DC power question

[Sticks McGee]

I asked this in off topic but will post here too. This time more simplified.

Golf cart. Six 8 volt batteries wired in series. Drive system is 48v. The other things on the cart run on 12v. They use an inverter/converter device to step the 48 down to 12. Main positive and negative from the batteries run to the controller for the motor then feed the inverter/converter and the 12v things are all fed from that.

My question is other than the main pos and neg coming off the batteries there are a few smaller ga wires connected with eyelets to terminals on a couple of the batteries. How do you determine if the terminals are positive or negative along the way in the series wiring? If it was parallel wired I know all the negative battery posts would be negative and all the positive posts would be positive. I found a service manual but other than the jumpers from battery to battery and the stuff coming off the main pos/neg(48v) they show no wiring attached to the batteries.

 

[Radix2]

The terminals along the way are negative or positive only relative to other terminals.

For example, the jumper connecting the first battery negative to the second battery positive is negative to the first battery positive, but positive to the second battery negative (and all the other batteries). The point is, it is the difference between the leads going to these points that is the voltage to your other circuits. Another way to look at it is that all these jumpers are positive compared to the system ground ( assuming the chassis is connected to the ground of the last batttery in series.)

My guess is that they are tapping some power from the pack for other uses or monitoring. Use a meter on these leads to see what voltage is across them. If the manual does not show these wires, are they used to power added on aftermarket stuff like radio or something?

 

[Falcon67]

That's how I haywired the lights on our old cart - 36V. Just ran wire from the switch and connected it to a pair that provided 12V.

 

[Sticks McGee]

Gotcha...I will get back down there soon to take another good look at it..Another question:

Let's say the negative main is also connected to the frame or chassis. Now you come along and ground something like a light to one of the terminals of a battery that is in the middle of the series. Is that a true ground or will it act like a ground?

My thought on this when I thought about it was that the jumpers between the batteries were neither positive or negative but just a carrier. I am not sure that is correct but the other guy there present Saturday (basically a brother in law) said he believed anything that was a ground would be connected to the negative post and anything feeding power would be connected to the positive even in the middle of the pack but my thinking is if a ground wire was connected to the negative post of the second battery and a power feed was connected to the positive of the third battery they essentially are both connected to the same wire. Now tapping power from number 2 battery to a load and grounding that load to the negative of that same battery, the voltage would be just the amount from that battery and not the whole series. In his case 8 volts as there are six 8 volt batteries.

 

[Jess]

Tapping off a single battery in a series string isn't a good idea due to the uneven discharge of one battery and the recharge problems that result. Lots written about this in marine and military vehicle forums, where 24V starting is common. The right way is to use your convertor for all 12V draws.

 

[handyman85]

Wiring the batteries in series adds the voltage but keeps the capacity (amp hours) the of each battery to make the total of 48v. Only one wire can be grounded, and it has to be the end of run that is attached to the load. The red arrows show the direction of electricity

Wiring the batteries in parallel adds the capacity (amp hours) but keeps the voltage the same; Similar to an RV or boat 12v setup. When one negative battery is grounded they all are essentially grounded because there's no device between each battery.

Grounding a battery midstream is very hazardous because electricity always takes the easiest path it can. In the example, Batteries 1&2 are basically being shorted and all that energy has a free path between positive and negative.

Voltage is measured as the potential difference between two points. If measured between two battery posts it measured the battery itself, if measured between the first positive terminal in line to the last negative terminal, it measures the combination of all those batteries.

Hope this helps,
Dave

 

[rlitman]

Tapping off a single battery in a series string isn't a good idea due to the uneven discharge of one battery and the recharge problems that result. Lots written about this in marine and military vehicle forums, where 24V starting is common. The right way is to use your convertor for all 12V draws.

Yes, OR use a charge equalizer.

 

[Radix2]

Gotcha...I will get back down there soon to take another good look at it..Another question:

Let's say the negative main is also connected to the frame or chassis. Now you come along and ground something like a light to one of the terminals of a battery that is in the middle of the series. Is that a true ground or will it act like a ground?

