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Wiring mixed age LiFePO4 batteries: best practices

mmahdi

New Member
Joined
Nov 27, 2024
Messages
7
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Ontario
Hello,

(all batteries I'll mention in this thread are LiFePO4)

Current setup
I have 2 x 12V batteries that are about 1 year old. I also have 2 x 12V batteries that are pretty new. I have a 24V system consisting of 2 batteries in series, which are then hooked up in parallel to another 2 batteries in series, i.e. 2S-2P. Each series is composed of 1 old and 1 new battery.

Considering changing to...
My question is will it be better to change this to parallel first, then series. The reason Is that I've read this is best practice when batteries are mixed age; I'll elaborate my understanding:

Why Parallel First, Then Series is Better if batteries are mixed age......or am I wrong?

  1. During Discharge:
    • Parallel First: The older batteries contribute less current due to their higher internal resistance, and the newer batteries pick up the slack. The system remains stable because each parallel group balances internally.
    • Series First: Both batteries in a series pair must pass the same current. The older battery’s voltage drops faster, reducing the performance and stability of the series pair.
  2. During Charging:
    • Parallel First: The newer battery in each parallel group absorbs more charge, while the older battery absorbs less. This prevents overcharging of the older batteries and ensures smoother charging for the group.
    • Series First: The older battery’s higher resistance causes its voltage to rise faster, triggering an early cut-off and leaving the newer battery undercharged.

Disadvantages of parallel first, then series
1) if a bank is affected or disconnected somehow, the entire system goes down. Right now I can disconnect 1 series while the other series continues to function, so there is redundancy

2) checking individual battery voltages: individual battery perofrmance can be measured in series when the system is being discharged/charged and an underperforming or overcharging battery can be identified immediately. In parallel, each battery must be removed from the bank individually to be tested.

Expansion in the future seems to tilt in favor of going parallel first...
Adding more batteries in the very near future is not a concern however if I was to expand by adding 2 more batteries in the not so near future, it seems I will have to go parallel as the older batteries will certainly have a higher internal resistance and less capacity, and will cause the issues mentioned above.

Looking for your feedback.

Thanks!
 
Hello,

(all batteries I'll mention in this thread are LiFePO4)

Current setup
I have 2 x 12V batteries that are about 1 year old. I also have 2 x 12V batteries that are pretty new. I have a 24V system consisting of 2 batteries in series, which are then hooked up in parallel to another 2 batteries in series, i.e. 2S-2P. Each series is composed of 1 old and 1 new battery.

When putting batteries in series, "SAME" is best, so string the two old together, and string the two new together.

Considering changing to...
My question is will it be better to change this to parallel first, then series.

No. Never. You lose the ability to measure ANY of the 12V batteries directly. In 2S2P, you can always measure all four batteries' voltages. In 2P2S, you can't measure ANY of them.

Expansion in the future seems to tilt in favor of going parallel first...
Adding more batteries in the very near future is not a concern however if I was to expand by adding 2 more batteries in the not so near future, it seems I will have to go parallel as the older batteries will certainly have a higher internal resistance and less capacity, and will cause the issues mentioned above.

Looking for your feedback.

Thanks!

Nope. Simply add a new 2S string. Even better, just get a 24V battery.

The most important thing in any of this is that series elements need to be the same in as many ways as possible. The last thing you want to do is mix batteries of different characteristics in the same string unless you have no other choice.

Battery wiring best practices in line #6 of my signature.
 
When putting batteries in series, "SAME" is best, so string the two old together, and string the two new together.



No. Never. You lose the ability to measure ANY of the 12V batteries directly. In 2S2P, you can always measure all four batteries' voltages. In 2P2S, you can't measure ANY of them.



Nope. Simply add a new 2S string. Even better, just get a 24V battery.

The most important thing in any of this is that series elements need to be the same in as many ways as possible. The last thing you want to do is mix batteries of different characteristics in the same string unless you have no other choice.

Battery wiring best practices in line #6 of my signature.
Thanks for the detailed responses.

Do you recommend a battery balancer? Or is it sufficient to ensure the bank is charged to full capacity I.e. cut-out voltage monthly?
 
Thanks for the detailed responses.

Do you recommend a battery balancer?

Yes. One per string.

Or is it sufficient to ensure the bank is charged to full capacity I.e. cut-out voltage monthly?

Monthly, no. Twice a week, maybe.

In either case, the 12.8V batteries need to regularly be charged above 13.6V for a couple hours to allow meaningful balancing to happen.
 
Yes. One per string.



Monthly, no. Twice a week, maybe.

In either case, the 12.8V batteries need to regularly be charged above 13.6V for a couple hours to allow meaningful balancing to happen.
That makes sense.

I took measurements today when the charge controller had entered the Boost stage for approximately an hour.

String 1: 14.25V + 14.35V=28.6V
String 2: 13.40V + 15.25V=28.65V
*some numbers rounded up/down at the 100th decimal place.

