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Parallel charging vs. series charging

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Parallel charging vs. series charging

Old 03-02-2021, 09:25 AM
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More info on parallel charging and various questions and misconceptions that arise. Also discusses series charging some.

On the topic of parallel charging of Lixx / PB packs.
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Old 03-02-2021, 09:34 AM
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Originally Posted by BaconRaygun
The problem is mostly when you have a pack with a cell that's a bit off from the others before you start charging. Assuming everything is perfect, then you are 100% right, it does not matter. Nothing is 100% perfect though, so in practice, it can be a problem. If you have two identical 6s packs, and each cell is at 3.75 except one... one is at 3.65, and you parallel charge, you may have a problem. The link you shared cautions the user about this.

Packs with parallel cells are not too common these days for anything aside from lower C TX or RX packs. I've taken apart hundreds of LiPo packs over the years to see how they are assembled. Last time I saw a flight pack that was wired in series and parallel was a 20C Align flight pack from 2008.
Yes it mentions best practice is to have cells within a couple tenths, but is not required. Some fly guys connect discharged packs and charged packs together to put them both in storage even, althou that might be pushing it for some wiring and connections.
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Old 03-02-2021, 09:39 AM
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Originally Posted by Dave H
I'm still trying to follow this bit:
The reason why this setup isn’t very good is because the packs will discharge down to different levels. This is due to the flow of current, the current from one pack must flow ‘through’ the second pack meaning that the current sees virtually double the resistance of the first pack.
I have personally experienced this first hand many years before SMC published their article... I had no idea what caused one of my packs to prematurely swell... I had no IR meter, nor was I rotating the packs, but I definitely experienced premature LVC issues and all kinds of reliability issues when running packs in series for 10 min mains... no issues with 5 min quals, but 2x2S-5Ah in series was a disaster and 1x4S-5Ah gave me no issues.

I believe the extra wiring/connectors for series wiring is what causes the additional IR on the circuit, totally different from having a single pack wired internally, plus a single pack has matched cells where you can't possibly guarantee matched cells from 2 separate packs. Simply put, running packs in series is just a bad idea all the way around
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Old 03-02-2021, 10:03 AM
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Originally Posted by billdelong
I have personally experienced this first hand many years before SMC published their article... I had no idea what caused one of my packs to prematurely swell... I had no IR meter, nor was I rotating the packs, but I definitely experienced premature LVC issues and all kinds of reliability issues when running packs in series for 10 min mains... no issues with 5 min quals, but 2x2S-5Ah in series was a disaster and 1x4S-5Ah gave me no issues.

I believe the extra wiring/connectors for series wiring is what causes the additional IR on the circuit, totally different from having a single pack wired internally, plus a single pack has matched cells where you can't possibly guarantee matched cells from 2 separate packs. Simply put, running packs in series is just a bad idea all the way around
You may have had issues, I just don't think that quote I pulled explains why. I see people run series packs without issues in all kinds of RCs. Sure the packs should be reasonably matched and kept together to avoid a low cell. Recommend a higher than normal LVC setting.

An option to provide additional protection when running series packs: CellShield
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Old 03-02-2021, 10:07 AM
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Could have also just been a dud pack. Or could well have been a cold joint or some other issue.

If running packs in series is so bad, then we should be seeing constant failures in the RC heli world. Helicopters are far more power hungry and running series is pretty much the norm for large helicopters. Even our single stick packs are technically twin packs in series that you can charge independently or in parallel.


Dave H
You're right, I should not have used the word "impossible." I should have clarified... it's impossible to get a true reading, since pairs (or triplets or whatever) of cells are seen as one. Sure, it's possible to get a good balance when parallel charging, but you have no way of knowing if there is a problem because you cant see individual cell voltage, so as a safety precaution, I will run them individually once a week and monitor the pack. I've had instances where one cell was giving me trouble but after balancing it independently, it would go back to normal for a while.

