diy solar

diy solar

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

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.
How does the FET of a BMS in series have the full string voltage across it?
 
How does the FET of a BMS in series have the full string voltage across it?
I believe whenever a bms meets a cutoff (high temp, hi voltage ect) if the batteries are charging or under a load the voltage at the bms will be whatever the charge voltage is or the combination of the other batteries voltage that has not been cut off. So 58 ish volts charging or 36 ish volts under load.
 
How does the FET of a BMS in series have the full string voltage across it?
My understanding is if BMS protection goes open then the FETs are exposed to the voltage of the string.

Many self-contained 12V batteries are not rated to be placed in series for this reason. For them to work without potentially damaging the BMS requires the FETs to be rated for the full series voltage.
 
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.
a BMS is only connected to the negative, the balance leads would burn out rather quickly. A "shorted" BMS would behave like a battery without a BMS.

The BMS is not worrying me. A shorted Cell would be my largest concern.
 
a BMS is only connected to the negative, the balance leads would burn out rather quickly. A "shorted" BMS would behave like a battery without a BMS.

The BMS is not worrying me. A shorted Cell would be my largest concern.
Yep!
 
The cells inside the battery are wired in series wouldn’t a shorted cell simply drop the battery’s voltage by 3.2 volts and shut down the bms?
We are talking about post # 16 saying that shorted BMS will allow current dump from other batteries that are connected in parallel with battery that has shorted out BMS, which is not correct, if BMS is shorted out then that battery no longer has protection by BMS but it will not cause current dump.
 
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?
when you look at the Diagram Amperetime and Chins supplies - there are no fuses and no busbars just wires, in this scenario it is well advised to stay with a maximum of 4.

While, when you know what you are doing - adding correct sized fuses and cables from parallel batteries to a busbar - there is not really a technical limitation how many parallel batteries you can have. It's usually a "people don't know what they are doing" limitation.

Series connection and parallel-series connection are completely different story- series connected batteries run out of balance, and you get into trouble fast.

If you need to go parallel - take the same exact battery - do not cheap out and do 2x 12V for 24V or 4x 12V for 48V Use a 24V battery for 24V or a 48V battery for 48V Read whatever the BMS can handle - in continues discharge (not maximum) - and use that as a fuse between the batteries and the busbar.
 
My understanding is if BMS protection goes open then the FETs are exposed to the voltage of the string.

Many self-contained 12V batteries are not rated to be placed in series for this reason. For them to work without potentially damaging the BMS requires the FETs to be rated for the full series voltage
If the load offered no resistance (eg short circuit) i suppose that is possible
 
What exactly is problematic? What issues will occur?
Math gets you. First let me restate my understanding, which still isn't scholarly by any means, 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.

The larger the number, the greater the chance that one is damaged by the overcharging or undercharging involved

nothing truly magic about the #4, but it is generally seen as the limit
 
So much miss information in this thread from the start....to much to list in my opinion....
 
Math gets you. First let me restate my understanding, which still isn't scholarly by any means, 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.

The larger the number, the greater the chance that one is damaged by the overcharging or undercharging involved

nothing truly magic about the #4, but it is generally seen as the limit
I believe you are referring to Batteries in series. Until recently I was always getting the 2 mixed up. Batteries in parallel will keep themselves balanced but they will all be affected if one battery should short out possibly in a catastrophic manner if protections are not in place. In series the batteries can walk away from each other in voltages over time and the entire banks amp hour capacity becomes the same as the lowest amp hour output battery.
 
I believe you are referring to Batteries in series. Until recently I was always getting the 2 mixed up. Batteries in parallel will keep themselves balanced but they will all be affected if one battery should short out possibly in a catastrophic manner if protections are not in place. In series the batteries can walk away from each other in voltages over time and the entire banks amp hour capacity becomes the same as the lowest amp hour output battery.
That too, but no. Batteries in parallel will self balance over time, this is true, but it is the over time that can get you if the power being pushed into the different batteries is done so unevenly, or even when the individual batteries in the string absorb / provide power unevenly. Having run multiple banks in in parallel, and even used mixed chemistries in series/para mix I've watched it happen, I've adjusted for it, and I've remediated it.

Top that off, with the fact that unless you're staying with 12-volts these are series connections as well and the need to just know what your in for becomes priceless

As I said there's no magic number "4", but the more you throw at it, the more you need to protect it, monitor it, and know how to react to it when things go wonky
 
I am currently running 6P/2S battleborns with no problem at all.
How do you know? Do you monitor the voltages of each battery individually during heavy charge / discharge cycles (especially at the upper / lower ends of the curve?)
 
Back
Top