BMS needed?

[Nujenj]

I was wondering if I need BMS for a 4S8P raw LiFePO4 batteries for the sole purpose of use for my travel trailer. These batteries will be charged by two 150v max 60 amp MPPT charge controllers. I am new to all this so forgive me if I am missing any information you need. Thanks for the help!

 

[John Frum]

4s8p is actually 8 12.8 volt batteries in parallel.
Normally that would require 8 BMSs.

 

[Nujenj]

4s8p is actually 8 12.8 volt batteries in parallel.
Normally that would require 8 BMSs.

yes there will be 8 12v batteries. I was told by the president of the https://www.electriccarpartscompany.com that I wouldn't need the BMS if I were to make sure to balance out all the cells properly. Just wanted to hear from others in the this community what the pros and cons were? The Battery specs are below: I will have 32 of these which make up 8 packs of 12.8v

100Ah Lithium Battery, 3.2V, 2C (6C peak, 10 sec)

Aluminum Shell Batteries

Lithium LiFePO4

5.4L * 1.6W * 12H in​

 

[Bob B]

You need the BMS .... do a search for "need BMS"..... or are you just here to stir the pot. Seems like there is one of these posts every day.

.....and don't trust anyone who tells you otherwise.

 

[Nujenj]

You need the BMS .... do a search for "need BMS"..... or are you just here to stir the pot. Seems like there is one of these posts every day.

.....and don't trust anyone who tells you otherwise.

I am not here to stir the pot, I read a bunch and it talked about portable batteries and some post had remade batteries. I am new to all this so I apologize.

 

[Bob B]

Do a search on the subject and you will find a LOT of discussion.

The bottom line is that a battery pack will tend to get out of balance over time .... or you may have a bad connection develop .... etc, etc.
The BMS is the only device that monitors individual cell voltages in invokes protections based on that.

Even if a pack total voltage is fine .... individual cells can be either high or low and can be damaged without some intervention.

 

[Nujenj]

Do a search on the subject and you will find a LOT of discussion.

The bottom line is that a battery pack will tend to get out of balance over time .... or you may have a bad connection develop .... etc, etc.
The BMS is the only device that monitors individual cell voltages in invokes protections based on that.

I have found some discussions, but thank you for the help explaining it to me. I just need to find the best size that works for my setup.

 

[Bob B]

Having 8 batteries in parallel creates issues of it's own.

Maybe at least consider going with 4 2P4S packs .... or even 2 4P4S packs .... or consider a 24V system.

 

[Supervstech]

IF, and I stress ONLY IF you decide to try and use a battery without a BMS... you are far better off tying 8 cells in parallel, then making a single 4 s string out of each bundle of 8 cells linked in parallel...
So, 8P4S config...
This way, more of the cells will be balanced to each other...

HOWEVER...
This is not wise.
The cells are not cheap, and no matter how well top, or bottom balanced, the cells WILL drift in ability to charge, or discharge, and pretty quickly you will find cells below, or above the limits for sustained operation of lithium, and they will either be turned to trash, bulge and fail, or simply quit functioning.

use a BMS... it may not prevent issues from evenly charging or discharging, but it WILL stop damage from the scenario by stopping the charge/discharge function.

 

[Nujenj]

IF, and I stress ONLY IF you decide to try and use a battery without a BMS... you are far better off tying 8 cells in parallel, then making a single 4 s string out of each bundle of 8 cells linked in parallel...
So, 8P4S config...
This way, more of the cells will be balanced to each other...

HOWEVER...
This is not wise.
The cells are not cheap, and no matter how well top, or bottom balanced, the cells WILL drift in ability to charge, or discharge, and pretty quickly you will find cells below, or above the limits for sustained operation of lithium, and they will either be turned to trash, bulge and fail, or simply quit functioning.

use a BMS... it may not prevent issues from evenly charging or discharging, but it WILL stop damage from the scenario by stopping the charge/discharge function.

I see! so if I have 32 cells and am trying to build a 12v system. what is the best way to do this with adding the least amount of BMS, since I see that the better ones cost about $120.

 

[Positronium]

Any particular reason why you sticking with 4S?

