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

Thread Starter

zemanekj

Joined Jan 31, 2019
57
It is my understanding that when 2 power sources are connected in parallel (for this question I will refer to the
3S 2200mAh LiPo Battery, 11.1 V) that the output would be 11.1v and 4.4Ah. Is this correct?

And if I connect the same two LiPo batteries in series the output would be 22.2v and 2.2ah? Is this also correct?

I know output may depend on how much power a motor draws, but that aside - are my presumptions correct, or off base here? I can't find very many clear cut answers for this kind of question online anywhere. Thank you, and I hope to be able to start contributing more to this website, instead of only asking questions all of the time.
 

dl324

Joined Mar 30, 2015
9,332
I can't find very many clear cut answers for this kind of question online anywhere.
Here's an article that should answer all of your questions. As noted in the article, there are some significant issues inherent in using parallel or series configurations. You'll get the best performance when the cells are matched; this is why devices that use multiple batteries in series, parallel, or series and parallel tell you not to mix battery chemistries or weak batteries with strong batteries.

EDIT: Here's an article on cell matching.
 
Last edited:

KeithWalker

Joined Jul 10, 2017
532
It is my understanding that when 2 power sources are connected in parallel (for this question I will refer to the
3S 2200mAh LiPo Battery, 11.1 V) that the output would be 11.1v and 4.4Ah. Is this correct?
And if I connect the same two LiPo batteries in series the output would be 22.2v and 2.2ah? Is this also correct?


You are correct but in the real world, there are other things that may affect your choice of battery configuration.
Both configurations will supply the same amount of power.
If you connect two batteries in parallel, make sure that they are both charged up to the same terminal voltage. Otherwise the one with the highest voltage will dump current into the other. This could damage both cells.
If you are using the battery to drive a load directly, the configuration is limited to the requirements of the load. If a boost convertor is used, you have a choice of how to configure them. In that case, the best choice is usually to connect them in parallel because it simplifies the charging requirements. No battery equalization circuit is required.

Keith
 

MrSoftware

Joined Oct 29, 2013
1,615
For most intents and purposes, if your batteries are the same model and age, then putting them in series or parallel will be fine. Just be sure to balance charge when you charge them. Any half decent lipo charger will have this feature built in.
 

Delta prime

Joined Nov 15, 2019
52
. Thank you, and I hope to be able to start contributing more to this website, instead of only asking questions all of the time.
Hi, I'm new here as well. This might seem silly but I am the exact opposite of you. I have been reading some of their threads all this time to various topics and various contributors and moderators hi everybody! Great place great topics excellent answers and very entertaining as far as personalities.Your right about the battery stuff... By the way.
 

Thread Starter

zemanekj

Joined Jan 31, 2019
57
Thank you for the response! You said that, "If you connect two batteries in parallel, make sure that they are both charged up to the same terminal voltage. Otherwise the one with the highest voltage will dump current into the other. This could damage both cells."

Now, what if I connected a diode to one of the batteries to prevent this from happening? For example, say one battery has an output of 5v/5a and the other battery has an output of 10v/5a. If I connect a diode to the 5v/5a battery wouldn't that diode then become a reverse polarity diode, preventing the more powerful battery from dumping into the weaker one? Also, the weaker battery wouldn't be able to get any power to a motor because of the diode correct?
 

Thread Starter

zemanekj

Joined Jan 31, 2019
57
For most intents and purposes, if your batteries are the same model and age, then putting them in series or parallel will be fine. Just be sure to balance charge when you charge them. Any half decent lipo charger will have this feature built in.
When you say balance charges are you just talking about voltage? For example, would a 20v/80a and 20v/10a battery get along just fine? Or does the current have to match too?
 

Thread Starter

zemanekj

Joined Jan 31, 2019
57
Thank you for all of the responses! When you all say that I must balance the charges of the batteries are you just talking about voltage? For example, would a 20v/80a and 20v/10a battery get along just fine? Or does the current have to match too?
 

