Lots of people ask about load testing the batteries in a golf cart. You can use a generic load tester (the kind you would get at an auto supply store) but it will only do so much. It only tests one battery at a time, and the battery is tested with a 100 amp load for the duration of the test. Unfortunately, that isn’t a great indication of what happens when you use the batteries in a golf cart, under a real load. The load tester might show a problem with a battery if it has an ‘obvious” problem, but not necessarily, if the problem is evasive. When you mash down on the accelerator of the golf cart, the batteries are loaded WAY beyond what a load tester will simulate. Instantaneously, the current requirement is up in the hundreds of amps. The batteries, the controller, the wiring, and the fuse (if it has one), couldn’t sustain such abuse for any length of time, but it is only until the cart starts moving. Then this thing called CEMF starts happening with the motor, and the load settles down to a manageable number.
The best way to get a REAL feeling for how the batteries are doing when trying to run the cart, is to use the cart as the load tester. Simply put a meter from B- to B+ and drive the cart while observing the meter.
B- means the lowest point of voltage potential in the battery pack and B+ means the highest. In most cases, you can look at the charge receptacle and find the – and + wires headed to these points. As the batteries are strung in series, these two points will be found on the two batteries that only have one wire going to the other batteries. Obviously, this takes a little routing of the meter probe wires under the seat to position the meter to where it can be read while driving, but that is no big deal. Let’s say, for example, that the fully charged state of charge for your 36 volt golf cart is 37.9 volts. That’s pretty close to where it should be, and if you’ve charged the batteries for a complete cycle (until the automatic charger has shut itself off) and your reading is much lower than that, you’ve got to have a problem with one or more of the batteries. But go ahead with the rest of this procedure, and perhaps we can isolate the problem to one or more, or even all of the batteries.
What you do, now that the meter is hooked up to the battery pack and you’ve got it to where you can drive the cart and observe the meter, is to do just that. In order to put some very REAL numbers together for a realistic example to talk about, I took my own 36 volt cart that I use at home and did some charging and driving and consolidated the following data. The batteries in this cart are about 3 years old and have never been left without charging for any length of time or have never suffered any other unusual abuse. First, I gave the cart a good charging with an automatic charger. I put the charger on in the evening (as a lot of people do) and it shut off some time during the night and I did this testing the next morning.
First, I put a meter across the battery pack and got a reading of 38.14 volts. The charger actually reaches higher voltages than that during the charging cycle, so if you test the battery pack right after the charger shuts off, you may get a reading even higher reading than my 38.14. But remember, my charger had shut off sometime during the night and then the battery pack had sat for probably a few hours before I took the reading.
Now, as we put the cart in motion, using it as a “real time actual load” tester, we’ve got to understand a little about how digital meters work. Digital meters (and that is about all that you can buy now) are great, but they have some limitations. One of them is that their response is NOT instantaneous. The higher priced the meter is, the faster (in general) the response time will be. This is because the meter has a sampling rate. It looks at the voltage in intervals of time and updates its display to reflect the latest thing it sees. The faster the sampling rate, the faster the information is updated on the display. So, if you are using a cheap meter, that is not a problem, just be patient and let the meter stabilize on a reading after it “flops around” looking for a good “average” to display.
To facilitate a reasonable bunch of readings, I carried a portable GPS with me so that I could monitor the cart’s speed during the test.
The next thing I did was to step very lightly on the accelerator to put the cart in motion. As I stepped down on the pedal, the meter jumped around a bit and then displayed 36.85 as the cart started gaining speed. Now remember, I did not slam the accelerator to the floor, I just took off in what I would call a normal fashion.
Next, I let the cart settle out at 6 mph. When it did, the display on the meter settled on about 37.00 volts.
Then I took the speed up to 10 mph and the meter read 36.40 volts.
Next, I put the pedal to the metal (Wide Open Throttle or “WOT”) and the meter dropped, just for a short time to 35.60 volts.
As the cart gained speed and reached it maximum of 14.8 mph, the meter went up to 35.96 volts for a little bit and then settled on 36.02 volts.
When I backed off of the throttle, the meter jumped up to 36.89 volts and then to 36.00 volts as I let it come down to 10 mph.
Next, I let the cart settle down to 6 mph and the meter went to 36.8 volts.
Next, I shut the cart off and let it sit for 10 minutes and then checked the meter and found it to read 37.9 volts.
Once again, these were not new batteries, but neither had they ever been abused.
Perhaps you can use this as a typical example to show what should happen with the battery pack’s voltage during a more or less typical charge of the battery pack and trip on the cart.
If I were to drive the cart for an extended time, let’s say 12 miles or so, the readings would start to drop off proportionally.
If you go through this routine and things are a lot different, you need to see why. Let’s assume that just one of the batteries has problem, and all of the rest of them are ok. In order to identify which one has the problem, we would just repeat the same procedure as above, but with the meter attached to only one battery at a time. You can simply divide the readings above by 6, and that will give you approximately what should be read across one battery at a time. With this information you can easily determine if one of the batteries is doing something bad. Batteries can check fine without a load right after a charge, but “fall out” when loaded. The batteries are strung in series, so all of the current to the motor has to go through all of the batteries, and any one of the batteries can become a restriction or even an “open” to the path. You may even see a battery make the display on the meter go to a negative (display a -). This may seem unusual, but it indicates that the battery has completely “fallen out” and the meter is reflecting a reading just like the battery wasn’t in the circuit.
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
One reply on “How to REALLY load test your batteries”
What “meter”. What is it? Where do I get one?