Why cant I run more than 4 lifepo4 batteries in parallel?

[aentrop]

Looking at Chins or Ampere Time batteries from amazon (12v200ah models) and they both say 4s4p MAX. Why is this?

How would the batteries even know if I set up for example 16 units all in parallel for a massive reserve bank?

Our Lifepo4 batteries can be connected in parallels and in series for larger capacity and voltage. Allow to be extended up to 4 in series and 4 in parallel (Max 4S4P) to get more capacity (Max 800Ah) and higher voltage (24V, 36V, 48V).

 

[sunshine_eggo]

Looking at Chins or Ampere Time batteries from amazon (12v200ah models) and they both say 4s4p MAX. Why is this?

How would the batteries even know if I set up for example 16 units all in parallel for a massive reserve bank?

Liability.

IMHO:

1) they don't trust the unwashed masses to actually connect them properly and verify that current is well shared

2) they want to limit their exposure to smaller systems.

 

[eabyrd]

My understanding, and realize it isn't scholarly by any means, is that it becomes increasingly hard to keep the individual batteries in balance as you parallel them. Pulling power from, and pushing power to each batter equally becomes more & more difficult as the number of batteries in parallel increases. That is compounded by the fact that each battery will have a unique internal resistance so will accept / produce power at a slightly different rate.

 

[Cajunwolf]

My understanding, and realize it isn't scholarly by any means, is that it becomes increasingly hard to keep the individual batteries in balance as you parallel them. Pulling power from, and pushing power to each batter equally becomes more & more difficult as the number of batteries in parallel increases. That is compounded by the fact that each battery will have a unique internal resistance so will accept / produce power at a slightly different rate.

Bingo! I have run four, but now only three for that reason. I have no problems with three but have had an issue with four. I see guys with up to eight batteries in parallel, but they have more money than sense and a lot of high-tech goodies to monitor them.

 

[DJSmiley]

And the short current adds up with multiple batteries.

With 6x a 100A BMS in parallel, 1 failed BMS will have to sustain 500A in the wires and connectors, without any of the BMSses tripping (since the other 5 provide their rated 100A thus don't trip)
It might even not trip a BMS, but just burn the failed BMS and the internal wires in the battery without any safety to prevent this

If you
- Fuse every single battery accordingly to its BMS rating
- Use proper wires and lugs to handle the total current of all BMSses together

you will be good (except for any balancing issues - batteries have to be pretty equal), but since the chances are high people are NOT aware how to handle 500A+ safely or have the tools to do this properly, they don't want to be responsible for it.

 

[rmaddy]

With 6x a 100A BMS in parallel, 1 failed BMS will have to sustain 500A in the wires and connectors

This would only be true if you were actually trying to pull 600A from the batteries. Assuming 12V batteries that would mean over 7200W. No one should be attempting to such usage. Just because 6 such batteries in parallel could support 600A of discharge current, that doesn't mean they will be subjected to anything anywhere near that much.

Someone using a 12V setup might, at most, have a 3000W inverter which could pull up to 300A if pushed to its limits.

 

[rmaddy]

I have run four, but now only three for that reason. I have no problems with three but have had an issue with four.

Would you mind sharing how you had the batteries wired up? There will certainly be issues if your loads are only connected to one battery. Three batteries are probably OK with the "diagonal" connection method. For 4 or more you really want to use the "posts" or "bus bars" wiring setups to keep them in balance better.

 

[Cajunwolf]

Would you mind sharing how you had the batteries wired up? There will certainly be issues if your loads are only connected to one battery. Three batteries are probably OK with the "diagonal" connection method. For 4 or more you really want to use the "posts" or "bus bars" wiring setups to keep them in balance better.

In my old boat, I had 4-12V 100AH Lead batteries in parallel for 200AH. They were wired in a standard parallel configuration, positive and negative, with load looped between positive on one end and negative on the other. I was always having an issue with one battery or the other, and without sophisticated tools and test equipment, you end up in an expensive guessing game. I had three 12V 100AH LiFePO4 batteries in the camper I sold early this year, all wired the same way, with very few easy-to-diagnose issues if sny. I just ordered three new 12V 200AHLiFePO4 batteries I'll be wiring parallel to go into the sweet little 28-foot class C I picked up in May.

 

[squowse]

And the short current adds up with multiple batteries.

With 6x a 100A BMS in parallel, 1 failed BMS will have to sustain 500A in the wires and connectors, without any of the BMSses tripping (since the other 5 provide their rated 100A thus don't trip)
It might even not trip a BMS, but just burn the failed BMS and the internal wires in the battery without any safety to prevent this

If you
- Fuse every single battery accordingly to its BMS rating
- Use proper wires and lugs to handle the total current of all BMSses together

you will be good (except for any balancing issues - batteries have to be pretty equal), but since the chances are high people are NOT aware how to handle 500A+ safely or have the tools to do this properly, they don't want to be responsible for it.

With the BMS in paralle - if a BMS fails it will either
1) go open circuit and no current will pass through it. The other BMS will have to allow the extra current or will trip out
2) go closed circuit and will probably have one sixth of the current passing through it, approx 100A in the given example.

