Obviously, there are lots of variables. There are different kinds of batteries on the market and several different kinds of chargers, and the cost of electricity varies from region to region. However, even with all of these variables, some general conclusions can be drawn, as I will attempt to do in this post.
Let’s start with the basic measure of electricity that your bill will be based on: the kilowatt-hour. It’s pretty simple to understand and I will use it to describe what goes on while charging the golf cart. A kilowatt, is defined as 1000 watts of power, and the number of watts is defined as the current being used by the appliance multiplied by the voltage that supplies the current flow. Let’s take some examples:
If we have an appliance, like a light bulb, that draws 1 ampere (amp) of current and is being supplied with 120 volts of force, the number of watts being used is 1 amp x 120 volts = 120 watts: power in watts is equal to the current in amps multiplied by voltage in volts. That is pretty simple. Now let’s say that you had 10 of the same type of bulbs plugged in at the same time. You would then be using 10 amps of current (instead of 1 amp), but the voltage would still be 120 volts, so, you would now be using 10 times the wattage: 10 amps x 120 volts = 1200 watts. This can also be expressed as 1.2 kilowatts (1.2 x 1000 watts).
Now, in order to decide how much our electric bill for using the 10 bulbs would be, we must relate things to time. It depends not only on how much wattage is being used, but also, how long it is used as to how much your electric bill is going to be affected. That’s where the kilowatt-hour comes in. By definition, 1 kilowatt-hour is 1000 watts being used for 1 hour. So, if we used our bulbs for one hour, we would be using 1200 watts (1.2 kilowatts) multiplied by 1 hour. So, we’ve actually used 1.2 kilowatt- hours of power. Had we only lit up 5 of the bulbs, but for 2 hours, we would have used the same amount of power, even though we were using half the wattage. In kilowatt-hours: 5 amps x 120 volts is only 600 watts, but since we used it twice as long, 600 watts X 2 hours still comes up 1.2 kilowatt-hours. Or if we had used 20 of the light bulbs (using twice the wattage) for only one half of an hour, we would still consume the same amount of energy: 20 amps x 120 volts is 2.4 kilowatts x one half hour = 1.2 kilowatt-hours.
In summary:
watts = current being drawn by load (our charger) multiplied by the source voltage
kilowatts = watts divided by 1000 (1 kilowatt = 1000 watts)
kilowatt-hours = kilowatts being used multiplied by time in use (in hours)
1 kilowatt-hour = 1 kilowatt being used for 1 hour
When it comes to the battery charger for a golf cart, many authors look at the label on the charger and conclude that the charger is roughly a 1200- watt appliance. That is because the label says 120 volt and 10 amps on it. And then, they figure that the average charge time for a cart is about 5 hours. So that makes the calculation of cost per charge pretty simple to figure out. 1.2 kilowatts multiplied by 5 hours equals 6 kilowatt-hours. Then we just need to multiply the number of kilowatt-hours by the local rate that we are charged per kilowatt-hour by our power company and bingo. Where I live, here in central Florida, we currently pay about 11 or 12 cents per kilowatt-hour, so I’ll use 12 cents for the following examples. So now, we multiply our 6 kilowatt-hours by $0.12 and we get $0.72 for the cost of charging the cart. You can get on the internet and find the rate for your local power, or just divide the amount of you last bill by the number of kilowatt-hours that is stated on the bill.
Here’s the problem with figuring it out that way:
The charger doesn’t use 1.2 kilowatts (1200 watts) the whole time the charge is occurring.
When a battery has been used for a while and is in need of a charge (low charge state), it will draw lots of current from whatever source tries to replenish it. In “electronics talk” (ohm’s law), the current that a device will let flow through it in a connected circuit, depends on the device’s resistance, and the voltage that is applied. In other words, I = E/R, where I stands for the current in amperes (amps), E stands for the voltage in volts, and R stands for the resistance in ohms.
