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BMS always necessary?

heirloom hamlet

heirloom hamlet

life my way
Joined
Feb 3, 2020
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Location
Savannah, GA
Okay, stay with me here.
If I have brand spanking new cells, perfectly balanced... connect a harness to each 24v battery set to routinely check and balance with a balancer as necessary...use an A-I-O MPPT set so the battery cutoff at the top and bottom is where you want it for battery health...never connect a very heavy load... in an environment that never gets below freezing...
Is it really absolutely necessary to get a BMS and set it up for each battery?
 
Well no. "IF" nothing goes wrong. Have you ever had things not go the way you expected? Considering the battery's expense seems like the cheap part of a build would be a BMS.
 
What chemistry?
For the lithium ion batteries of the exploding variety I would say yes.
For lifepo4 not so much.
 
BMS's have 2 main functional areas.
Cell balancing and protection circuits.
You are handling the cell balancing manually.
You are also handling low voltage and high voltage via alternate means.
I assume you will have discrete over current protection as well which is the only thing that is absolutely necessary.
 
I forgot about low temp charge cutoff.
Which is superfluous if you live in a place that is warm year round.
 
smoothJoey said:
BMS's have 2 main functional areas.
Cell balancing and protection circuits.
You are handling the cell balancing manually.
You are also handling low voltage and high voltage via alternate means.
I assume you will have discrete over current protection as well which is the only thing that is absolutely necessary.
Click to expand...
I'll have to check and see if the LV2424 has an over-current protection setting. Thank you.
 
ecualibrium said:
I'll have to check and see if the LV2424 has an over-current protection setting. Thank you.
Click to expand...

Even if it does you want a fuse/breaker at the battery end of the cable.
I like marine rated battery fuses because they attach directly to the battery terminal.
 
smoothJoey said:
Your response to this and other questions makes me think you should go with a BMS.
Click to expand...
That's extreme. I was aware of that possibility, just couldn't find a simple clear example. I'm new that's true. But I've learned a lot lately and have a ton to learn... which I do daily. But I'm thorough and hard working. I wanted to be sure it was not a major no-no. As we all know there are a thousand scenarios, directions and options one could go in this field. No single lid, for any one pot. I really appreciate your help continuing my training.
 
ecualibrium said:
Is it really absolutely necessary to get a BMS and set it up for each battery?
Click to expand...

Well..... Lets say one of your batteries in " series " goes bad and shorts internally.

The math..

Say you have a 4s pack. 3.65v * 4 = 14.6v ( this is probably around your charging voltage )

One cell goes out... now all 14.6v is being put into 3 cells. 14.6 / 3 = 4.86v per cell..

I'm sure I don't have to tell you what happens when you charge at 4.86v per cell.
 
pinbrew said:
Well..... Lets say one of your batteries in " series " goes bad and shorts internally.

The math..

Say you have a 4s pack. 3.65v * 4 = 14.6v ( this is probably around your charging voltage )

One cell goes out... now all 14.6v is being put into 3 cells. 14.6 / 3 = 4.86v per cell..

I'm sure I don't have to tell you what happens when you charge at 4.86v per cell.
Click to expand...
This?
 

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smoothJoey said:
Click to expand...

This video scared me. Yes, a 300A fuse will protect a thin wire against a very short 1000+A spike before it has a chance to melt. However it will not protect the wire against a 250A sustained current (for some reason) that will melt the thin wire. Fuses should be rated lower than the wire, unless you only want to protect against a short catastrophic event.
 
carlos1w said:
This video scared me. Yes, a 300A fuse will protect a thin wire against a very short 1000+A spike before it has a chance to melt. However it will not protect the wire against a 250A sustained current (for some reason) that will melt the thin wire. Fuses should be rated lower than the wire, unless you only want to protect against a short catastrophic event.
Click to expand...

It may have only worked because he used a long, 8 gauge wire, protecting the fuse.
Did you catch his comment that the battery's 20,000 A output capability exceeded interrupting capability of the fuse?
15' of 8 AWG Total Resistance: 0.0094 Ω
Was that 12V? 1277A. Twice or 4x that for 24V or 48V, but no where near 20 kA.
If he'd used a short 4/0 cable, outcome could have been different.
As he said, "Don't try this at home"

Here's what you want. Class T, 350A, 125VDC, 20,000A interrupting:

Class T Fuse - 350 Amp - Blue Sea Systems

Use with Class T Fuse Blocks for circuit protection of devices including inverters. high interrupt capacity for large battery banks including Lithium-ion and TPPL batteries.
www.bluesea.com www.bluesea.com

That's what I've got, fuses right at the battery not supposed to ever blow (except in a bonded fault), plus breakers in the inverters which will trip first in an overload.
 
Cell level protection is required here for ensuring charging safety and, should any cell drift towards an unacceptable voltage, charging must stop. This can involve automatically disabling the charger(s) and, if this doesn’t appear effective, the battery must be automatically disconnected from all charging sources: this typically means disconnecting the charge bus, an event referred to as a HVC disconnect. This should always be treated as a serious issue: a HVC event indicates that something is not in order in the installation and the charge bus should not reconnect unless the battery is significantly depleted.

Since cell-level voltage measurement is mandatory for charging safely, it can just as well be used for over-discharge protection too. Cell-level protection is the minimum standard for operating a lithium bank.

copied from here; http://nordkyndesign.com/protection-and-management-of-marine-lithium-battery-banks/
 
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