 

[Cajunwolf]

With the BMS in paralle - if a BMS fails it will either
1) go open circuit and no current will pass through it. The other BMS will have to allow the extra current or will trip out
2) go closed circuit and will probably have one sixth of the current passing through it, approx 100A in the given example.

Never underestimate the foolishness of a foolish person.

 

[MJSullivan]

My understanding, and realize it isn't scholarly by any means, is that it becomes increasingly hard to keep the individual batteries in balance as you parallel them. Pulling power from, and pushing power to each batter equally becomes more & more difficult as the number of batteries in parallel increases. That is compounded by the fact that each battery will have a unique internal resistance so will accept / produce power at a slightly different rate.

Parallel is never a problem within a string. Basically paralleled cells self balance. The issue is balancing between strings. For that you need a BMS.

 

[eabyrd]

Parallel is never a problem within a string. Basically paralleled cells self balance. The issue is balancing between strings. For that you need a BMS.

Not true. While the op was talking a out batteries not cells, even with cells there is risk in placing them in parallel. The BMS is not designed to sense the state of or to balance a group of cells with each lead. It will not know the condition ofthe individual cells at each point in the series. Yes cells and batteries will eventually self balance. The problem is the differing rates of charge and discharge. 2 is fine. Up to 4 manageable with care. more than that is problematic

 

[JRUD]

Simple. Depends on how much voltage the BMS can handle. The better ones will do 48v. The cheap ones will fry.

 

[eabyrd]

Simple. Depends on how much voltage the BMS can handle. The better ones will do 48v. The cheap ones will fry.

I think you may have misread the subject. The bop is asking about paralleling batteries, not placing cells in series

 

[Bobert]

I am new to this forum I have a similar question. I have a 48v solar setup in my travel trailer. The lifepo4 batteries I am using state 4 in series or 4 parallel as their limitations. Some manufacturers allow for a combination of series and parallel. I assume that the parallel limitations are due to combined amps. However in my situation the largest continuous amp draw I can create is about 70 amps and the largest theoretical charge rate I can provide is 65 amps. Practically though 40 amps is the highest charge rate I have ever used. If I was to build 2 48v banks and connect them in parallel I would still have the same amps as 2 12v batteries in parallel. I don’t understand the reason for the 4 series or 4 parallel limitation and have not found the answer why and until this thread not even anyone willing to guess. Does anyone understand the science being the 4 or 4 limitation?

 

[DJSmiley]

Series: Used fets rated voltage is the limitation, since they need to be able to handle the total voltage of the series string

Parallel: Wires are the primary risk, although small. You can run 6 batteries in parallel, no problem.
However, worst case situation, without separate fuses on each battery, a failing BMS (dead short) in 1 battery means the wires in the battery to the BMS will have to handle the max current of all the other remaining batteries. Thus, in case of 6 batteries, each 100A max, 1 dead short BMS, the wire between the BMS and the primary busbar has to handle 500A..
Since its only made to handle the 100A it was designed for, 500A is going to fry it. And if you're unlucky, the resistance of this wire is just enough to keep the current slightly below 500A, thus none of the other batteries BMSsses will trip and they will continue to fry this wire.

Unlikely, but it CAN happen.

 

[Bobert]

So then since my systems max continuous draw is less than 70 amps and is fused at 100 amps a 100 amp fuse between battery banks could prevent this possibility and still give me additional storage capacity.

 

[time2roll]

I would go more than four in parallel but fuse each battery separately at half capacity.

 

[Bud Martin]

Series: Used fets rated voltage is the limitation, since they need to be able to handle the total voltage of the series string

Parallel: Wires are the primary risk, although small. You can run 6 batteries in parallel, no problem.
However, worst case situation, without separate fuses on each battery, a failing BMS (dead short) in 1 battery means the wires in the battery to the BMS will have to handle the max current of all the other remaining batteries. Thus, in case of 6 batteries, each 100A max, 1 dead short BMS, the wire between the BMS and the primary busbar has to handle 500A..
Since its only made to handle the 100A it was designed for, 500A is going to fry it. And if you're unlucky, the resistance of this wire is just enough to keep the current slightly below 500A, thus none of the other batteries BMSsses will trip and they will continue to fry this wire.

Unlikely, but it CAN happen.

Please clarify 'Dead short BMS'. Are you talking about Switched MOSFET shorted out (or welded relay contact) which is in series with the battery? I cannot see how 500A will be flowing in the dead shorted BMS. I see high current if batteries shorted out.

 

[WYtreasure]

Bingo! I have run four, but now only three for that reason. I have no problems with three but have had an issue with four. I see guys with up to eight batteries in parallel, but they have more money than sense and a lot of high-tech goodies to monitor them.

I wonder if this high-tech goodie would balance batteries and prevent the multiple battery imbalance issue.

$23 at alibaba.
I have seen it recommended here:

Battery Equalizer PLC-10 with display from Mazava (Unboxing)

Arrived today so wanted to share with you initial unboxing on this Battery Equalizer PLC-10 from Mazava Purchased from AliExpress for around $35, it looked to be exactly what I was looking for. I have 4x12V in a 2P 2S setup for 24v. I have matched batteries but purchased at a different time...

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