okashira wrote:Let's say you have 10 flatpacks in series programmed for 50v 30A
So, the whole string will go to 500V 30A.
If you limit one to 5A. Once that ONE flat pack sees 50V it will try to taper current. Only one, though.
Let's say battery is at 300V. All flat packs will provide 50V and 30A max except the one programmed for 5A. Just 6 flat packs can provide 300V and 30A.
Each supply will push 30A until it's individual 30A limit is reached, until 300V.
Once 300V and 30A is reached, that one "5A" flat pack will reach its current limit. But there are 9 other flatpacks setup for 30A and 50V. The other nine, ignoring the current limited one, can do 450V and 30A. So even if the limited power supply went to 0v, the other nine would force current through it.
So 6 PSU can do 300V 30A. The other 4 must have 30A going thru them per Kirchoff's current law. So... This supplys may end up with a negative voltage!
Thus, current limit must be applied to all PSU in series or bad things will happen!
Voltage limit can be applied so only some PSU to achieve desired limit ( ie 50+50+50+30 = 180)
Conclusion, current must be applied to all psu in series. On the other hand, voltage limit can be defined by all psu in series added.
B careful with this though. All it takes is for one PSU out of 10 to malfunction and produce higher then 50v and damage your battery without a BMS.
Total summary, use a well designed BMS! Lol.
As i said many times, the thumb rule for serie psu for charging is :
The only thing you need is to have ONE of the psu in serie that have enough voltage range to take all the delta V of the 0-100% battery SOC and that is also the psu with the lowest current limit ( in fact it only need to have let say 1 less amp than others)
For a typical 400v system of 96s. this means you may need 4 "50V" flatpack's configured.
Min voltage of 96s: 240V
Max voltage: 403.2V
So one flatpack is not enough. For a higher voltage system, one might need a way to ensure the flatpacks are all communicating in some way so they properly share voltage.
flatpacks might work great up to systems of 3s ~150V or so, but higher you risk funny things happening, unless you want to babysit them all.
At this level it's looking more like just using an 400V salvage EV charger setup (with a custom micro or even a CANBUS hack) makes more sense.