Am I missing something? BMS's

[onmebike]

I've just ordered some some 3000mah Li-ion cells to build a 12ah pack[40] and I'm wondering just how important having a bms is?
Because the pack will be constructed of 10 x 4 parallel wired cells, in series, its impossible for the bms to monitor each individual cell.
If one cell was to go down, the bms couldn't tell, having 3 other cells in parallel, the voltage would remain the same.
Niether would the bms know the state of charge in each of the 4 cells in parallel, one may be 3,7v and the others 3.2v, it would only register the one with the higher voltage.
I can see it will protect against over-voltage but the charger should also do that to a point and it will cut off at low voltage which the controller also does.
As far as I can see this also applies to Lifepo4 and Li-poly packs where parallel wired cells are used.
So, other than for packs constructed of single cells in series, what use is the bms?

 

[NRG]

yes, if you have sealed packs made up of paralleled cells then you can't break out each cell to monitor voltage. I don't make my packs that way for this very reason.

 

[flecc]

Yes, this isn't a very suitable way to make up packs for precisely that reason.

It usually works for NiMh if the cells are carefully pre-matched since they never use BMS anyway, but I'm not keen on this with lithium types at their present stage of development. Over time lithium cells they may become as repeatable and reliable as other types and one day may not need a BMS, but I'm not sure we are there yet.
.

 

[onmebike]

This appears to be the way many Lithium based packs are constructed, even the newer lifepo4 cells seem to use parallel cells to increase Ah.
The only way I can think of doing it is to isolate each of the parallel wired cells with diodes, then monitor each cell before the connection to its diode, which in my particular case would then require a 40 cell bms. What sort of diode would handle the power though?

 

[flecc]

It's true that they are increasingly doing this, but those I've seen are the highest grade ones like the 10 Ah Panasonic unit one using 2 paralleled banks of 7 cells each, and the 14 Ah one from Advance that Wisper and eZee use, having 2 paralleled banks of 10 cells. Maybe it will prove safe for all grades of cells, but I'm still inclined to be a bit cautious.

If doing this though, I wouldn't use diodes due to the losses involved.
.

 

[onmebike]

The only other way would be to make 4 separate 10cell packs at 3000mah, include balance connections and use an r/c type balance charger which I already have.
Then I could simply link them before use.

 

[NRG]

That's what I do with my Rc Lipo packs, I have 12 series cells in a pack and parallel them with either another 12 series or even a third for longer distance.

Each is 5Ah nominal, so I can have 5, 10 or 15Ah pack configuration. IE 12s1p, 12s2p or 12s3p.... I bulk charge with a CC/CV Meanwell PSU but carefully monitor individual cell voltages....you can soon spot a weak cell and eliminate it from the pack or make provisions for it during charging. I find the current RC Lipo cells to be be very close in capacity and performance between cells so only have to resort to using the balance charger perhaps once every 5 to 10 charges.

I also only charge to 4.1v and never go below 3.2v if possible (3v absolute minimum) this increases life greatly. Interestingly although the cells are rated 5Ah they are more like 6~7Ah so curtailing the votages top and bottom seems to have little effect on usefull capacity.

 

[xplo5iv]

Niether would the bms know the state of charge in each of the 4 cells in parallel, one may be 3,7v and the others 3.2v, it would only register the one with the higher voltage.

The parallel cells will all be at the same voltage, by definition. With practically zero resistance between the connections any voltage difference would lead to an almost infinite current until the cells were at the same voltage. Assuming the cells are always kept parallel they will always be at the same voltage, even if their capacity varies greatly.

The only danger of parallel charging wouuld be if the internal resistance of the cells vary out of proportion to their capacity and safe charging current, e.g. if a low capacity cell with a low safe charging current, but low internal resistance were connected in a parallel charge then it would get more of the total current than it should.
(I'm talking Lithium here - the charging algorythms are different with Nickel)

I'm guessing, but I think that's pretty unlikely, as low internal resistance is a good thing and likely correlates to high safe charging speeds and good capacity.

Am I missing something? (I could be...)

Either way, definitely use a BMS, or you will kill your cells

There is a distinct 'knee' in a lithium going flat, meaning the one with a slightly lower capacity can drop voltage off really quickly once below a certain point, while the others still stay up. As the cells get older they gradually tend to become less matched and then...gotcha

Guess how I know...

Good luck & enjoy the bike

Jonathan

 

[onmebike]

Guess how I know...

Good luck & enjoy the bike

Jonathan[/QUOTE]


Could it be? an xplo5ion

 

[xplo5iv]

Thankfully not with Lipo's, that comes from model rockets.

I've yet to blow up a Lipo, even when badly crashed in model aircraft and then the resulting mangled battery shot with an air rifle to see if it would blow...

 

[onmebike]

Thankfully not with Lipo's, that comes from model rockets.

I've yet to blow up a Lipo, even when badly crashed in model aircraft and then the resulting mangled battery shot with an air rifle to see if it would blow...

A friends garage burn't down with the loss of all his planes which included some jets. Another had a house fire and I've seen a lipo explode when being charged with the wrong charger.