As the battery starts to receive a charge (absorb current), its resistance starts to increase (it’s less thirsty). So, the I goes down as the R increases and the voltage remains the same. In our case, the voltage does start to rise a little, because it is easier for the charger to provide a little higher voltage, but the resistance going up is the main factor. This change in resistance is the driving force in making the charger do its job. There is no effort made on the part of the charger (at least this type of charger) to regulate the voltage or current flow. It just reacts to the batteries’ changing resistance (due to its changing charge state) to make things work. The logic board that is part of the charger does monitor the voltage as it changes over the length of the charge cycle, but only for the purpose of determining when the time is right to shut the charger off. All of this changes quite a bit when we go to one of the newer chargers (called a “smart” charger) as we will discus in a little bit, but for now, we’re still talking about the older automatic charger, like I was using in this test.
So, when the charger is first plugged in to the cart (assuming it is an automatic charger that is plugged into a normal 120- volt AC outlet), the batteries draw much more current at the beginning of the charge cycle than at the end. This is best indicated by using one of the older automatic battery chargers that has a current meter on its front panel. By “automatic charger” I mean the type that turns on automatically when the cart is connected to it, and the charger shuts off automatically when the cart is fully charged. With the current meter, you can easily see what is happening, if you watch it over the course of the entire charge cycle. How much wattage is being used by the charger (our appliance) varies as the charge cycle progresses. It might be a 1.2- kilowatt appliance at the beginning of the charge cycle, but certainly not at the end of the cycle.
At this point, it is important to understand that the charger has two things going on. One is the amount of wattage that it draws from the 120 AC outlet. That is the basis of the electric bill and is measured on the AC side of the charger (input side). The other thing, is that while the 120 AC outlet is supplying it with AC wattage, the charger is supplying DC wattage to the battery pack of the cart. The charger’s job is to convert the 120 volts AC into a usable DC voltage that can be used to charge the batteries. On that side of the charger, the DC voltage varies as the charge cycle proceeds, but for a 36 volt cart, it will start out at just above 36 volts (maybe 38 or so) and then raise up to a higher reding (maybe 45 or so) toward the end. A part of its being automatic, is that there is a logic board of some sort in the charger that senses these voltages changes, and keeps the charger tuned on until it is happy that it has been long enough at the “finishing” voltage, to allow the batteries to “equalize” out in order to get the batteries charged to nearly the same amount. We will always see a higher voltage being supplied by the charger to the batteries than we would see if we read the voltage of the batteries without the charger connected. If the charger is to replenish energy to the batteries, it has to maintain enough of a potential to offer current flow to the batteries. If the batterie’s voltage was higher, the batteries would be trying to “charge” the charger. Remember, here we are dealing with the DC side of the charger.
Typically, at the beginning of the charge cycle, you will see the current meter on the charger jump up quite high on its scale, probably in the range of 25 amps (if the charger has that high of a rating). If the charger is only capable of supplying 15 amps, it jumps up to close to that number of amps. But let’s, for the sake of discussion, use the number 25. If you had a voltage meter connected to the cart’s battery pack (which you usually wouldn’t), you would see it start out at the beginning of the charge cycle something in the neighborhood of 38 volts or so and work its way up to perhaps as high as 45 volts toward the end. So, if we see that the charger’s output is 25 amps at 38 volts, we would say that .950 kilowatts (950 watts) are being consumed. But we’ve got to remember that now we are reading the charger’s current meter, and the voltmeter that we have installed, and that we are on the output side of the charger’s circuitry, measuring DC components. Not like the AC components that we have been talking about before. We can still calculate the wattage being supplied to the batteries with the same formula, by multiplying the current by the voltage, but that is not a true indication of how much wattage the charger is drawing from the power source (our 120- volt plugin). If chargers had a 100% efficiency rate, the output wattage and the input wattage would be the same. We’d be supplying 950 watts to the batteries and the charger would be using 950 watts of power from our 120- volt outlet. But chargers AREN’T 100% efficient, so we would be using more AC power than the amount of DC power that we are providing to the batteries.
So, that’s where, in my opinion, the whole thing gets quite interesting. In order to get a more detailed accounting of what really goes on during the charge cycle with wattage on both the AC and DC sides of a typical charger during the charge cycle, I rigged up my old Hyundai 36 volt golf cart and its automatic charger with some extra meters and monitored them during a charge cycle from beginning to end.
