LIFEPO4 BMS Question

MKalman

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Hey there,

If I am creating two separate 24V 230ah batteries using EVE cells and have two 8S JBD BMS, one in each battery, is it okay to combine the two 24V batteries to produce 48V? Is this problematic?
 

Ampster

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It is not optimal because each 24 volt battery is in series with the other one an there nothing making sure they are at the same voltage.
 

HRTKD

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Hey there,

If I am creating two separate 24V 230ah batteries using EVE cells and have two 8S JBD BMS, one in each battery, is it okay to combine the two 24V batteries to produce 48V? Is this problematic?

For lead acid batteries, that's fine. For batteries with a BMS, it's not.

If you need 48 volts, then build a 48 volt battery.
 

MKalman

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So I'd be better off creating a 48v 230ah battery? And what happens if I want to make two of these (one in the future), then what do I do?
 

LithiumSolar

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Hey there,

If I am creating two separate 24V 230ah batteries using EVE cells and have two 8S JBD BMS, one in each battery, is it okay to combine the two 24V batteries to produce 48V? Is this problematic?

It's not problematic as long as the BMS support being put in series like that to 48V. It's not optimal though and would definitely be better to just use a 48V BMS.

So I'd be better off creating a 48v 230ah battery? And what happens if I want to make two of these (one in the future), then what do I do?

Yes. If you create a second one in the future, it's not a problem to connect both in parallel (set BMS parameters the same).
 

MisterSandals

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And what happens if I want to make two of these (one in the future), then what do I do?
Just out of curiosity what is the big picture of what you are trying to do?
Sounds like you built a couple 24V batteries and are looking to move up to 48V?
 

Ampster

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.....if I want to make two of these (one in the future), then what do I do?
You can buddy pair the cells (2P16S) or buy another 16S BMS if you want redundancy. I have a 3P16S pack and think redundancy is overrated. But I have an $800 BMS so it is an economic issue for me.
 

MKalman

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Just out of curiosity what is the big picture of what you are trying to do?
Sounds like you built a couple 24V batteries and are looking to move up to 48V?
I'm looking to make a future proof system and am working with 16 230ah EVE cells,

I don't want a ridiculously huge case that weighs 140 lbs as I will be moving provinces up here in Canada

That's why I'm wondering if I can make two 70 lb pelican cases into batteries and it'll be easier to manage and less likely to break the handles.

You can buddy pair the cells (2P16S) or buy another 16S BMS if you want redundancy. I have a 3P16S pack and think redundancy is overrated. But I have an $800 BMS so it is an economic issue for me.
So a 8S2P BMS exists?
 

Ampster

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So a 8S2P BMS exists?
No, but a 16S BMS will see the buddy pairs as one cell so it will read a 2P16S pack the same as a 16S pack. The designation 2P is what the buddy pairs are. That is 16 pairs of 2. Each pair is wired in series with other pairs.
 

MKalman

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No, but a 16S BMS will see the buddy pairs as one cell so it will read a 2P16S pack the same as a 16S pack. The designation 2P is what the buddy pairs are. That is 16 pairs of 2. Each pair is wired in series with other pairs.
So this would mean I would have two packages of 16 cells, each 48V 230AH, totalling 48v 460ah, correct?

But the same will not apply if I have two packages of 8 cells and then connect the positive and negative leads in series?
 

Ampster

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But the same will not apply....
The same what will not apply? Voltage, kWhs or use of BMS?
To clarify, two packs of 16 cells is 32 cells and two packs of 8 cells is 16 cells. Yes the total kWhs of the first will be double the kWhs of the second. A 16S BMS will work for either scenerio depending how you wire them. I use the term pack to describe a group of cells in a configuration were there is a single negative and single positive terminal at the end of each pack.
Just to be clear, the original question was about two packs of 8 cells each with their own BMS.
 

MKalman

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The same what will not apply? Voltage, kWhs or use of BMS?
To clarify, two packs of 16 cells is 32 cells and two packs of 8 cells is 16 cells. Yes the total kWhs of the first will be double the kWhs of the second. A 16S BMS will work for either scenerio depending how you wire them. I use the term pack to describe a group of cells in a configuration were there is a single negative and single positive terminal at the end of each pack.
Just to be clear, the original question was about two packs of 8 cells each with their own BMS.
Ahhh, I get it, the packs are in two cell couples, 32 total, that would be fine, but the whole goal is to keep the weight of each package less, as 144 lbs is too heavy.