You may be able get the same charging power with one 60A solar charge controller by going 8s4p pack.

 

[Supervstech]

I see! so if I have 32 cells and am trying to build a 12v system. what is the best way to do this with adding the least amount of BMS, since I see that the better ones cost about $120.

For a 12V setup, bundling 4 packs of 8 paralleled cells, then adding a single quality BMS will give a decent margin of safety for the pack.

how big is each cell in capacity?

 

[Nujenj]

Any particular reason why you sticking with 4S?

You may be able get the same charging power with one 60A solar charge controller by going 8s4p pack.

I am using 4s because that was what the president of the batteries recommended me to do. He designed my whole system because I was trying to use Will's 24v system and integrate a step down dc to dc transformer to power my 12v equipment and lighting in my camper. I will have 1650 watts solar array wired in 2 sets of parallel and each set will have its own charge controller. The sets of solar array will have 4x100 watt panels wired in series. the solar charges are 150v/60amps.

 

[Nujenj]

For a 12V setup, bundling 4 packs of 8 paralleled cells, then adding a single quality BMS will give a decent margin of safety for the pack.

how big is each cell in capacity?

each individual cell is suppose to be 100Ah @3.2v. What BMS rating would work for this and would solar charger go into the bus bar then the BMS go from batteries to bus bar? I was under the impression I would have to have a bus for each pack.

 

[Luthj]

The single biggest factor you need to consider is what your max sustained current is. From there you can determine how many separate batteries you want to build. I doubt you need 8. Instead You can probably do 2 or 4 batteries.

What size cells are you using?

 

[Supervstech]

WOOF! 800Ah bank... that is a honking large BMS!
let’s change gears... how big a demand will you have on the bank? What loads will you be powering?

 

[Positronium]

WOOF! 800Ah bank... that is a honking large BMS!
let’s change gears... how big a demand will you have on the bank? What loads will you be powering?

I was just going to ask this. From the dual 60A charge controllers BMS will need handle up to 120A of charging current.

 

[Nujenj]

The single biggest factor you need to consider is what your max sustained current is. From there you can determine how many separate batteries you want to build. I doubt you need 8. Instead You can probably do 2 or 4 batteries.

What size cells are you using?

I will be living full time in my 37-1/2ft travel trailer with my wife and 3 kids. as of right now I did a practice run staying in the trailer plug into a kWh meter and because we will be stying in some colder regions around 30s-40s for a month or two and might not haver full hook ups. I ran a quartz electric heater in tandem with the propane heater and used the TV at night time only and did not even use the bathroom and we were pulling about 3 kWh per day with the biggest loads probably pulling a little under 20 amps at a time. I used the calculator Will had in his book as well as other videos and in order to have 3 days worth it said I needed that many batteries. however I am new to this whole thing and sometimes feel like I am in over my head....

 

[Supervstech]

I will be living full time in my 37-1/2ft travel trailer with my wife and 3 kids. as of right now I did a practice run staying in the trailer plug into a kWh meter and because we will be stying in some colder regions around 30s-40s for a month or two and might not haver full hook ups. I ran a quartz electric heater in tandem with the propane heater and used the TV at night time only and did not even use the bathroom and we were pulling about 3 kWh per day with the biggest loads probably pulling a little under 20 amps at a time. I used the calculator Will had in his book as well as other videos and in order to have 3 days worth it said I needed that many batteries. however I am new to this whole thing and sometimes feel like I am in over my head....

When showing the loads, don’t use amps... amps are a confusing metric... use Watts...
20 amps at 12V is 240 watts... 20A at 120V is 2400Watts...

 

[Nujenj]

WOOF! 800Ah bank... that is a honking large BMS!
let’s change gears... how big a demand will you have on the bank? What loads will you be powering?

The powered jack in the front hitch uses 12v, the motors for the slides uses the 12v, all the lights in the trailer uses 12v, the propane heater uses the 12v and the fridge uses the 12v. other than that the electric heater uses the 110v but it will also draw from the battery bank if I don't have shore hook up. So far the electric heater running non stop with the propane heater coming off and on as well as the 12v fridge has not tripped my 20 amp breaker from the garage outlet.