Test post…..You gotta have electricity while on the road. It’s one of the main things that makes it feel more like home, whether you’re on the road for a few weeks or a few years. This post is a quick run through of camper van battery electricity basics, looking at how does a battery work and how to calculate volts, amps, watts, amp hours, watt hours and more!! Here’s what’s discussed below:
- What is a battery?
- Volts, amps & watts
- Lead acid battery calculations
- Lithium battery calculations
- Calculate amp hours & watt hours
A battery is essentially a chemical reaction in a box. On the outside of a car battery we only see the positive and negative terminals poking out. On the inside there are sets of positive and negative plates (called electrodes) and a porous insulator between them called a separator. All of that is submerged in a solution called electrolyte. The electrolyte can be liquid, semi-liquid or a solid, depending on the battery type.
When a battery is charged, an electrochemical reaction between the electrodes and electrolyte occurs and electricity from the charger is converted and stored as chemical energy.
When power is drawn from the battery, a reverse reaction between the electrodes and electrolyte converts the chemical energy back into electricity, with negative charged particles (electrons) flowing out from the battery’s negative terminal, through the wires attached to the battery, to your electronic appliances and back to the battery’s positive terminal. Upon recharging, electrons flow back to the battery from the positive to negative terminal, creating a chemical reaction once again and restoring charge to the battery.
So the battery is not really generating electricity. It is just storing it to be used later. The amount of energy a battery can store is referred to as the capacity. It’s like water in a hot water tank, sitting there until you’re ready to start using it. The larger the tank or battery, the more that can be stored. Similar to when you turn the tap and let the water flow from the water tank, hitting your electrical device’s ON switch opens a path that lets electrons (electricity) to flow out of the battery.
If we take the water and electricity analogy further:
The flow of electrons in a circuit through a wire creates an electrical current. This is similar to water flowing through a hose or pipe. The power the battery delivers is meaured in watts. With our water tank analogy, power would be the total amount of water flowing through the system. How fast and how many electrons are moving through the electrical circuit is measured in amps. This equates to the rate at which a volume of water is flowing through the hose. The force with which the electrons flow through the circuit is measured in volts. This is similar to the pressure the water exerts pushing it’s way through the hose or pipe.
The battery’s capacity is measured in amp hours, abbreviated Ah. In terms of battery usage, this measure tells you how many amps or electrical current the battery can deliver in one hour. The greater the amp hours the longer you’ll be able to run your electronic devices. Much like the total amount of water available is limited by the size of the water tank, the amount of amps available per hour is limited by the size of the generator or battery.
Battery Electricity Calculations
The above water-electricity analogy allows us to visualize what has to be taken into account in order to figure out how many amps, volts and watts will be required to run all our electrical gadgets and devices.
We can now move on to some basic battery electricity calculations by comparing your home’s 110 volt electrical system with the 12 volt battery you’d have in a campervan. We can think of the house’s electrical system as being like a fire hose, while the van’s 12 volt battery is more like a small garden hose.
Because the house has a bigger water tank (capacity), it will have a greater volume of water flowing per hour (amps) with a higher water pressure (volts). This results in the house being able to deliver a greater amount of power (watts) than the van. The more amps and volts you have, the more power you’ll be able to generate. This is reflected in the formula:
Watts = Amps x Volts
If we take the example of a 60 watt light bulb, whether you’re using the house’s electrical system (fire hose) or the 12 volt battery (garden hose), the amount of power coming out of either hose has to be 60 watts for that light bulb to work. For the garden hose to have the same amount of power coming out of it as the fire hose, the volume of water going through it has to be flowing at a faster rate than for the fire hose.
Translating this into electrical terms, in order to generate the same number of watts, the amperage has to increase if the voltage decreases. In other words, the number of amps required by a 12 volt battery to operate the same 60 watt light bulb is going to be greater than that needed by the 110 volt household system.
The number of amps multiplied by volts gives you the total power in watts. If you want to know how many amps an electronic device will draw or use, then divide watts by volts:
Amps = Watts / Volts
Let’s take the 60 watt light bulb again. Your household electrical system is 110 volts. So the amount of amps that light bulb will draw is 60/110 = 0.5 amps. But if you use that same light bulb in your van with a 12 volt battery, the calculation is 60/12 = 5 amps. The battery will have to provide ten times more current to run that same light bulb.
Realistically, however, you won’t be able use the full 100 percent capacity of a lead acid battery. As we’ll see in our discussion of types of batteries, to prevent damaging your battery you never want to drain it below the 50 percent level. The percentage amount a battery has been discharged from full is referred to as the depth of discharge (DOD, for short). So with lead acid batteries you can only utilize half of their capacity. With lithium ion batteries, as we’ll see below, they have a greater depth of discharge which makes them quite advantageous.
In order to determine realistically how long a 12 volt lead acid battery will operate the 60 watt light bulb, you have to take the battery’s rated capacity (amp hours, Ah) and divide that in half because of the 50 percent depth of discharge limit. You then take that amount and divide it by the number of amps that the electrical device draws:
12v Usage Hours = (Rated Ah x 0.5) / Amps
For our 60 watt light bulb drawing 5 amps and a 12 volt battery rated at 24 Ah, the calculation is 24 x 0.5 = 12/5 = 2.4 hours. So the battery will give you enough power to use that lightbulb for just under two and a half hours. If you had a bigger battery with a capacity of 100 mAh, then the light bulb would last 100 x 0.5 = 50/5 = 10 hours.
But it’s not as straightforward as that, so it’s time to ratchet up our battery electricity calculator.
One important factor when using household items with your 12 volt battery is that you’ll need an adapter called an inverter. The 110 volt household device uses alternating current (AC), so named because the current switches direction 50 to 60 times each second. A 12 volt battery uses direct current (DC) which flows in one direction only.
In a camper van or RV setting, your electrical appliances would be plugged into the inverter which would be connected to the 12 volt battery. The inverter simulates an AC current by changing the direction of the battery’s DC current 60 times per second, allowing you to operate your AC device. But the inverter is only 80 to 90 percent efficient and the battery is ultimately required to provide a bit more extra power.
A conservative inverter inefficiency factor of 1.2 has to be applied to calculations for 110 volt AC devices. That is, the device’s AC wattage has to be multiplied by 1.2 to determine the equivalent wattage that the DC battery will have to generate. We can also calculate how many amps the AC device will draw from the battery:
DC watts = (AC watts x 1.2 inefficiency)
DC amps = DC watts / Voltage
For example, if you have a couple of appliances going through the inverter that have a total power requirement (referred to as load) of 140 AC watts, the calculation is 140 x 1.2 = 168 DC watts / 12 = 14 DC amps. That means the 12 volt battery will have to produce 168 watts and 14 amps of current in order to keep the 140 watt appliances running.
With lithium ion batteries the above calculations are little different because the lithium battery can be discharged to a greater degree than the lead acid battery before causing any damage to the battery. With lithium batteries you can drain them down to the point where there’s only 5 to 15 percent charge left, which translates to a maximum depth of discharge of 95 percent.
Now that we can calculate how long we can run our electronic devices on a battery of a specific size, we can also do the reverse calculation to figure out how large a battery we’ll need to run our electronic devices for a set period of time.
Watt Hours = Amp Hours x Volts c c
In a future post I’ll expand on the above battery electricity calculator basics to figure out how much power your electrical system will have to generate to meet your complete electrical needs.