Yes, two separate cases with 8 cells each at 24v, connected in series to make 48v, what kind of BMS will work for this?

Would it be better to just make two separate 24V batteries and connect them in parallel? This wouldn't pose a problem for the BMS right?
 

Ampster

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Yes, two separate cases with 8 cells each at 24v, connected in series to make 48v, what kind of BMS will work for this?
A 16S BMS is the only thing that will keep all 16 cells balanced. It is possible to have two groups of eight cells connect into one 16S battery. The only tricky part will be the BMS wires and the bus bars when you want to separate those packs. But you would only have to remove one bolt if designed to accommodate that pack separation.
 
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MKalman

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A 16S BMS is the only thing that will keep all 16 cells balanced. It is possible to have two groups of eight cells connect into one 16S battery. The only tricky part will be the BMS wires and the bus bars when you want to separate those packs. But you would only have to remove one bolt if designed to accomodate that pack separation.

Long term 48 volt inverters are more plentiful and a better long term solution for your new house. For a low capacity portable pack 12 or 24 volts could work.
So I can find a way to have loose wires that come out from the pack that join together to create the single 48v battery, that's an interesting thought, would it simply be the one cable that connects the + terminal from one battery to the - of the next? This and the BMS wires, how would I keep the BMS in one box and have it connect to the cells in the other enclosed box?
 

Zwy

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Hey there,

If I am creating two separate 24V 230ah batteries using EVE cells and have two 8S JBD BMS, one in each battery, is it okay to combine the two 24V batteries to produce 48V? Is this problematic?
You are limited to the max amperage rating of one BMS when in series like that, 100A if I remember right. There also is the question of keeping cells balanced, might not be that easy when in series.

You would do better with one 16S pack.
 

RCinFLA

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The risk of using two 24v BMS's in a 48v 16S battery array is the 24v BMS may be built with 40v breakdown MOSFET switches.

When one of the BMS's do shut down for low/high cell voltage, overcurrent, or over temp, that one open BMS MOSFET switch can be exposed to 55v or greater voltage which exceeds their MOSFET breakdown voltage rating.

Some 24v BMS are built with 80v to 100v MOSFET's but unless you are sure best not to risk it. You won't know you have a problem until one of the two BMS's actually shuts down. It can cause BMS to short out then damage battery cells.
 

MKalman

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I see, very interesting! Okay, I'll stick to one battery I guess. Now I just need to figure out how to do it in two separate packages.
The risk of using two 24v BMS's in a 48v 16S battery array is the 24v BMS may be built with 40v breakdown MOSFET switches.

When one of the BMS's do shut down for low/high cell voltage, overcurrent, or over temp, that one open BMS MOSFET switch can be exposed to 55v or greater voltage which exceeds their MOSFET breakdown voltage rating.

Some 24v BMS are built with 80v to 100v MOSFET's but unless you are sure best not to risk it. You won't know you have a problem until one of the two BMS's actually shuts down. It can cause BMS to short out then damage battery cells
 

TorC

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I would design them with a 16S BMS. They make connectors suitable for the set of BMS wires so you can disconnect them to separate the cases for travel. You just have to make sure the order is right and, if you make a 16S2P total configuration it would be prudent to avoid mixing up the 16S halves.
 

400bird

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If you need 48v and portability, I agree with all the recommendations for a 16s BMS with the cells in multiple boxes.

If your lucky, your BMS is like this one with two sense lead connectors. Connect one to the negative half along with the main B - connection on the BMS. Run the second connector to a different box with the positive half of the cells and the B+ connection.

That way you just have one large cable and the BMS connector running between the two boxes.

Or wheels, wheels on one large box is the easiest.
 

MKalman

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I would design them with a 16S BMS. They make connectors suitable for the set of BMS wires so you can disconnect them to separate the cases for travel. You just have to make sure the order is right and, if you make a 16S2P total configuration it would be prudent to avoid mixing up the 16S halves.
Yes, I will use a labeller to ensure that the batteries are labelled (Pack #1 Part #1, Pack #1 Part #2) etc.

Can you link me this connector that you speak of for managing the BMS wires between two separate cases?
 
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