited amount of power.
Simply put, if you want a faster cup of coffee you need an appliance designed to draw more power.