Is it sane to straddle balancing leads?

[ulidtko]

Hi everyone! Another first-time poster here.

I'm trying to get through the planning/design phase of my first solar system. For the batteries, I really like and admire what people do with those hundreds of 18650 — however, for a first build I'd prefer something less involved and simpler. So to say, I want to first see a working solar system, test and "feel" its strengths & weaknesses — before embarking onto the journey of custom-built packs. You gotta keep your "helloworld" ambitions sane

I'm soft-decided on LiFePO₄ chemistry for that reason (added safety + longer service life matters more to amateur me than top-notch capacity).

By a good chance, I can get 4× 12V 50Ah cells (ehhh, batteries, these should be 4S LFP cells internally) locally for about $900. Is that price any good, isn't it?

Although many reviews report there's protective cut-off circuitry built-in — no information yet if there're balancing circuits inside those 12V packs. I'm still undecided which overall arrangement (4S, 2P2S, 4P) to use, but should I pick the 48V 4S — I'm certain I'm gonna need a BMS.

Here's where I'm getting to my actual question. In LFP BMS's, the balancing wires are set nominal 3.2 volts apart — but I'll only have 12V-spaced inputs. Can I leave 3/4 of balancing wires unconnected? I'm sure BMS should be able to tolerate 1 or 2 cell dropouts — but not so sure if a 16S BMS will run smooth on only 5-wire connection (GND, +12V, +24V, +36V, +48V, skipping 3 leads between each).

Couldn't find this question even stated (lest answered) anywhere, including this forum — so doing it the old-fashioned way, asking myself.
Advice appreciated, and thanks in advance for any response.

 

[DThames]

Is there a reason you are not building your battery from 16 cells, where you have access to all points of the battery circuit?

 

[ulidtko]

Is there a reason you are not building your battery from 16 cells, where you have access to all points of the battery circuit?

Well yes, the reason being the 12V "cells" I'm almost sold on. Very likely they're just prepackaged 4S inside... but I'd want to avoid stripping the extra packaging.

Perhaps a much simpler answer is to connect those in parallel. That'll yield natural balancing at 12V, right? And there's no need for BMS in that case, simplifying the build.

 

[Steve_S]

How about we use the KISS principle here.

First off the batteries. LFP is the closest to FLA profile of the Lithium Family, and you already learned the safest. They are not hard to assemble and work with, require no special welders or preps that Li-Ion and others need. I would even go as far as saying that LFP is the most Idiot Proof formulation to use. $900 for 12V/50AH = INSANE ! a BattleBorn 12V/100AH is < $950 USD and cheaper on sale ! and that is for a Great Prebuilt Drop-In.

HEADS UP ALERT: NOT all "prebuilt" batteries can go in series, some can't do parallel either. This is BMS related.

BMS'. Huge Area but KISS applied. Many BMS's can handle less cells than they are designed for. (NOT ALL !!!) For example some 16S BMS's can be used in an 8S or even 4S config. BUT they must be the correct type of BMS to do so and you have to be aware of the potential pitfalls. That depends on the actual device, they vary too much between makes & models.

Building Packs. (12V) 4S, 24V (8s), 48V (16s) there is no 5s,7s with LFP.
Simple Series Packs are the easiest to deal with and manage. each cell gets a lead and can be managed & monitored.
You cannot mix different cells within a pack. So if your building 100AH pack, then it's all same 100AH cells, no intermix.
You can have different capacity "packs" within a bank (* conditions apply with regards to charge/discharge capacity handling.)
BTW: No BMS = very silly and certainly risking your investment. It IS the Guard-Dog watching & protecting the cells.

IF you want to build a system, you need to figure out the Watts / Amps you want to be able to deliver and at what voltage (120 Single Phase or 240V split phase or ? depends on where you are). That will help determine the most suitable voltage for your battery bank. 12V battery system will NOT run 6000W Inverter (well it will run it but terribly and with big risks).

When you know the Target Voltage of your intended system then start considering your battery build, sizing it (you never want to do things more than once, unless you just have piles of cash and want to play because you are bored to tears).

