Connecting two 12V batteries -BMS question

Awsmits

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I am building two separate 12V batteries from eight 280Ahr prismatic cells, but I plan to connect them in series to create one 24V battery. I’m not connecting all eight cells in series directly because I would like to keep the weight down, taking them in and out of my boat As 12V batteries.
Question: do I use a single 8S BMS, or two 4S BMS units?
 

FilterGuy

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I am building two separate 12V batteries from eight 280Ahr prismatic cells, but I plan to connect them in series to create one 24V battery. I’m not connecting all eight cells in series directly because I would like to keep the weight down, taking them in and out of my boat As 12V batteries.
Question: do I use a single 8S BMS, or two 4S BMS units?
Given your objective of making them movable, you need two 4S BMSs. However, make sure the BMSs are rated for wiring two in series.

When a BMS turns off the current, the total series voltage appears across the internal FETs of the BMS. If the voltage rating of the FETs is lower than the total series voltage, they can burn out. (This is why you see pre-built batteries place limits on the number that can be put in series)
 

Awsmits

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How does one determine the voltage rating of the BMS FET? Is it always in the specs of the BMS?
 

FilterGuy

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How does one determine the voltage rating of the BMS FET? Is it always in the specs of the BMS?
Sadly.... many (most?) BMSs don't document what the serial limitation is. If it does not say in the specification, you have to contact them to find out......and since you are usually dealing with china resellers it is sometimes hard to understand the answer and always hard to trust it.
 

Awsmits

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Right, hope so. They only respond to emails, and I have my query submitted.

However, just to be clear, isn't it "electrically" OK to use a single 24V 8S BMS with these two serially-connected 12V batteries? It seems that to move them as separate 12V batteries, I just need to disconnect the power cable that joins + and - of each battery, and remove the 4 BMS sensor wires from one of the batteries. Yes?
 

MisterSandals

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However, just to be clear, isn't it "electrically" OK to use a single 24V 8S BMS with these two serially-connected 12V batteries? It seems that to move them as separate 12V batteries, I just need to disconnect the power cable that joins + and - of each battery, and remove the 4 BMS sensor wires from one of the batteries. Yes?
You are describing building a regular 8S battery and using a 8S BMS. And you are describing disassembling it into 2 parts in order to move it. You could just as well disassemble it into 4 parts or 8 parts to move it and its the same concept.
 

FilterGuy

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Right, hope so. They only respond to emails, and I have my query submitted.

However, just to be clear, isn't it "electrically" OK to use a single 24V 8S BMS with these two serially-connected 12V batteries? It seems that to move them as separate 12V batteries, I just need to disconnect the power cable that joins + and - of each battery, and remove the 4 BMS sensor wires from one of the batteries. Yes?
Yes, you could do it that way....

How often are you going to be moving the batteries? If it is very often I would think doing it this way would be a big PITA.

Also, do you have welded studs on the cells? If you have the threaded holes in the aluminum pads, you really should avoid playing with them any more than absolutely necessary.
 

Awsmits

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Take them out in the fall, put them back in the spring (although the cells would probably be fine on the boat during the cold CT winter, as long as I don’t charge them below freezing). I prefer them to sit in my 60 degree F basement over the winter where I can monitor them.

Yes, welded studs on the cells!
 

Just John

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I plan to use a BMS from Overkill.com. I’ll call them up to make sure. Thanks!
JBD (the manufacturer of Overkill) told me they are designed for up to 80v. I would verify with Overkill, but likely you will have no problem with two in series. Daly on the other hand does not rate their BMS for use in series.
 

Just John

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Wow! That would work for a 48V system! The more I learn about the JBD, the more I like it.
I just noticed yesterday that they apparently came out with a new higher rated (amps) model in August. I ordered one to check it out last night. I really like the power connectors.


Sorta on topic, it also has an interface for a battery heater. JBD apparently started out as a manufacturer of BMS for medical equipment, and it shows in the quality.
 
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FilterGuy

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I noticed that board too. I hope it is as good as the older models. One thing intriquing is that they have controls for powering heat pads.
 

Just John

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The advertising for their 150 amp model specifically states it supports 4 in series and up to 80 volts.

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Awsmits

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So, just a follow up query from this newbie, if I have two 12V batteries, each with their own 4S BMS, and I connect them in series (for 24V), can I charge and discharge them at 24V? Will each BMS handle 24V? And if one BMS reaches the set charge and shuts off, won’t this stop charging of the other 12V battery?
 

Just John

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So, just a follow up query from this newbie, if I have two 12V batteries, each with their own 4S BMS, and I connect them in series (for 24V), can I charge and discharge them at 24V? Will each BMS handle 24V? And if one BMS reaches the set charge and shuts off, won’t this stop charging of the other 12V battery?
If the BMS are rated for it, yes you can use two 12v batteries as a 24v battery. With LiFePO4, you really only know when they are full or empty, so get them both full individually, put them in series, and use them and charge them in series. That way they are synchronized, and yes, if one BMS disconnects, it means the same as if you had a single 24v BMS, one cell is full or empty.
 

Awsmits

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Thanks, John. So would you advise separating the two 12V batteries occasionally to check the balance of the 4 cells in each battery, or is that over-diligence?
 

Just John

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Thanks, John. So would you advise separating the two 12V batteries occasionally to check the balance of the 4 cells in each battery, or is that over-diligence?
I would use the Bluetooth app and monitor each battery. If they get out of balance, then you can separate them and "top balance" the string when/if necessary. Depending on the cell quality, that might be never. I don't know yet, I haven't experimented yet. I will have more data in about 6 months. I wouldn't worry about cells in a 12v pack getting out of balance any more than I would from a 24v or 48v pack. Just make sure connections are good.
 
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