LifePo4 batteries in 2S2P vs 2P2S for my 24V inverter setup?

[Scoob-SanDiego]

BACKGROUND:

I have 4x LiTime 12V 100AH batteries and my MPP inserter runs at 24V.
It looks like most setups configure things in 2S2P (two strings of 2x12v inseries) and the two strings connected in parallel.

Recently I have been learning about the problems that unbalanced 12v batteries in series can cause with an inverter and the capacity of your battery bank.
Ian at watts247.com has some GREAT videos on the subject explaining the benefits of using a battery equalizer for your 12v batteries (NOT active balancer).
watts247 carries two 12 batteries equalizers:

A 2x12v batteries model: https://watts247.com/product/eq-24-2-24v-battery-balancer-balances-2-x-12v-batteries-in-series/
And a 4x12V batteries one: https://watts247.com/product/eq-48-...-batteries-or-any-x-3-2v-batteries-in-series/

The videos explaining what those devices do and why they are useful / needed are very good:

Note that his equalizers look like they might be white labels from NEEY (on aliexpress)
https://www.aliexpress.us/item/3256805739573881.html

Naturally you can save a few bucks by buying from China but if you have questions, need support or warranty, a knowledgeable US based vendor has its advantages


QUESTION:

Is there ANY advantage in running my batteries in 2S2P. vs in 2P2S
i.e. two stinrgs of 2x12V then put in parallel to get 24V 200Ah
vs 2 x 12V batteries in parallel (creating a 12V/200Ah battery). THEN put in series to create a 24V 200AH battery.

In the context of using a battery equalizer, the two 12V batteries in parallel are already self equalizing between themselves.
You can then use the smaller/cheaper battery equalizer (the one for 2 batteries instead of 4) to balance between the two 12v/200Ah batteries.

So my question again: Other than using a cheaper equalizer, is there any benefit of 2s2p over 2p2s?
Thanks!
- Christian

 

[DIYrich]

If you do 2s2p, if one battery goes down, you loose 1 bank. If you do 2p2s, then you loose all of them.

 

[Scoob-SanDiego]

If you do 2s2p, if one battery goes down, you loose 1 bank. If you do 2p2s, then you loose all of them.

Actually you would not loose power.
In 2S2P and you loose a battery, you loose a whole string.... so I Would end up with 24V 100AH.
In 2P2S, if you loose a battery, it means that one of. your two batteries unit is now is only 100Ah ..... Unless my understanding is not correct and the faulty battery would actually take down the voltage of the other one.

I was under the understanding that LifePo4 batteries are normally solid for at least 10 years.

My main concern has to do with the general individual and total battery health.
Considerations such as if you have two batteries in series and they are not balanced, then it will end up damaging the weaker battery ... at some point the the delta is such that the weaker battery gets over charged.

 

[Akane]

Thank you for bringing this up. My friends and I are literally building the same kind of system right now with four 12v 100ah li time batteries.

We got a 24v inverter and want to wire the batteries in 2s2p.

But we’re confused about it because in the battery manual, it doesn’t say anything or show any diagrams for wiring in 2s2p. They specifically recommend 2p2s.

But my question is: WHY?!

Scouring the internet and it seems impossible to find anyone explaining why that would be more beneficial over 2s2p, or why li time recommends it in the first place.

I’m wondering how this hasn’t come up as a question before and especially within this solar forum.

So far we’ve fully charged all the batteries and wired them together in parallel, and are now waiting approximately 18 hours for them to balance out before configuring them into 2s2p.

We have a victron smartshunt and are hoping it will give us all the information about the batteries that we’ll need when the system is up and active.

But I still can’t understand how this hasn’t been clearly addressed here lol. Sorry for the rant. I’d be very happy if anyone has any resources/videos to share that can help us wrap our brains around the concept better.

Thank you! And I wish you the best with the system you’re building :D

 

[Rocketman]

I had a similar problem on my MotorHome way back when I was using GC2.

I had 4 6v batteries hooked up in 2s2p (1+2 in parallel with 3+4). Over time (including a complete discharge) battery 1 and battery 4 ended up significantly lower voltage than the other 2. That brought everything down.

