First off, 6 volt, 8 volt, and 12 volt batteries are all made up of 2 volt cells, internally strung together in a series fashion to make up their total voltage. 3 of these 2 volt cells in series = 6 volts, 4 of them = 8 volts, and 6 of them = 12 volts (the three common packages used golf carts). But how does a 2 volt cell really work?
Most engineers agree that, in theory, a 2 volt cell can be charged to about 2.15 volts, therein the magic begins. However, this charge state is rarely achieved due to temperature variations, ageing of materials (cycling), and characteristics of the chargers we use to restore their energy. So, for this discussion, let’s use a more realistic maximum charge for a 2 volt cell, 2.105, which is what I have found through years of working with them is more common. Now we are assuming here that we are talking about a GOOD fully charged cell in a GOOD battery. We’ll get to testing and charging later, but for now, let’s use 2.105 and do a little math.
A 36 volt golf cart uses 6 of these 6volt batteries for its energy source, so here we go.
The 6 volt battery is made up of 3 cells, so if we take 2.105 x 3 we get 6.315 volts. Now that’s not 6 volts at all, so why do we call it a 6 volt battery? For convenience. I like to refer to this as the battery’s “NAMED” voltage.
Next, we know that the 36 volt golf cart has 6 of these batteries so:
6 x 6.315 = 37.89 volts (I’ll round that to 37.9 volts)
Therefore, 37.9 volts will become, for the purposes of this discussion, our standard for a fully charged 36 volt golf cart with 6 fully charged 6 volt batteries.
The next thing we need to do is establish what is “DISCHARGED”. On this subject you can find volumes of information and lots of disagreement among different engineers and authors, so I am going to use some numbers here that are a combination of what I have observed through experience and averaging the many published document that I have studied.
The general consensus is that once the 2 volt cell is discharged down to about 1.984 volts, it’s done and has no practical charge left in it. I’ll call that 2.00 volts.
So, our charge RANGE for the 2 volt cell is from a fully charged 2.105 to 2.00 volts and it’s “dead”.
If we take that up to the battery level, it means that 3 of these cells add up to a RANGE for the battery of 3 x 2.105 = 6.315 volts (fully charged) to 3 x 2.00 = 6.00 volts (fully discharged). So, its RANGE is only .315 volts, and that’s it.
Now, most golf cart battery manufacturers do not recommend using more energy out of the battery than that which would take the battery down to below a certain THRESHOLD. Battery life is directly related to how deeply a lead acid battery is discharged in a cycle (fully charged to discharged to some degree and then recharged). Most of the manufacturers that I have studied wouldn’t recommend going below around 30% to 40% of the batteries charge RANGE. The important part of this is to recognize that this means we can USE around 65% of that charge RANGE (leaving around 35%) and still keep our batteries healthy. Another way to discuss the discharge process is by using a term called DOD (Depth of Discharge). It is just the opposite of SOC (State of Charge) and describes the energy that we have used as a percentage of the RANGE used, not as a percentage of what is left. So, a battery that has been discharged down to 35% of its RANGE has been discharged to a 65% DOD (35% SOC). As previously indicated, the less energy that is taken out of them, the more cycles they will live through and therefore the longer they will last (to a certain degree). Studies have proven, however, that if at least 5% of the RANGE isn’t used before recharging, it is actually detrimental to the life of the Battery Pack, so use some common sense here. So, let’s use 35% SOC (65%DOD) and do the math.
What we want to do is to see what the RANGE for the golf cart as a whole is, not just for just one cell or one battery. So, if we take a RANGE for one battery and multiply it by 6 batteries, we get 6 x .315 (the RANGE for one battery) = 1.89 volts. We’ll use 1.9 volts to make things a little easier.
Now, going back to our manufacturer’s recommended THRESHOLD of only taking 65% of this RANGE away, (65%DOD) we get this:
Fully charged Battery Pack of 37.9 volts – 65% of it’s 1.9 volt RANGE (1.24 volts) = 36.66 volts (rounded)
So when your Battery Pack voltage gets down to 36.66, you should be headed back to the charger.
This does NOT mean, however, that if you see your charge meter dip below that number while under normal operation that you have a problem. To properly test the fully charged state voltage, you would have to start with a fully charged Battery Pack and then remove what is called a “surface charge” by either waiting several hours before testing or by loading the batteries with a light load for 3 or 4 minutes and then letting the batteries sit for a few minutes before doing the testing. I do this loading by simply driving the cart at a slow speed for the 3 or 4 minutes. I then let the cart sit for just a few minutes and with that technique, I seem to get very consistent and reliable results. The charge state at any time after loading has occurred for any reasonable amount of time must be tested after the golf cart has sat for a while (at least 5 minutes) so as to stabilize.
I often have people think that the 65% discharge means 65% of the NAMED voltage of the battery. In other words, they think that you can take the Battery Pack down, for instance, to 36 volts – 65% of 36 volts (23.4 volts) = 12.6 volts. WRONG, WRONG, WRONG!!!
Another thing I see is that people walk up to a Battery Pack that they have no idea what is going on with, stick a meter on a battery and think they now know everything they need to about the battery. They haven’t even started until they have tried to complete a normal charging of the batteries, removal of surface charge, etc.
But now we need to talk about how the batteries do under a load. To load test a battery and really get any good information about it, the battery needs to be fully charged (all of the steps taken above) before the testing occurs. Lots of people use a common load tester like the one you can buy at Harbor Freight and that’s ok, but remember that this type of load tester, generally speaking, only supplies about a 100 ampere load to the battery and does not accurately indicate what happens when you put the pedal to the metal while driving the golf cart. It is, however, in most cases a good way to get a general idea about the health of the battery. When the battery is loaded with the tester, the needle on the tester should only move slightly (certainly no more than .5 volts for a 6 volt battery) with the 100 ampere load. If the needle drops way down when loaded, the battery is not healthy. Of course, to perform a load test on a battery that hasn’t been charged fully is worthless.
Another thing that I see a lot, is when someone monitors the voltage of a battery in the pack while driving the cart, they might see their meter drop down on a battery to around 5.8 volts for just a second or two. That doesn’t mean that the battery actually became discharged to a lower than normal charge state. It just reflects a normal drop in voltage while the battery is starting to supply the high current level required to get the Motor going at first demand. The important part is that the voltage doesn’t STAY down in that range for a long period of time. As the current starts to flow and the CEMF of the Motor increases (that’s another whole story) so that the load gets stabilized out, the voltage should come back up in the 35% to 100% up RANGE.
I have only done the math here for a 36 volt golf cart with 6 batteries (6 volts each). But you can follow the same pattern for a 48 volt golf cart with 6 batteries of 8 bolts each or 4 batteries of 12 volts each: (in abbreviated form)
2.105 x 4 x 6 = 50.52 or 2.105 x 6 x 4 = 50.52 Charged
2.00 x 4 x 6 x 48 or 2.00 x 6 x4 = 48 Discharged
50.52 – 48 = 2.52 Range
65% of RANGE = 2.52 x .65 = 1.64
50.52 – 1.64 = 48.88 = time to head to the charger
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