I've also been parallel charging for many years, safely, with no problems. My biggest words of caution are, let the packs equalize for a few minutes and definitely get a fused paraboard. Usually not a problem if you don't let them equalize, but it only takes one unseen problem to cause a fire. Stay safe out there!
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Old 03-02-2021, 11:39 AM
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Originally Posted by BaconRaygun
No, because you are still balancing them. The charger dictates the end voltage, not the way the pack is constructed. Assuming the balance lead is wired correctly and the charger is able to charge each cell independently, then the main contributor to how your packs balance is the charger. This is of course assuming the pack itself is healthy.

That's the whole point of balance-charging. If it didn't work that way, then why would we pay extra money for a charger that can also balance? Back in the day when LiPo tech was new to the RC world, you would have a separate device to balance the packs. Charging LiPo batteries was a lot more of a pain in the ass back then.
So are you are saying there is no point of having all 4 cells the same voltage in a 4s battery?
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Old 03-02-2021, 12:02 PM
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parallel charging is meant more for smaller packs for like Drones 1300mah or 1500mah packs etc. as you only get 3-5 min flight times so you have many packs 6 -12 is normal. so after you burn through your packs you can recharge them all at the same time. providing they are all the same cell count, MAH size, and preferably C rating.
when you get into bigger car batteries 5000mah and up most chargers do have the power to charge more than 2 at a time so its really irrelevant.

to charge a 1500mah at 1c is 1.5amps to charge 6 1500mah in parallel it is 9A
5000mah at 1c is 5Amps to charge 2 in parallel it is 10Amps!

better off getting a charger that can charge dual port(2 lipo at the same time)

you can't charge more than 1 lipo in series there is no way to balance it properly and you will damage the lipo at the very least. you have to make a harness that can take the lipos and connect them so that the charger can see the lipo as a single lipo (IE: 2 3S and make it a single 6S that the charger can recognize) but again once you get into bigger lipos most chargers cant handle the load. 2 5000mah 3s in series would be a 6s 10,000Mah load and 1c is 10Amps. bigger than that and your getting into 8s load which most chargers cant do, or over 10amps which most chargers cant do(not alot cant even do 10amps)
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Old 03-02-2021, 01:09 PM
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Originally Posted by mrreet2001
So are you are saying there is no point of having all 4 cells the same voltage in a 4s battery?
No, not at all what I am saying. How are you coming to that conclusion based on everything I have said?

Why would the two packs be at different voltage per cell? If they are identical packs, in identical health, and charged with the identical parameters, why would their balance differ if not charged together?
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Old 03-02-2021, 01:30 PM
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Originally Posted by BaconRaygun
Could have also just been a dud pack. Or could well have been a cold joint or some other issue.

If running packs in series is so bad, then we should be seeing constant failures in the RC heli world. Helicopters are far more power hungry and running series is pretty much the norm for large helicopters. Even our single stick packs are technically twin packs in series that you can charge independently or in parallel.


Dave H
You're right, I should not have used the word "impossible." I should have clarified... it's impossible to get a true reading, since pairs (or triplets or whatever) of cells are seen as one. Sure, it's possible to get a good balance when parallel charging, but you have no way of knowing if there is a problem because you cant see individual cell voltage, so as a safety precaution, I will run them individually once a week and monitor the pack. I've had instances where one cell was giving me trouble but after balancing it independently, it would go back to normal for a while.

I've also been parallel charging for many years, safely, with no problems. My biggest words of caution are, let the packs equalize for a few minutes and definitely get a fused paraboard. Usually not a problem if you don't let them equalize, but it only takes one unseen problem to cause a fire. Stay safe out there!
Fairly simple way to check on the fly, connect each pack in turn and check each cells discharged voltage, before connect all and charge away. A good idea for any cell or pack charge. Agree monitoring each pack occasionally can also be useful. And for others don't do anything you are not comfortable with, the links I posted earlier include concerns to be aware of and times it may not make sense to parallel.
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Old 03-02-2021, 02:12 PM
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Originally Posted by Roelof
I see one problem with parallel charging.
If you have one empty and one full battery ad lets say you charge 2C basec on the capacity of both batteries toegether then the empty one will get almost 4C charge.

