Help with Understanding BMS amps

WOLF 1

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Need some assistance please. I am building 2 800-amp 12 volt batteries then series them for 24 volts. They are 3.2 volt cells lithium iron phosphate. The question is I have 800 amps but will only ever use 150 amps at a given time, so could I get away with a 300 amp BMS? Thanks in advance
 

Dzl

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Need some assistance please. I am building 2 800-amp 12 volt batteries then series them for 24 volts. They are 3.2 volt cells lithium iron phosphate. The question is I have 800 amps but will only ever use 150 amps at a given time, so could I get away with a 300 amp BMS? Thanks in advance

So it looks like you are confusing amps with amp hours. The BMS rating is refers to amps (a measure of current/flow) the batteries capacity is measured in amp hours (a measure of capacity/stored energy). The BMS could care less about amp hours, it is only concerned with the maximum amps that will be flowing through it.

But to answer your question, yes, you would be fine using a BMS rated for 300A Continuous Discharge.

I'm curious to learn more about your project, 800AH @ 24V is a pretty hefty battery bank, what are you powering and why the decision to go with 24v instead of 48v for a system that size?
 

ghostwriter66

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I'm curious to learn more about your project, 800AH @ 24V is a pretty hefty battery bank, what are you powering and why the decision to go with 24v instead of 48v for a system that size?

I'm too am curious with 24 vs 48 .. normally anything over 200aH folks start doubling up on voltage and not aH... The cable gauge/size alone would start scaring me
 

Steve_S

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I'm too am curious with 24 vs 48 .. normally anything over 200aH folks start doubling up on voltage and not aH... The cable gauge/size alone would start scaring me
Again Amp Hours is not Amps being pushed from the battery packs. 1000 Amp Hours @ 24V is still 24V and still requires cable rated to the maximum amperage you will be calling for. 4000W inverter can demand up to 166Amps @ 24V to delivery the 4000W (plus a little for it's own overhead which is minimal [my samlex uses 18W]) Now charging a 1000Ahr battery pack will require serious oomph, the other side of dealing with big capacity banks. It doesn't matter if you use a bunch of 200Ahr cells to make a 1000Ahr pack (gosh that would be big) or four Winston 1000Ah for 12v/1000Ahr (they are monster sized as well & rare plus $$$$)
Never seen one that big ?
Look Here: https://www.ev-power.eu/Winston-40A...FeYPO4-3-2V-1000Ah-Special-product.html?cur=1

4/0 Cable can handle a LOT of amperage but once you start pulling more than 200A / 250A with surges, better to step up to 48V which would drop that to 100A/125A. Full on house installation are 48V or better at 72V simply to handle the loads and keep the copper reasonable.
 

smoothJoey

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4/0 Cable can handle a LOT of amperage but once you start pulling more than 200A / 250A with surges, better to step up to 48V which would drop that to 100A/125A. Full on house installation are 48V or better at 72V simply to handle the loads and keep the copper reasonable.

+1
 

Supervstech

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Need some assistance please. I am building 2 800-amp 12 volt batteries then series them for 24 volts. They are 3.2 volt cells lithium iron phosphate. The question is I have 800 amps but will only ever use 150 amps at a given time, so could I get away with a 300 amp BMS? Thanks in advance
So, a 3600 watt load?

what duration?

3600W pulled for 8 hours is 28,800Wh, or at 24v is 1200 Ah
 

Steve_S

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BTW a "ponderance" just to consider on amps. I'm 24V, I use a coffee maker (not a warmer type, it has a thermos carafe) but for the 6 minutes it takes to brew a pot it's using 60A off the battery pack. It does not take much to push the Amp demand up but one also needs to consider the duration of that demand level too, a thinner wire can handle it but it will warm up and if too much, it will heat & possibly fail if overloaded.
 

nosys70

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you better would use several MPPT device, each one with its own battery.
if your load are smalls but many, you would split the circuit (by floor or type or phase ), like one for the lights, one for the fridge...
if you load is huge and unique (a big device like AC) , you can still mount some MPPT in parallel (special model needed).
usually the salve ones triggers only if the load exceed the master one.
And if you need a 24V battery, assemble a 8S battery with a 8S BMS instead building 2x 12V battery.
 

WOLF 1

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So it looks like you are confusing amps with amp hours. The BMS rating is refers to amps (a measure of current/flow) the batteries capacity is measured in amp hours (a measure of capacity/stored energy). The BMS could care less about amp hours, it is only concerned with the maximum amps that will be flowing through it.

But to answer your question, yes, you would be fine using a BMS rated for 300A Continuous Discharge.

I'm curious to learn more about your project, 800AH @ 24V is a pretty hefty battery bank, what are you powering and why the decision to go with 24v instead of 48v for a system that size?
So it looks like you are confusing amps with amp hours. The BMS rating is refers to amps (a measure of current/flow) the batteries capacity is measured in amp hours (a measure of capacity/stored energy). The BMS could care less about amp hours, it is only concerned with the maximum amps that will be flowing through it.

