what happens when 2 LiPo batteries are connected in parallel and in series?

BobTPH

Joined Jun 5, 2013
4,753
If for some reason the voltages are not equal (in the real world they won't be exactly equal) and I put a diode on the less powerful battery to protect it - will (if the diode starts to have a reverse polarity) the weaker battery also be unable to conduct electricity to the motor because the diode is reserved biased?
Putting a diode on one cell would prevent it from ever charging.

If the two cells are at different voltages when you want to put them in parallel, charge the lower one until they are the same. Once in parallel, they can never be at different voltages again.

Bob

Tonyr1084

Joined Sep 24, 2015
6,866
Thank you again for the answers. I'm finally able to understand what's going on with my circuit. My last question for this thread, if you would indulge me haha.

If for some reason the voltages are not equal (in the real world they won't be exactly equal) and I put a diode on the less powerful battery to protect it - will (if the diode starts to have a reverse polarity) the weaker battery also be unable to conduct electricity to the motor because the diode is reserved biased?

I've read over and over that reverse diodes will block electricity going in the wrong direction, but nothing on if electricity is going in both the wrong direction and correct direction from two different power sources at the same time. I presume the weaker battery will not be able to get electricity to the motor while this is happening.....
Thanks for the drawing. To answer your question about diodes - they DO have a voltage drop called the Forward Voltage (or Vf). In the drawing you made you have two diodes, both typically drop about 0.7 volts (Vf). They're not needed. And yes, batteries are rarely equal in voltage.

In the instance you drew - removing the diodes - the higher voltage battery will naturally want to impart some electric pressure (voltage) to the other battery. Overall, in parallel, they have an average voltage. Say one battery is at 19.95 volts and the other is at 20.05. The average voltage would be 20.00V. As the motor draws current, the stronger battery will have the greater amperage while the smaller battery will do very little to drive the motor. When the stronger battery (higher amperage) begins to deplete its charge the smaller battery will play more of a role in powering the motor. But the voltage will continue to be an average of the two batteries.

You can try an experiment with three AA batteries. Two of them in series (3 volts) and one parallel to the two series wired batteries (1.5 volts). The voltage will average out to be somewhere around 2.25 volts. The single battery will be trying to take a charge. If you leave the test rig connected for any length of time the smaller battery will likely leak acid due to excessive charging and heat. So don't perform the experiment for an extended period of time. Just make the connections, take a reading, then disconnect the rig.

As for amperage, again, the stronger (higher) will carry the bulk of the load until the voltage drops enough to make the smaller battery work harder. Again, keep in mind averages.

BobTPH

Joined Jun 5, 2013
4,753
Overall, in parallel, they have an average voltage. Say one battery is at 19.95 volts and the other is at 20.05. The average voltage would be 20.00V.
It will only be the average of the two if the internal resistance of both batteries is the same. Try a car battery and an AAA battery, (only as a thought experiment, do not really do this!) Do you really think the resultant voltage would be 6.75V?

Bob

Tonyr1084

Joined Sep 24, 2015
6,866
Do you really think the resultant voltage would be 6.75V?
I suppose not. Of course I had in mind batteries that are closer in value. 12V vs. 1.5V is a bit of a stretch. But you're right. Internal resistance plays a part.

Poppy ANn

Joined Mar 18, 2015
2
Asking questions is the way we learn. Making mistakes is another way we learn. As long as those mistakes are not catastrophic.
I have always said you learn more from making a mistake than from doing it correctly as making a mistake tends to stick in your mind easer than than doing it correctly and not thinking about it.

KeithWalker

Joined Jul 10, 2017
2,298
When you connect two batteries in parallel, the voltage does not average out across the internal resistance!
The one with the highest terminal voltage will quickly discharge into the other, charging it until the two voltages are equal. The current flowing will only be limited by the internal resistance of the batteries and the resistance of the connecting wires. If the voltage difference is large enough, this can produce a lot of heat and can quickly destroy both batteries.
That is why you must charge both batteries to the same voltage before connecting them in parallel.

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