Tonyr1084

Joined Sep 24, 2015
3,942
Not really. As both batteries discharge, their voltages will be affected differently. And if you draw more amperage than the smaller battery is rated for you could overheat it. That could be bad for the battery's health.
 

Thread Starter

zemanekj

Joined Jan 31, 2019
57
Not really. As both batteries discharge, their voltages will be affected differently. And if you draw more amperage than the smaller battery is rated for you could overheat it. That could be bad for the battery's health.
How is the voltage affected differently? I understand the answer may be pretty complicated so if you know any resource I could read, I'd would really appreciate it.
 

Tonyr1084

Joined Sep 24, 2015
3,942
As a battery drains, it's reserve energy drops. With two batteries of different size (same voltage) the smaller battery will deplete faster. Take for instance two batteries, a D cell and an AA cell. Both produce the 1.56 volts when new, but the D cell has much more energy density than the AA cell. I believe the AA will drop voltage faster than the D will. I could be wrong, if so - someone will correct me. That's the good thing about this website. When someone is wrong others make sure to clarify the issue. So I'm not afraid of admitting to being wrong - it happens. But I believe what I'm saying about different size batteries.

[edit] I think I'm going to build a rig and test my theory.
 

KeithWalker

Joined Jul 10, 2017
532
As a battery drains, it's reserve energy drops. With two batteries of different size (same voltage) the smaller battery will deplete faster. Take for instance two batteries, a D cell and an AA cell. Both produce the 1.56 volts when new, but the D cell has much more energy density than the AA cell. I believe the AA will drop voltage faster than the D will. I could be wrong, if so - someone will correct me. That's the good thing about this website. When someone is wrong others make sure to clarify the issue. So I'm not afraid of admitting to being wrong - it happens. But I believe what I'm saying about different size batteries.

[edit] I think I'm going to build a rig and test my theory.
You are correct if the batteries are connected in series. They will both discharge at the same current so the one with the least energy will become discharged first. If the current continues to flow from the battery with the highest charge, the other one will become reverse charged.
If the batteries are connected in parallel, they will maintain the same voltage. The one with the highest charge will supply more current than the other. They will both become discharged at the same time.
Keith
 

Tonyr1084

Joined Sep 24, 2015
3,942
Yes, Keith, that's exactly what I meant to say regarding series connected batteries. And yes, with batteries in parallel, they will both maintain the same voltage and the one with the higher capacity will handle the lion share of the current being drawn.
 

MrSoftware

Joined Oct 29, 2013
1,615
If you're connecting the cells in series, you really should make sure the cells are as closely matched as possible. Using the same model and capacity cells would be ideal.

If you're connecting the cells in parallel, it's a little less critical IFF the maximum current for the system is safely below the maximum current rating of the weakest cell. Note that you also have to make sure not to charge too fast for the weakest cell. In short, if you're not confident in what you're doing, use matching cells.

Balance charging means monitoring each individual cell and making sure they all charge to the same voltage. Try as we might, the world is not perfect and even battery cells of the same make and model are not identical. When charging them in series, some will charge faster than others. Left unchecked this can cause some cells to over-charge and others to over-discharge, neither of which are good. Balance charging means monitoring all cells individually. The most common method (probably because it's the cheapest/easiest method) is to charge in series, then any cells that get too high are pulled down by dumping a little energy from that cell through resistors. Look into how balance chargers work.
 

Thread Starter

zemanekj

Joined Jan 31, 2019
57
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.....
 

Tonyr1084

Joined Sep 24, 2015
3,942
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.....
Going to have to ask you to rephrase your question, at least for me anyway. I'm having trouble following your meanings regarding diodes. Also, please specify the battery configuration (series or parallel). Without that info it's impossible (for me) to give you a good answer. And a drawing or two would help as well.
 

Thread Starter

zemanekj

Joined Jan 31, 2019
57
Going to have to ask you to rephrase your question, at least for me anyway. I'm having trouble following your meanings regarding diodes. Also, please specify the battery configuration (series or parallel). Without that info it's impossible (for me) to give you a good answer. And a drawing or two would help as well.
Here is a picture!
 

Attachments

Top