But to answer your question, yes, you would be fine using a BMS rated for 300A Continuous Discharge.

I'm curious to learn more about your project, 800AH @ 24V is a pretty hefty battery bank, what are you powering and why the decision to go with 24v instead of 48v for a system that size?
Thanks for the reply, I've been living totally off grid for 16 years. 10 years ago we build a new house and put in a nice Outback 3600 flexware with 1 80 amp and 1 60 amp mppt Outback charge controllers. I have 16 Trojan l16 that are finally given up not bad for 10 years.
Don't want to go back to let acid so I'm building my pack with lithium iron phosphate and have no idea how BMS work but I've done a lot of research and I am purchasing a chargery 16 . Battery packs are large because of the BMS being 16 wires.. this forum has been great along with a bunch of others. I just hope I'm doing it okay, lead acid was a lot easier.
I thank all you guys for your input
 

Steve_S

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Thanks for the reply, I've been living totally off grid for 16 years. 10 years ago we build a new house and put in a nice Outback 3600 flexware with 1 80 amp and 1 60 amp mppt Outback charge controllers. I have 16 Trojan l16 that are finally given up not bad for 10 years.
Don't want to go back to let acid so I'm building my pack with lithium iron phosphate and have no idea how BMS work but I've done a lot of research and I am purchasing a chargery 16 . Battery packs are large because of the BMS being 16 wires.. this forum has been great along with a bunch of others. I just hope I'm doing it okay, lead acid was a lot easier.
I thank all you guys for your input
The math is the painful part.
24V "Series packs" so 8x200AH cells for a 24V/200AH pack with 1 8S-BMS .
24V Series / Parallel pack" (8s2p) 16x200AH = 24V/400AH with 1 8S-BMS (cells are paired) (Avg Weight 150 Lbs / 72Kg +/-) * can a 16S be used ?
* Yes but cells being paired negates individual cell management as such.
"Strictly Series Packs" are simpler to manage, lighter to manipulate and apply KISS rules which is a plus if something goes wrong.

Remember that with your Trojan's only 50% is useable but with LFP 80% for a safe & happy long life, plus faster charging as well.

Look here for Big Bang for Buck ! S&H to USA similar
Deligreen-quote.jpg


COMPARE with 280AH (small vendor ? ) $324 difference for an extra 160Ahr ** bigger & heavier
2x 24v/280Ahr Series packs, paralleled = 560Ahrs (or one 8s2p) Pack @ 560Ahrs)

Capture.JPG
 
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Bleedingblue

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Thanks for the reply, I've been living totally off grid for 16 years. 10 years ago we build a new house and put in a nice Outback 3600 flexware with 1 80 amp and 1 60 amp mppt Outback charge controllers. I have 16 Trojan l16 that are finally given up not bad for 10 years.
Don't want to go back to let acid so I'm building my pack with lithium iron phosphate and have no idea how BMS work but I've done a lot of research and I am purchasing a chargery 16 . Battery packs are large because of the BMS being 16 wires.. this forum has been great along with a bunch of others. I just hope I'm doing it okay, lead acid was a lot easier.
I thank all you guys for your input


Im in the same boat and its kinda confusing trying to learn about the BMS? Do you hook up the 4/0 cable to the bms itself? Or does the bms just balance the cells? What size BMS? What options do I need? Etc

Im redoing my solar and also use the trojan l16 batteries. I have 24 of em in 3 strings of 8 for a 48 volt system. Aims 10k converter.

Will be going bigger with 2x the solar panels and using a solark 12k
 

smoothJoey

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Im in the same boat and its kinda confusing trying to learn about the BMS? Do you hook up the 4/0 cable to the bms itself? Or does the bms just balance the cells? What size BMS? What options do I need? Etc

Im redoing my solar and also use the trojan l16 batteries. I have 24 of em in 3 strings of 8 for a 48 volt system. Aims 10k converter.

Will be going bigger with 2x the solar panels and using a solark 12k
It depends on the bms but in the common case the B- lead goes to the battery negative and your 4/0 cable goes to the p- lead.
Yer bog standard BMS does cell balancing and has protection circuits for over current, over voltage, under voltage etc.
For a 10k inverter I assume you are using 48 system volts.
10k watts / .85 efficency factor / 48 system volts = 245.098039216 dc amps
245.098039216 * 1.25 = 306.37254902 fuse amps.
Depending on the round trip circuit lengh I can see 4/0 wire being appropriate.
You won't find a commodity grade BMS that can handle that kind of amperage though.
You are well and truly outside the pareto envelope.
There are options to explore though.
 
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Dzl

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Im in the same boat and its kinda confusing trying to learn about the BMS? Do you hook up the 4/0 cable to the bms itself? Or does the bms just balance the cells? What size BMS? What options do I need? Etc

Im redoing my solar and also use the trojan l16 batteries. I have 24 of em in 3 strings of 8 for a 48 volt system.