On the AC side, I didn’t have to concern myself with the 120 volts AC. That is a constant, so all I needed to do was put an inline current meter in the circuit to its input, so I could check the current level being drawn from the 120- volt AC source as the cycle progressed. On the DC side, the charger had a current meter already on the front panel, so I could easily monitor it during the cycle. All I needed to do was to add a voltmeter across the battery pack so that I could keep an eye on it also. Here is what I found:
I plugged the charger into the 120- volt outlet, and as soon as I plugged the golf cart into the DC side, the charger came on and started the charge cycle (as it was supposed it to), and here were my readings:
AC current immediately went to 8.3 amps
DC voltage at the batteries went immediately to 37.76 volts
DC current went immediately to 22.5 amps
So, at that point, the wattage on the AC side was 120 volts X 8.3 amps, or 996 watts.
Interestingly enough, the label on the charger said 120 volts and 9.5 amps. So that would be 1140 watts, if I had just gone by that information (quite a ways off of than actual readings would indicate).
The wattage on the DC side would be 37.76 volts x 22.5 amps, or 849.6 watts.
When we compare the 996 in (on the AC or input side) to the 849.6 watts on the output side (to the batteries), we see that at this point in time, the charger is operating at about 85% efficiency. (.85301 x 996 = 849.6).
The entire charge cycle took just over 4 hours and during the cycle the charger went from using its initial 996 watts of power down to using around 252 watts toward the end of the cycle.
Since I, of course, couldn’t stand and watch the meters ALL through the 4 hour charge cycle, I simply took the readings at the beginning and end of each of the 1-hour periods. I then “averaged” out the current and voltage readings on AC and the DC sides for each 1-hour period to calculate the wattage usage for each period. During the charge cycle, the AC wattage always decreases, as I mentioned before. On the DC side, I watched the DC current steadily decrease and, of course, the DC voltage steadily increase.
During the first hour of the charge cycle, the charger averaged out using 694.87 watts for 1 hour, or .69487 kilowatt-hour. During the second hour it went down to .48272 kilowatt-hour. During the third hour it continued on down to .37367 kilowatt-hour and then on down to .24922 kilowatt- hour in the fourth. Adding these four totals up, we arrive at 1.8 kilowatt-hours. At my local power company’s current rate of about 12 cents per kilowatt-hour, that means that the entire charge cost me about 22.6 cents. So, had I just used the information on the label of the charger, I would have come up with a lot different result. The label said 9.5 amp and 120 volts AC. So, if you multiply 9.5 amps by 120 volts to get 1140 watts, and then multiply that by the 4 hours you get 4.560 kilowatt-hours. At my 12 cent per kilowatt-hour rate, the cost of the charge would be 54.72 cents, instead of 22.6. That’s over twice as much (150% more than that actually measured).
The type of charger that I have used in this example is becoming extinct, but I used it because it had a current meter on its panel, which made it easier for me to make the point. The newer chargers (referred to as “smart” chargers), however, are much more sophisticated and actually get involved in current and voltage regulation to create at least three different “phases” of the charge cycle to provide a more efficient charge. They still rely on the fact that the batteries themselves drive the process through their changing their resistance to current flow depending on their state of charge, but they use algorithms programmed into their “smart” microprocessor that controls their performance. I could have used one of these smart chargers and got very similar results but because most of them don’t have a meter to read, I would have had more work to do to monitor the operation. Most of the newer chargers not only have no meter, but rely on communicating their progress during the charge cycle to the operator through LEDs on their front panel. If something goes wrong, they can flash the LEDs in certain sequences so that the operator can “look up” the code it is flashing and see what the charger thinks is wrong.
Another thing that is changing the whole picture is the coming of a new generation of batteries called Lithium Ion. They are much lighter than the old “lead acid” type batteries that we have been dealing with in this article and offer many other advantages including faster charge times, longer performance with no need for water to be added, no corrosion of battery terminals, and even more. They, of course, require a different kind of charger with a different type of algorithm to offer the best performance. They will even farther reduce the cost of a charge cycle, as the technology advances.
Ron Staley has published the following books, and you can get more information about them by just clicking on each title below:
Electric Golf Cart Repair 101 (and a half)
Techniques, Tips, Tools and Tales
Gas Golf Cart Repair 101 (and a half)
Techniques, Tips, Tools and Tales
4-Stroke Golf Cart Engines Explored
What Makes Them Tick
By an old Slot Machine Mechanic