In seril you do not have that problem.
That's not quite how it works. When you plug it in, the charged pack will discharge into the empty pack before you even turn the charger on. You want them to be at similar voltage levels before you plug them in.
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Old 03-02-2021, 02:22 PM
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Originally Posted by billdelong
I have personally experienced this first hand many years before SMC published their article... I had no idea what caused one of my packs to prematurely swell... I had no IR meter, nor was I rotating the packs, but I definitely experienced premature LVC issues and all kinds of reliability issues when running packs in series for 10 min mains... no issues with 5 min quals, but 2x2S-5Ah in series was a disaster and 1x4S-5Ah gave me no issues.

I believe the extra wiring/connectors for series wiring is what causes the additional IR on the circuit, totally different from having a single pack wired internally, plus a single pack has matched cells where you can't possibly guarantee matched cells from 2 separate packs. Simply put, running packs in series is just a bad idea all the way around
Additional wiring won't affect the IR, which is the Internal Resistance. By definition it excludes any external wiring. Extra external resistance won't damage the batteries or cause puffing.
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Old 03-02-2021, 07:51 PM
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Clearly the difference between 2 packs wired in series vs a whole single pack has some sort of effect, otherwise many people wouldn't notice this phenomenon. Not like the guys at SMC are inexperienced, clearly they know something about power systems.

There's definitely a significant difference in what the cells see in the 2 scenarios. Cells soldered tab to tab vs 2 connections (could be high resistance if dirty/loose) a bit of wire on top of the cell tab connections would look different under a big load.

That leads to some cells discharging more than others and could cause some to dip below 3-3.2v and get damaged.

End of the day if everything goes right, and you swap the packs so they arent in the same position, there shouldn't be a problem. But thats not always the case.
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Old 03-03-2021, 04:58 AM
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Originally Posted by Alexv2024
Clearly the difference between 2 packs wired in series vs a whole single pack has some sort of effect, otherwise many people wouldn't notice this phenomenon. Not like the guys at SMC are inexperienced, clearly they know something about power systems.

There's definitely a significant difference in what the cells see in the 2 scenarios. Cells soldered tab to tab vs 2 connections (could be high resistance if dirty/loose) a bit of wire on top of the cell tab connections would look different under a big load.

That leads to some cells discharging more than others and could cause some to dip below 3-3.2v and get damaged.

End of the day if everything goes right, and you swap the packs so they arent in the same position, there shouldn't be a problem. But thats not always the case.
Adding resistance in series won't make certain cells discharge more than others. If there's no parallel path for the current to flow then the exact same current will flow through the cells.

The main advantage to pre assembled packs in series is that they can match cells that have comparable capacities and resistances. Unmatched cells can lead to unbalanced voltages. No 2 cells are identical due to manufacturing tolerances.
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Old 03-03-2021, 05:22 AM
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Originally Posted by BaconRaygun
No, not at all what I am saying. How are you coming to that conclusion based on everything I have said?
If you are running 2 2s packs in series then it would be the same as a 4s pack. If you need to balance all 4 of the 4s cells then you should also need to balance all 4 cells of the 2 2s packs. Im not seeing the difference here.

Originally Posted by BaconRaygun
Why would the two packs be at different voltage per cell? If they are identical packs, in identical health, and charged with the identical parameters, why would their balance differ if not charged together?
Same reason you would have 1 or two cells off in a 4s pack. They should in theory be identical as well, but you still have to balance them so they are the same.
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Old 03-03-2021, 06:37 AM
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Originally Posted by mrreet2001
If you are running 2 2s packs in series then it would be the same as a 4s pack. If you need to balance all 4 of the 4s cells then you should also need to balance all 4 cells of the 2 2s packs. Im not seeing the difference here.
Umm... man, I'm sorry, but you are starting to confuse me. I'm not sure why you think I said you don't need to balance two s2 packs but would need to balance one 4s pack. I did not say that, nor did I imply it in any way. I just went over everything I posted and I've been really clear, so I really have no idea what you are talking about at this point.


Originally Posted by mrreet2001
Same reason you would have 1 or two cells off in a 4s pack. They should in theory be identical as well, but you still have to balance them so they are the same.
I was referring to packs that have already been balance charged. Why would there be a difference, and more importantly, why would balancing them together have an impact on their final resting voltage?
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