I also took measurements under discharge conditions (~1kw load, ~250w solar input)
String 1: 13.2V + 13.2V=26.4V
String 2: 13.2V + 13.2V=26.4V
*some numbers rounded up/down at the 100th decimal place

Am I right to be concerned about the voltage differential at boost charge of the battery at 15.25V?
 
That makes sense.

I took measurements today when the charge controller had entered the Boost stage for approximately an hour.

String 1: 14.25V + 14.35V=28.6V
String 2: 13.40V + 15.25V=28.65V
*some numbers rounded up/down at the 100th decimal place.

I also took measurements under discharge conditions (~1kw load, ~250w solar input)
String 1: 13.2V + 13.2V=26.4V
String 2: 13.2V + 13.2V=26.4V
*some numbers rounded up/down at the 100th decimal place

Am I right to be concerned about the voltage differential at boost charge of the battery at 15.25V?
Yes. The mentioned balancer should help with that, if it actually functions properly.
 
That makes sense.

I took measurements today when the charge controller had entered the Boost stage for approximately an hour.

String 1: 14.25V + 14.35V=28.6V

This string is probably okay, but it MIGHT be in protection mode. 0.1V isn't bad, but it's not great.

String 2: 13.40V + 15.25V=28.65V

This string is horrifically imbalanced, and the 15.25V battery is in protection mode. You are measuring the votlage of the parallel battery/charger, not the actual string. The 2nd battery is at 0V.

Recommend you dismantle this string and charge both 12V to full individually or in parallel (preferred).

I also took measurements under discharge conditions (~1kw load, ~250w solar input)
String 1: 13.2V + 13.2V=26.4V
String 2: 13.2V + 13.2V=26.4V
*some numbers rounded up/down at the 100th decimal place

These don't really mean anything as LFP votlages are very tight in the 3.1-3.4V/cell operating range. Very common to see imbalanced batteries have great voltages when discharging.
 
This string is probably okay, but it MIGHT be in protection mode. 0.1V isn't bad, but it's not great.



This string is horrifically imbalanced, and the 15.25V battery is in protection mode. You are measuring the votlage of the parallel battery/charger, not the actual string. The 2nd battery is at 0V.

Recommend you dismantle this string and charge both 12V to full individually or in parallel (preferred).



These don't really mean anything as LFP votlages are very tight in the 3.1-3.4V/cell operating range. Very common to see imbalanced batteries have great voltages when discharging.
Going to do as you recommended and I have a balancer coming in tomorrow as well.

Will post results after the process is complete.

Thanks for all the advice!
 
Going to do as you recommended and I have a balancer coming in tomorrow as well.

Will post results after the process is complete.

Thanks for all the advice!

Confirming this is the config/plan:

String 1 is 2S old batteries.
String 2 is 2S new batteries.

You will individually or in parallel charge the 12V in each string.

Once charged and reformed as 2S, you will install 1 balancer per string.

If the above is correct, I endorse it! 😁
 
Well, I thought I was confident with my plan....seems I still have some confusion.

After our last discussion, the tech from eco-worthy advised to connect parallel first, as "it weaker battery doesn't then dictate how the string behaves. In parallel the stronger battery will absorb charge before the weaker battery cuts out, and can also fully discharge. Reduces stress on weaker battery and allows utilization of full capacity".

So I read the entire manual again and went down the rabbit hole. Here's my understanding of the 'issues' of each method:

- series connection: weaker battery works harder and the series takes on it's limitations. Solution is battery balancer. Benefit over parallel is less connections, ability to remove a string and ability to troubleshoot individual batteries.

- parallel connection: current sharing means "closer" battery works harder, charges faster. Solution is bus bar and equal conductors from EACH battery. Benefit over series is the claimed better balancing.

Question 1) But it seems based on what everyone suggested above, the series connection would be balanced within the series, and hence a "stronger series" would provide more current and absorb more and the overall system works better...is this right?

Question 2) in the diagram (#3) from the manual there's only 1 balancer needed for 3 strings of series. Is this correct? I can't make the jump from balancing 1 string to all 3 at the same time.

Question 3) I've attached 3 pictures of the possibilities I'm considering; please let me know if my understanding is correct, and which wiring method you'd recommend.

Thanks as usual!
 

Attachments

  • Parallel #1.png
    Parallel #1.png
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  • Parallel #2.png
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  • Series from manual.png
    Series from manual.png
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Any one of them will work, but the second picture is the most "perfect". I would test each battery for capacity and try to match two in series so the voltage will track better. Then parallel the 3 low batteries and 3 high batteries and connect them like pic2 and use one balancer. You will end up with same capacity in the lower set and upper set this way, even though the capacity in each battery is different in the parallel set. Like if you had 2 each of 50ah, 100ah, and 200ah. You would put one of each rating in the low set and one of each rating in the high set. In your case, the differences in capacity are just going to be smaller.
 
If the capacity of all 6 are significantly different, just connect them so that total capacity of the low set is as close as possible to the high set. Connect as pic2 with one balancer. That is the best you can do with what you have.
 

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