My thought on this when I thought about it was that the jumpers between the batteries were neither positive or negative but just a carrier. I am not sure that is correct but the other guy there present Saturday (basically a brother in law) said he believed anything that was a ground would be connected to the negative post and anything feeding power would be connected to the positive even in the middle of the pack but my thinking is if a ground wire was connected to the negative post of the second battery and a power feed was connected to the positive of the third battery they essentially are both connected to the same wire. Now tapping power from number 2 battery to a load and grounding that load to the negative of that same battery, the voltage would be just the amount from that battery and not the whole series. In his case 8 volts as there are six 8 volt batteries.

If you have six 8V batteries ...(are they really 8v batteries?)

first battery is connected neg term to ground, other terminal/jumper to next battery is 8V to ground

second battery is connected neg term to bat 1, pos terminal/jumper to next battery is 16V to ground

third battery is connected neg term to bat 2, pos terminal/jumper to next battery is 24V to ground

fourth battery is connected neg term to bat 3, pos terminal/jumper to next battery is 32v to ground

fifth battery is connected neg term to bat 4, pos terminal/jumper to next battery is 40v to ground

sixth battery is connected neg term to bat 5, pos terminal, connection to motor control is 48v to ground

you can tap off any of those voltages to ground. The only concern would be charge imbalance as noted above, but if the battery is fully charged often the float period will equalize the charge.

 

[Sticks McGee]

Thanks Dave. That is essentially what I was kinda thinking. That makes it clearer to me. I figured if one or more of those small leads were actually ground and connected mid stream that would be how you explained it.

Yes they are 8 volt batteries as it is a 48 volt system. I measured voltage at main pos and neg and got a 50 volt reading. His key on lights up his headlights nicely so I assume his step down to 12 volts is correct. I'm not sure if his other small wires connected are factory or something added. I will find out soon when I go back to look at it.

Thanks to all the responses.

 

[Sticks McGee]

UPDATE....

I went back down there yesterday to make heads or tails of the golf cart. Here is what I know. There was a death in the family. The original owner owned the cart in Florida and we are in Indiana. The cart was then given to the current owner. My girlfriend's father. He has had the cart now this being the second summer. A month or so ago it started giving him problems. He called a place but nobody in the area had any experience with this brand. He got a name of a place to get batteries and they sent a guy out. He tested the batteries and they needed replaced. The guy was not a tech but just a battery guy. He swapped out all 6 batteries for new ones. Cart worked for a day then wouldn't go. Owner flipped up the seat to check connections on battery. Found a couple connectors in bad shape. Battery guy says he doesn't work on the carts, just batteries. Owner gets new eyelet connectors but does not have crimpers so the day I was there I brought a few things and changed 4 eyelet connectors but afterwards it still would not move.

Yesterday..

After getting all the control box remounted and the one battery back in place I looked over all of the wiring. Aside from the jumpers on the batteries and the two main cables going to the controller. I found 6 smaller wires. Some were in various places so I traced them down. One set of wires is about a 10-12 ga set. One black and one red. I followed these and they went to the charge receptacle so those went onto the positive and negative main cables. A second set of larger wires (again around 10-12 ga) I traced down. These went to the converter that steps the 48 volts down to 12 for all the 12v stuff on the cart. I put the red on positive and the black on negative mains. That left 2 wires that were around a 16 ga. The red one had an inline fuse and the black one just went into a piece of split loom under the cart and up into the dash area along with the red one. These two wires were definitely not factory and definitely were not connected to the two main terminals. It looked as tho they ended up running up the front pillars of the cab but were not connected to anything. Most likely prior lights or something added. These wires we decided to leave disconnected. If they were for something 12 volts you can't get 12 out of the batteries. You have either 8,16,24,32 or 48. When I was happy with all the wire positions and connections we flipped the seat back down and wouldn't you know it, the cart worked like new again. I think it was a combo of bad batteries followed by bad connections and possibly wrong placement by the battery guy of the small cables. He was ecstatic to have his cart up and running again..I was glad it was a simple fix.

 

[prostreetamx]

We use 24v carts at work. They use 4-6v batteries in series to provide the 24v. They tap into the middle of them to provide the 12v needed for the lights.. None of the cables are connected to the chassis on any of our carts. Over the years some have been rigged up differently but I believe they are all the same now after I fixed them. I like to tag all the smaller wires when I swap out batteries so everything works correctly. We get about 2-3 years out of a set with heavy use and lots of dry heat. They tend to get low on water and most of the guys never check. You ruin a battery pretty quick charging them when low on water.