As I was learning about lithium’s I realized it would have been better to have 1 & 3 paralleled into a massive 6v battery and 2 & 4 paralleled then put them in series. I also should have used my Victron BMV712’s mid-point tool to keep an eye on it. Oh well I am happier with my diy lithium battery anyway.

Good Luck

 

[Scoob-SanDiego]

Thank you for bringing this up. My friends and I are literally building the same kind of system right now with four 12v 100ah li time batteries.

We got a 24v inverter and want to wire the batteries in 2s2p.

But we’re confused about it because in the battery manual, it doesn’t say anything or show any diagrams for wiring in 2s2p. They specifically recommend 2p2s.

But my question is: WHY?!

Scouring the internet and it seems impossible to find anyone explaining why that would be more beneficial over 2s2p, or why li time recommends it in the first place.

I’m wondering how this hasn’t come up as a question before and especially within this solar forum.

So far we’ve fully charged all the batteries and wired them together in parallel, and are now waiting approximately 18 hours for them to balance out before configuring them into 2s2p.

We have a victron smartshunt and are hoping it will give us all the information about the batteries that we’ll need when the system is up and active.

But I still can’t understand how this hasn’t been clearly addressed here lol. Sorry for the rant. I’d be very happy if anyone has any resources/videos to share that can help us wrap our brains around the concept better.

Thank you! And I wish you the best with the system you’re building :D

The one advantage I can see of doing 2P2S (i.e. you create TWO 12V batteries of 200Ah) is that the two batteries in parallel will be balanced between the two of them. There can still be an imbalance between those two batteries in series. Where if you had a 2S2P, there can be an imbalance between the two batteries of the first 24V string AND there can be yet another imbalance between the two batteries of the second 24V string... so 2 possible imbalance vs a single one in 2P2S....

Here is my observation on my system:
If I first TOP balance all my 12V batteries (i.e. fill them up to 100% BEFORE I connect them), and if I don't discharge them below 12V, then even in my 2S2P configuration, they have so far stayed within 0.02V of one another.

But over time, they CAN (and presumably WILL) drift.
A manual solution might be to periodically measure the individual batteries voltage and turn of the whole thing to top balance them when they drift too much.
Or a more helpful solution is to simply add a battery equalizer like this one:

• https://watts247.com/product/eq-48-...-batteries-or-any-x-3-2v-batteries-in-series/
  • Note:
    • Easy to configure. You just connect the 4 red/black wire pairs from the device to the 4 +/- terminal of your batteries REAGARDLESS of how you connect your batteries.
    • It IS a good idea to have your batteries somewhat close to the same voltage BEFORE you connect the balancer.

• You can also get what looks to be the exact same device directly from AliExpress (here, a NEEY brand model HC02 (4 batteries w voltage reading display) for $45 delivered. Of course, buying the white label unit from watts247 gives you the ability to talk to someone for questions and easier return/exchange if anything is wrong with the order.
  • https://www.aliexpress.us/item/1005005925888633.html

Finally, if it is not too late, I would (from my limited but growing experience) STRONGLY recommend that you change your inverter selection immediately to a 48VDC from your current 24V selection. You ALREADY have invested in 48V worth of batteries. The higher voltage inverters will give you:

• Higher PV input voltage / Watt
• Higher Load power

Personally, I *love* my MPP LV2424.... BUT, it is very limited. It cost me about $550 from ebay (with shipping). It is limited to 145V VOC input (30-80V operating PV voltage) and 2000W output. During the holiday season, I could have purchased an EG4 3000 for $600-$650. In comparion, the EG4 can handle 500V / 5000W PV input!!! And 3000W output. For the extra $100-$200, it should have been a no-brainer!!! But that's my first system and every penny seemed like a lot... I'm now $2500 into it and now the extra $200 is laughable... I would have probably saved that much by having a single solar string instead of 6 like I have to do now (because of the limited PV input of my inverter).

Enjoy your solar adventure. It's a lot of learning and a lot of fun!

 

[Akane]

Wow I must say these replies are incredibly helpful. Thank you so much!!