Why BMS ?

[Razumpel]

Hello together,

i understand, that some Cells need a BMS, because they aren't
fine selected - so they become a drift in single voltages.
I think in the cheap-cell-packs from alibaba for example there
is a BMS needed...

But if you would use Sony Konion Cells - there is theoretical no BMS needed.
Several Vehicles i know drive with these Cells without looking at any single-cell.
Only the Main-Voltage is under observation. Since Years !

Why uses no one these Cells in an EUC ?
OK, little bit more expensive... but they have nice technical Data :
Generation 5 is about 2.600mAh - with 30A current...
Generation 6 is about 3.000mAh - with 35A (so far i know)

If there is no BMS - it could not shutdown the EUC while errors
occour in it measurement...
Not getting hurt - cause BMS failes - sounds good for me.

Just my 2 cent

 

[sbouju]

BMS is not only for equilibration between cells during charge, but (I think that it...) prevents against over overcharging (with charger, or going downhill already fully charged) and dangerous undercharging while riding till the end of the battery, no ?

[Chriull]

41 minutes ago, Razumpel said:

Hello together,

i understand, that some Cells need a BMS, because they aren't
fine selected - so they become a drift in single voltages.
I think in the cheap-cell-packs from alibaba for example there
is a BMS needed...

But if you would use Sony Konion Cells - there is theoretical no BMS needed.
Several Vehicles i know drive with these Cells without looking at any single-cell.
Only the Main-Voltage is under observation. Since Years !

Why uses no one these Cells in an EUC ?
OK, little bit more expensive... but they have nice technical Data :
Generation 5 is about 2.600mAh - with 30A current...
Generation 6 is about 3.000mAh - with 35A (so far i know)

If there is no BMS - it could not shutdown the EUC while errors
occour in it measurement...
Not getting hurt - cause BMS failes - sounds good for me.

Just my 2 cent

 

Reasons for a BMS that come into my mind:

- also nicely selected cells can drift apart after time

- Without a BMS there is no cell balancing happening to prevent any potential drift (as for RC modells cells without BMS (no "internal balancing") are commonly used, there the charger does the balancing (needs 'number of cells in serie + 1' extra cables + 2 charge wires), which can lead to major degradation of the batterie pack. If i remember correctly especially in high load situations in cold weather (imho somewhere described in http://batteryuniversity.com/learn/)

- The BMS shuts down in case of a short-circuit (bridge shoot through, wheel/motor stuck, current limiting at low speed does not work correctly, motor cable insulation melted -> short circuit). Without a BMS vast amounts of energy could be set free in such a short-circuit case.

On the other side in http://forum.electricunicycle.org/topic/459-electric-unicycles-bms-problem-and-solution/ for example many users have the opinion, that any over current shutdown by the BMS is bad and should be disabled - better the rare case of overheat/fire from shorted batteries than an accident caused by shut-down.

Another shut-off condition of most/all BMS used for EUC is over-voltage while regenerative breaking.

Batteries:

Nominal Capacity and max nominal/peak current allowed is only one part of the picture that shows the characteristics of an cell. For low capacity packs (2p for example) such "high performance" cells could be of advantage since they are able to deliver needed peak power. For higher capacity packs (more cells in parallel - not as much peak current per cell needed) lower nominal capacity cells with lower max nominal/peak performance could be advantegous. "High performance" cell tend to deliver much less than the nominal capacity at same currents than some "low perfomance" cells. So for a 6p battery pack a 10A 2900mAh pack could deliver a much higher "real world" performance than a 30A 3.500 mAh pack (fully fictional example without any investigation).