[chbot] BMS - How they work?

[Helmut Walle]

Well, if the charger clamps the voltage at 4.2 V per cell the current will decrease to zero as 
the cell is approaching that voltage. And if the charger's maximum current is less than the 
cell's maximum charging current there won't be a need for any extra current limiting at the 
beginning of the charging procedure. So the simplest (and cheapest...) charger could simply be a 
constant voltage source with limited current. In practice this may lead to problems, however, as 
the cell voltage will drop somewhat once the charge current stops, and then current would flow 
again if the charger were simply 4.2 V constant, which could eventually damage the cells. So 
some shutdown would be required. In practice this could be achieved by shutting down based on a 
combination of voltage and current (or maybe even current alone) - if the charging current drops 
below a certain level the charger should simply shut down. Cell balancing in series-connected 
batteries would still have to be considered, though.

Also, most of these rechargeable Li-ion technologies do not particularly like being fully 
charged anyway - so in principle it might be preferable to only ever (or mostly anyway) charge 
them to an end voltage slightly below 4.2 V instead. However, this would not work if the cells 
are always connected in series, because in that case full charging may be required to ensure 
balanced charging between cells...

And what is the issue with the "vehicle or bike"? Is this something existing with a nominally 12 
V charging system for lead-acid batteries? Or some other context?

Kind regards,

Helmut.


On 07/12/2022 20:58, Marshland Engineering wrote:
> The problem I have (I think) is that you have some sort of regulated charging
> circuit. From what I have read, once a cell is at 4.2 v a shunt is activated
> to stop further charging. So what happens in the case of a vehicle or bike as
> the voltage and current for charging is often much greater than the shunt and
> the voltage pack can handle.  How does it stop 'charging'
> 
> If the BMS is set to 4.2 volts, the fully charged batter of 4 cells is 16.8.
> Too high for some aftermarket ignitions.
> 
> 
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