Batteries in series (how would the BMS do) - Page 1
Forum section: electronics / projects

author: fredjh
author_rank: Newbie
timestamp: January 04, 2023, 02:47:05 pm
content: Hi,How would an Einhell battery's BMS do when it is put in series like shown in attached for a total output of 80 volt?It is 4 batteries in series. Each battery has aprotectionpower disconnect Mosfet (rated Vds max : 60V) but the pack has 80V. Will each battery be exposed to higher than 60 volts in this setup? Does it work as is you think or not?I can't analyse the circuit enough with my knowledge. Any help is greatly appreciated.Thanks,Some history,-In a previous post Dave helped me by suggesting an ideal OR to skip the LED. Awesome!-I am making a battery pack for my ebike project by using modular tool batteries. Einhell batteries to be precise. These are used by manufacturer in two in series with no problems. However I would like to put 4 in series. Idea is to be able to move around my battery pack into things and be able to add remove capacity between things. As crazy as it sounds I'd like to try it

author: coromonadalix
author_rank: Super Contributor
timestamp: January 04, 2023, 03:41:04 pm
content: The bms can or could work in serial and parallel stages it depend how the manufacturer designed itI would be careful to use portable tools packs, some charger system need to communicate with the bms to do their job, some of them sniff out the cell temperatures too etc ...You have to find systems with independent BMS controllers for each pack, aka not relying on the charger. That way you can use servers psu'sMy friend built an Ebike with an 300v pack, just need to have the right bms modules put into serial and parallel ... lots of research before ending with the right stuffYou can find lots of tutorials on the web ...

author: fredjh
author_rank: Newbie
timestamp: January 05, 2023, 01:36:26 pm
content: Thanks Coromonadalix,I can work out the rest. Frist step now is in fact this easy question. I made a simplified schematics attached. Is the Vds max of a MOSFET in such a circuit is a limitation or not is what I cannot know. Can the attached work?I wonder what if one moset is off and others on what then though?

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2023, 02:50:46 pm
content: Just add parallel diodes so the BMS will never see a large reverse voltage. However, power tool batteries are far more expensive per Wh than EV modules, so you'll probably want to reconsider.

author: fredjh
author_rank: Newbie
timestamp: January 07, 2023, 09:50:15 pm
content: Looking at my own schematics I see now that if one MOSFET turns off the same one will see the whole 80V across of it once the load is connected. That means a point of failure. So it won't work I guess

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 08, 2023, 12:52:48 am
content: Quote from: fredjh on January 07, 2023, 09:50:15 pmHence the parallel bypass diodes.

author: fredjh
author_rank: Newbie
timestamp: January 10, 2023, 10:52:17 pm
content: I suppose you mean a parallel diode across the MOSFET? Then I have to change the BMS circuit and solder parallel diodes on them. The problem is each battery can pass out up to 20A which makes it to perhaps let the magic smoke out quickly!Alternatively I read more about how a BMS works, here is a link to how the BMS is more actually like:https://www.powersystemsdesign.com/articles/application-of-power-mosfet-in-battery-management-charge-discharge-system/35/19032Since I do not see one of these funny back to back mosfets on my board by not detailed glance, I suppose I might have some other way to control the charge side. I know these batteries are charged at 3 amps by default. So the charge route can't be a simple diode or else it will dissipate like +1.5 watts.Not getting anywhere but to just skip 80volts and stick up to a max of 60volts.