 

[Zil]

$900 for all four? If you plan to use these pre-packaged batteries due to cost and availability. I would not trust the existing BMS if there is one, or even the hidden wiring quality. You would have sixteen 50 amp hour cells at 3.2 volts. There are many ways to do that. One way, as example only, connect 4 cells parallel, then those four series. for 12 volts, 200 amp hours. Use Overkill BMS treat each parallel set as one cell.

 

[ulidtko]

$900 for all four? If you plan to use these pre-packaged batteries due to cost and availability. I would not trust the existing BMS if there is one, or even the hidden wiring quality. You would have sixteen 50 amp hour cells at 3.2 volts. There are many ways to do that. One way, as example only, connect 4 cells parallel, then those four series. for 12 volts, 200 amp hours. Use Overkill BMS treat each parallel set as one cell.

Yes, $900 for all four. Removing the "drop-in replacement" cases, built-in bms's and wiring, extracting the cells and building my own 16S is something I consider doing... but still a bit reluctant. Mostly because such a rebuild is destructive and no going back from it.

 

[Ampster]

$900 for 2.4 kWatthours is $375 per kWh. That is significantly less that Battleborn which is close to $1,000 per kWh.
However if you are going disassemble those and add a 16S BMS you may want to just buy 16 generic LFP cells and assemble your own 48 v battery. Then you could be in the $200 or less range per kWh. That $900 could buy you at least 4 kWhs

 

[Steve_S]

Simple Numbers, the watt hours are actually higher due to the volts. (Volts X AmpHours = Watt Hours.)
12V/100AH battery = 1200 Watt Hour / 1.2 kWh.
12V/200AH battery = 2000 Watt Hour / 2.0 kWh.
12V requires 4 LFP cells.

4 Pcs 3.2V 105Ah Total US $416.24 delivered Duty & Taxes Paid to USA. (DPP)
4 Pcs 3.2V 202AH CATL Total US $557.00 delivered DPP
4 Pcs 3.2V 280Ah EVE Total US $457.60 delivered DPP

I believe this is self-explanatory.
Source: https://szluyuan.en.alibaba.com/productlist.html?


LFP Voltage Table or Reference. Consider the voltages when calculation your kWh.

State-Of-Charge​	Voltage at rest (zero current)​	Voltage under load (0.25C)​
100%​	14.0 Volt​	13.6 Volt​
99%​	13.8 Volt​	13.4 Volt​
90%​	13.4 Volt​	13.3 Volt​
70%​	13.2 Volt​	13.2 Volt​
40%​	13.2 Volt​	13.1 Volt​
30%​	13.0 Volt​	13.0 Volt​
20%​	12.9 Volt​	12.9 Volt​
17%​	12.8 Volt​	12.8 Volt​
14%​	12.6 Volt​	12.5 Volt​
9%​	12.4 Volt​	12.0 Volt​
0%​	10.4 Volt​	10.0 Volt​

 

[DThames]

Yes, $900 for all four. Removing the "drop-in replacement" cases, built-in bms's and wiring, extracting the cells and building my own 16S is something I consider doing... but still a bit reluctant. Mostly because such a rebuild is destructive and no going back from it.

Rather than tearing them all apart and back to your original question (sort of), if you can open the cases enough to all wires to the connection points between cells, you can bring out all leads needed for a 16 BMS. Just leave all of the internal wiring in place. Are they sealed plastic cases? What do they look like?

 

[Supervstech]

First... the price is fine for prebuilt, not fantastic, but ok.

Second, if they are prebuilt, they have a built in BMS... you need to ask if they can operate in series.


If not, they have to be opened up and arranged into 16S config.

 

[ulidtko]

if you can open the cases enough to all wires to the connection points between cells, you can bring out all leads needed for a 16 BMS

Hey, yes, that's a nice idea too. The cases are like this... looks like they have the internal bms board and bus-bars in the top section. But I'm not sure how serviceable that case is.

 

[Supervstech]

Looks like a typical glue on battery top case.

 

[DThames]

Hey, yes, that's a nice idea too. The cases are like this... looks like they have the internal bms board and bus-bars in the top section. But I'm not sure how serviceable that case is.
View attachment 22164
View attachment 22165

My general response is "Someone put it together, I should be able to take it apart". I was blamed for "Taking everything apart as a child".

 

[Zil]

Will has a video where he cuts similar plastic case apart.