BMS is a lithium thing, no need to worry about it if your using lead acid batteries. ✌
 

Bleedingblue

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Trojan batteries are lead acid, confirm?
Lead acid batteries don't require a BMS.


Yes they are but I'm converting over to 280AH lithium. Going to order 48 of em to have 3 strings on a 48 volt system. I'm just trying to piece things together before I buy a BMS that I can't use.
I have 12-335 watt panels now and going to put up 24 more for a total of 36 panels
 

Bleedingblue

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BMS is a lithium thing, no need to worry about it if your using lead acid batteries. ✌


Yes I know i that. but I'm in the process of buying lithium just like the guy I was quoting saying I'm in the same boat as him.
 

Dzl

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Yes I know i that. but I'm in the process of buying lithium just like the guy I was quoting saying I'm in the same boat as him.

Gotcha, that wasn't clear to me from your earlier post, but now I see what you were saying. With a project as large (and expensive as yours) its a lot easier to justify the cost of a top quality BMS as it wouldn't add much to the overall cost of your system. That's a segment of the market that doesn't get a ton of discussion or attention here.

It might be useful to think of a BMS as a system or design concept instead of a device, it can be a device, and as used/discussed on this forum, it usually is a device, but it can also be a decentralized system.

As to your questions:
Im in the same boat and its kinda confusing trying to learn about the BMS? Do you hook up the 4/0 cable to the bms itself? Or does the bms just balance the cells? What size BMS? What options do I need? Etc
1. How you hook the cable up depends on the BMS, Some BMS' are 'inline' (i.e. in the main current path), others (like chargery or SBMS0) are not and control dis/charging indirectly using relays or remote control of devices. I would assume with a system powerful enough to need 4/0 cable at 48v, you would not be using an inline FET based BMS.
2. The BMS does much more than just balance the cells, but that is one function
3. If by size you mean current rating, this is determined by the maximum current that will flow through the BMS at any one time. If the BMS is not in the current path, current rating isn't relevant (apart from rating of external components like relays/shunts/etc)
4. I know this is an unsatisfying answer, but the options you need will depend on your situation, needs, and goals. I highly reccomend you read the two bottom links in my signature, (nordkyn design and marinehowto) there is a lot of good info in those articles about how to protect and take care of lifepo4 batteries, and how to design a system around them, with some good discussion on what a BMS is and should do.
 

Bleedingblue

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Gotcha, that wasn't clear to me from your earlier post, but now I see what you were saying. With a project as large (and expensive as yours) its a lot easier to justify the cost of a top quality BMS as it wouldn't add much to the overall cost of your system. That's a segment of the market that doesn't get a ton of discussion or attention here.

It might be useful to think of a BMS as a system or design concept instead of a device, it can be a device, and as used/discussed on this forum, it usually is a device, but it can also be a decentralized system.

As to your questions:

1. How you hook the cable up depends on the BMS, Some BMS' are 'inline' (i.e. in the main current path), others (like chargery or SBMS0) are not and control dis/charging indirectly using relays or remote control of devices. I would assume with a system powerful enough to need 4/0 cable at 48v, you would not be using an inline FET based BMS.
2. The BMS does much more than just balance the cells, but that is one function
3. If by size you mean current rating, this is determined by the maximum current that will flow through the BMS at any one time. If the BMS is not in the current path, current rating isn't relevant (apart from rating of external components like relays/shunts/etc)
4. I know this is an unsatisfying answer, but the options you need will depend on your situation, needs, and goals. I highly reccomend you read the two bottom links in my signature, (nordkyn design and marinehowto) there is a lot of good info in those articles about how to protect and take care of lifepo4 batteries, and how to design a system around them, with some good discussion on what a BMS is and should do.


Thanks for the input. I was highly questioning how i would even hook up a bms to 4/0 cable. Didnt understand why someone would want to run the current through a bms. Seems like a good way to fry it. Thought the charge controller/ inverter would be managing that aspect.

My setup would be 840 amps/ 40,320 wh of battery power.

Where my batteries will be will have no internet. I dont have internet but i do on my phone and the batteries inside the shop you have no signal of any kind. I barely have cell signal here. Have to get in my bedroom to even use my phone. No signal everywhere else.
The shop acts like a faraday cage.
 

Ampster

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I was highly questioning how i would even hook up a bms to 4/0 cable. Didnt understand why someone would want to run the current through a bms. Seems like a good way to fry it. Thought the charge controller/ inverter would be managing that aspect.
I agree with @Dzl with that size pack you may want to use large contactor controlled by a BMS. The charge controller/inverter can manage volts and Amps but if something goes wrong you want a backstop to protect your pack. I set my BMS to open the contactor at volts just beyond my HVD on my charger setting and just below the LVD on my inverter. Also if one of my cell groups in my pack goes south and the pack voltage is within specs I know that the contactor will shut down and I can fix the situation. I turn off the balancing but use the reporting function to tell me about the health of my pack.
 
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