I have a golf cart that uses 3 12v batteries in series. With the batteries connected to each other, but not connected to the load, I get 36v. When I connect the load, I get 0v. I measure each battery and get 12v, 12v, -24v. Can anyone explain this?

 

When I connect the load, I get 0v.

Hi ak,
Welcome to AAC.
When you measure the 0v, where exactly are you measuring the voltage.
E

 

I measure the voltage from Point A to Point B on the diagram. (Sorry about the poor drawing). Without the load attached, it's 36v. With the load attached, it's 0v.

 

The answer is simple, which is that at some point in the circuit there is a resistance great enough to drop the full 36 volts at some current. To locate that point you will need to connect some load, possibly even the motor, and then measure the voltage across each connection in the circuit. I have had to do that a few times, it is tedious but not that hard to do.
Within the circuit shown in post #3 are six connections, two on each battery, and four connections of the wires between the batteries.

 

“I measure each battery and get 12v, 12v, -24v.”

If these voltages have been measured correctly, one of the cells has been over discharged and its polarity reversed.
It is very likely that is ruined. Try to recharge it individually out of the cart to see if it recovers.

 

When I measured the batteries at 12v, 12v, -24v, this was when the load was connected. (load meaning that it was connected to the motor controller, but the motor was not running and the switch was off). Without the load connected, the -24v battery reads 10.6v (with proper polarity)

 

Does the battery measure fully charged when the load is disconnected? (at least 12.5V).
If so, one of the connectors between the cells has gone open-circuit. It is quite a common failure on lead-acid batteries.
Each cell has a lead post, and a lead connecting bar is press-fitted on to two posts, and it has become disconnected.
You need to buy a new battery.

 

When I measured the batteries at 12v, 12v, -24v, this was when the load was connected. (load meaning that it was connected to the motor controller, but the motor was not running and the switch was off). Without the load connected, the -24v battery reads 10.6v (with proper polarity)

You are telling us that the positive marked terminal of the battery is now negative? And the negative terminal is now positive?? Try it with at least one terminal of that battery disconnected, because something is quite wrong.

 

When I measured the batteries at 12v, 12v, -24v, this was when the load was connected. (load meaning that it was connected to the motor controller, but the motor was not running and the switch was off). Without the load connected, the -24v battery reads 10.6v (with proper polarity)

There are two probability for this issue
1- bad batteries, with 12v lamp check every battery , if the lamp light is strong enough and that the voltage across the battery not fall down a lot, not under 10v, and that it’s steady at some value above 10v, if not then the battery is bad and don’t hold charge for the load
2- you may have short circuit in the load circuit?, which Is strange for me , as if there is a Short circuit something would damage In the circuit and you could notice,
What happen when you loading the motor?, can you give us the readings of volt across the batteries?, when loading the motor?
How about charging voltage?, is it normal ?, increasing with time across the batteries?, and for one battery can reach to 13.7 v with time?, or no?

Can you send photos for the readings you take with multimeter?

Or you have connections loss between the batteries?

 

MisterBill2 - Yes, that's exactly what I'm telling you. Once the battery is isolated or the load removed, it reverts back to normal polarity. It goes against all I've learned about electricity (50 years as a Radio Amateur). These batteries are LiFePO4. Made by Ogrphy. All 3 were purchased as a matched set. They've only been in the cart for 1 or 2 days.
For Ian0 - the voltage when isolated was 10.6v.
I'm using the NOCO genius GENPRO10X3 charger. It charges each battery separately. And before anyone says it, All the voltage measurements I've mentioned were with the charger leads disconnected.
We tried to operate it with a load, but since we had 0 net volts nothing happened.
We put a load tester on the battery and it registered 0v. This is where it gets a little weird. We attached the load tester, but didn't press 'test'. Then attached a voltmeter and got 1.77v. And, like I said, with nothing attached, the voltage is 10.6v.
I'm pretty sure the battery is toast. I've written to Ogrphy to get it replaced.

 

OK, the battery appears to have developed a rather high internal resistance.Low enough to register on the ten megohm input of your DMM but not low enough to deliver any current, So what you have is a 12 volt battery in series with a few thousand ohms. That sounds like a candidate for replacement under warranty.

 

MisterBill2 - Yes, that's exactly what I'm telling you. Once the battery is isolated or the load removed, it reverts back to normal polarity. It goes against all I've learned about electricity (50 years as a Radio Amateur). These batteries are LiFePO4. Made by Ogrphy. All 3 were purchased as a matched set. They've only been in the cart for 1 or 2 days.
For Ian0 - the voltage when isolated was 10.6v.
I'm using the NOCO genius GENPRO10X3 charger. It charges each battery separately. And before anyone says it, All the voltage measurements I've mentioned were with the charger leads disconnected.
We tried to operate it with a load, but since we had 0 net volts nothing happened.
We put a load tester on the battery and it registered 0v. This is where it gets a little weird. We attached the load tester, but didn't press 'test'. Then attached a voltmeter and got 1.77v. And, like I said, with nothing attached, the voltage is 10.6v.
I'm pretty sure the battery is toast. I've written to Ogrphy to get it replaced.

Right
You have bad battery

 

Thank you all for your input. It's been most illuminating.

 

The battery that measures -24V is opening under load and no current is flowing through the load.
However, the load (most likely motor winding, i.e. simply wire is providing a path such that you are measuring the voltage across the other two batteries, but with reversed polarity. Red is what you are doing when measuring the bad battery. Because the load/motor has low resistance, you are essentially measuring blue which the the other two batteries connected in series with the meter leads reversed (i.e. + to -), hence the -24V.

 

The battery that measures -24V is opening under load and no current is flowing through the load.
However, the load (most likely motor winding, i.e. simply wire is providing a path such that you are measuring the voltage across the other two batteries, but with reversed polarity. Red is what you are doing when measuring the bad battery. Because the load/motor has low resistance, you are essentially measuring blue which the the other two batteries connected in series with the meter leads reversed (i.e. + to -), hence the -24V.View attachment 327369

Not that the battery is opening under load, but that it has a high internal resistance at all times. That is a common failure mode for rechargeable batteries.

 

As it is a lithium battery, it could be that the BMS has switched off. That could be because the BMS has failed, or that the battery has failed and the BMS has switched it off.

 

As it is a lithium battery, it could be that the BMS has switched off. That could be because the BMS has failed, or that the battery has failed and the BMS has switched it off.

Agood point. But does this battery even have a BMS?? (Battery Management System) Not every battery contains one.

 

I don't see where anyone said it was a lithium battery.

 

I don't see where anyone said it was a lithium battery.

Post #10, second line

 

The golf cart batteries I'm familiar with are wet cell batteries. This may seem like a dumb question, but are all the battery cells topped off with fluid? Fluid because some people think of it as "Water" while others think of it as "Acid" when actually it's a mixture of both. A low cell can act like a weak battery. An empty cell is going to be an open circuit. Hence, quite possibly the reason why you're seeing -24V just as wraujr suggests.

Just to clarify, an open circuit is a circuit who's resistance is approaching infinity. There may be the very slightest bit of connection which will have extremely high resistance but still be able to test at a full 12 volts (when removed from circuit). And that's probably going to be the best way to test the batteries, to remove them and test them under a 12 volt load of some sort. An old automotive bulb, even a turn signal (35W) should be enough to determine if a battery that reads a good voltage may in fact be dead. Dead doesn't necessarily mean 0V. I had an 80V battery that could barely light an LED with no resistor at all. It measured 80V but struggled on a 2Vf LED.