inside the BMS of a Lithium battery pack

[chantelauze]

after opening a BMS , these are the main elements in photo that i can peer ... is that questionning you ?
this BMS is attached to a Lithium manganese battery pack made of 7 cells of 8Ah capacity each ( 24v 8Ah battery pack )

 

[flecc]

You can see similar on my Torq site chantelauze, the photographs accompanied by a description of the content of the eZee Li-ion battery:

li_ion
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[chantelauze]

Tks so much Flecc...it's most informative
so i suppose here again ( like with the eZee torq ) the BMS is separated into 2 control boards , 1 for the the first bank alignment of 5 cells , and the 2nd one for the last 2 cells ...as in underneath photos

 

[Footie]

Excuse me chantelauze for jumping onto your thread but I have some questions about BMS'.

I don't know if it's me but this whole BMS thing is beyond me.

Even flecc's page says:
"And here's a close up of those printed circuits, five in all, giving individual monitoring of each cell. Again these are populated with miniature surface mount components, meaning repair isn't practical, but these components are very reliable anyway."

So, are these BMS circuit boards available to buy?
Where would one look for them and wiring information, etc?
Or is this a big mystery that amateurs should leave well alone
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[flecc]

Tks so much Flecc...it's most informative
so i suppose here again ( like with the eZee torq ) the BMS is separated into 2 control boards , 1 for the the first bank alignment of 5 cells , and the 2nd one for the last 2 cells ...as in underneath photos

Yes chantelauze, that's necessary for a 26 volt battery of course, 37 volts allowing symmetry with ten cells used.

It seems battery manufacturers like to keep the monitoring circuits very close to the cells with short connections.
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[flecc]

Excuse me chantelauze for jumping onto your thread but I have some questions about BMS'.

I don't know if it's me but this whole BMS thing is beyond me.

Even flecc's page says:
"And here's a close up of those printed circuits, five in all, giving individual monitoring of each cell. Again these are populated with miniature surface mount components, meaning repair isn't practical, but these components are very reliable anyway."

So, are these BMS circuit boards available to buy?
Where would one look for them and wiring information, etc?
Or is this a big mystery that amateurs should leave well alone
.

In general terms best left alone for manufacturers only, due to the risk of fire and even explosion. Lithium cells can be very nasty things if misused.

Some suppliers like Batteryspace.com sell lithium cells with the associated BMS circuitry, so obviously when matched in that way it's ok to buy, but even they have very strict warnings on site about the usage.

You'll have noticed also the threads on the other Li-ion type, Li-FePO4, where the pack supplier also sells the BMS to go with it.

The purpose of the management is just to protect the cells and guard against accidents, it carefully controlling the charge and delivery rates according to how the cell is responding, rather than with normal chargers which operate with reference only to all the cells together as a batch, which can damage a cell which is responding differently.

Ideally all chargers should have individual monitoring of cells, but with lead-acid, NiCads and NiMh, we could get away with a cheaper approach, though it wasn't ideal.
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[Footie]

You were right flecc, found BMS' at batteryspace - still don't understand it all though

Li-Ion Cells / PCM - PCB for Li-Ion Battery Packs DIY
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[Ian]

...still don't understand it all though
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You don't need to Footie, any more than you need to understand the chemistry of the battery or the physics of the motor.

Just ride and enjoy.

 

[Footie]

Just ride and enjoy.

That’s what I am hoping to do when my new battery arrives
The arrival of my new LiFePO4 (from Ping - being tested now) should herald great adventures and travels – my present lead acid limiting me to 7.5 miles.
Unfortunately, I always want to know how things work.
So, I’m trying to find out as much as I can, in advance.
Although, I think the BMS has stumped me, for the moment
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[flecc]

Although, I think the BMS has stumped me, for the moment
.

Basically it's just that Li-ion cells can't be trusted to run themselves. All cells contain chemical reactions, and with older battery types the reaction is stable and fairly constant.

Li-ion by contrast have chemistry that can go unstable, the penalty of a better potential, so the chemistry needs to be strictly controlled. Since the chemistry running away produces suddenly higher output, a monitoring circuit can detect that rise and cut the charge to that cell that's stimulating the reaction. In that way it damps down the reaction to return it to normal. A rise in output isn't the only way BMS can detect abnormal reaction, but I give that as an example.

The cell reaction can equally be monitored while it's supplying current to drive the bike, again to prevent it's chemical reaction running away.
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