# Two deep-cycle batteries connected in parallel by a single terminal

From what I understand, if two 12v batteries with different charge levels are connected in parallel with both terminals, the charges of the two batteries will even out. The one with the higher voltage will raise the voltage of the lower one and lose voltage in the process.

I'm confused about what would happen if two 12v batteries at different charge levels (or voltages) are connected by just one terminal. I think the confusion stems from the fact that batteries don't just have positive and ground terminals, but they actually have positive and negative terminals. So, if I understand correctly the positive terminal "pushes" positive charge and the negative terminal "pushes" negative charge.

Is the negative terminal of a 12v battery always the same voltage as the positive terminal, regardless of how discharged it is?

Let's say there's two 12v batteries at different voltages. The first battery is discharged to 12.5v. The second battery is discharged to 11.9v. They each have their own loads attached. In my case the load is a power inverter for first battery and lighting for the second battery.

What happens if both positive terminals are shared by each battery at some point? In other words, if the negative is separated but the positive is shared, does the voltage of each battery change? What happens to each of the positive terminal voltages? What happens to each of the negative terminal voltages?

What if both negative terminals are shared by the batteries? Does it have the same effect as sharing the positive terminals? Is it because of the negative charge "push" effect? What happens at each negative terminal in this case? Do they become balanced out at the negative terminal, causing the positive terminals to each have different voltages but the same negative voltage?

Lastly, what if the batteries share their negative terminal by a common ground? Do the negative terminals even out their voltage? Would the ground voltage change in any way? Would the ground voltage change to the negative voltage of the most charged battery? Would the ground voltage change to the negative voltage of the least charged battery?

Sorry for so many questions. Maybe the answer is simple and explains all of them. Thank you so much for your answer.

EDIT: When I refer to sharing only one terminal on each battery, it should be assumed that that each terminal is also still connected in a way that completes the circuit. So when both positives are shared, the positives are also still connected to each load circuit and each load circuit still ends at the corresponding negative terminal. When both negatives are shared, the positive terminals are each connected to their load circuits and the load circuits are still connected to each corresponding negative terminal. I'm not sure if that changes anything.

A 12 volt battery has 12 volts between its terminals. Voltage is always measured between two points - there is no "absolute" voltage.

If you have a 12 volt battery and an 11 volt battery, and connect the negative terminals, the voltage between the positive terminal of the 11 volt battery and the positive terminal of the 12 volt battery will be +1 volt, assuming the negative lead of your meter is on the positive terminal of the 11 volt battery.

If you connect the positive terminals instead, there will be a 1 volt difference between the negative terminals.

The term "Ground" is much misused in the electrical field. Occasionally it does mean a connection to the Earth, but most often it is just the point in a circuit that the designer chose to call "zero volts", and uses as a reference when measuring voltage elsewhere in the circuit.

• Thanks! I hope I have this right. Let's say battery one is 12v and battery two is 11v. I join both of the positive terminals by two wires. I connect the positive lead of my meter to the wire. I then connect the negative lead of my meter to the negative terminal of the 12v battery. Does the meter read 12v? If I connect the meter from the wire to negative terminal of the 11v battery, does the meter read 11v? – rchipka Apr 29 '15 at 15:22
• Yes, but the batteries are not connected together, they are just sharing a common reference point. – R Drast Apr 29 '15 at 15:25
• Okay cool, so no matter what's going on with the common reference point, the voltage of each terminal at the each battery remains the same. I think I've got it. Thanks for your patience with me :p – rchipka Apr 29 '15 at 15:32

Without a complete circuit of positive and ground hooked together in some fashion between the two batteries, nothing will happen. Just connecting the two positives (or two negatives) together will do nothing. You can even hook the positive of one battery to the negative of the other, and once again, nothing will happen.

In order to affect the batteries in question, they must have a complete path for current to flow.

Ground is just a convention. You can have a negatively grounded system, or a positively grounded system. With two batteries, you could have the positive of one tied to a ground reference point, and the negative of the other tied to the same point. Again, as long as there is no complete circuit between them, there is no way for one battery to know if another were connected or not.

• I see, thank you very much. So the only single terminal, positive or negative, that can affect a battery's charge is the opposite terminal on the exact same battery? Do I have that right? – rchipka Apr 29 '15 at 15:13
• What do you mean by charge? If you connect the positive of a battery to its negative, you will certainly affect the charge, you will discharge it. – R Drast Apr 29 '15 at 15:24