Multi-BMS Setup! Will it work?

SakisMx

1 mW
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Jun 27, 2020
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17
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Volos
Hi!
I found genuine 10s4p packs of Samsung ICR18650-26F of only 90€ each(and 2.25€ per cell).I also had one same pack from a previous ebike build so I ordered 3 of these packs and I now have a complete battery pack for my e-moped.I have wired 2 of these in parallel and another 2 of these in parallel and connected the groups together in series to give me a 20s8p pack! I charge the parallel groups separately with 2 10s chargers while they are not connected in series so I think there are no issues with charging!

Each pack however has its own bms so I have 4 different bms's in my complete pack. This doesn't cause problems with the series packs,however I think there is a huge issue with the parallel packs.When I twist the throttle there is a voltage sag of around 5 volts which is pretty big for Lithium-Ion so I think a bms has cut off in one of the parallel packs thus only one of the 2 packs in the parallel group gives power.Also first time I ran this setup there was very little voltage sag(around 1 volt).

So my question is:Can somehow this setup work by just bypassing the bms's(and only using them for balancing) so they will not cutt off and cause problems or I need to just have 1 bms per parallel pack(also bypassed as I am certain it will not be able to handle the amps)

I think the best way is to have only 1 bms per parallel pack but this requires removing all shrink wrap and insulation,and connecting each parallel group of the 2 10s4p packs I had started with,together,so I will get a 10s8p pack at the end with 1 bms.Then I have to insulate the pack again.This is going to take a lot of time and also requires me soldering the parallel groups of each 10s4p pack together as I don't have a spot welder, which is pretty dangerous in my opinion

So what do you guys think? Can I just get away by bypassing the bms's of the parallel packs or I must keep just one bms per parallel pack?

Thank you for time!
 
if you connected the individual groups of each pack together there is no difference what each bms sees.

ps: the voltage MUST drop otherwise no current can flow. and nobody knows if 5V is a lot of drop because it depends on the current being drawn.
 
flippy said:
if you connected the individual groups of each pack together there is no difference what each bms sees.

ps: the voltage MUST drop otherwise no current can flow. and nobody knows if 5V is a lot of drop because it depends on the current being drawn.

HI! Thanks for your reply!
To avoid connecting each individual group of each pack together, Will there be an issue if just bypass all the bms'es and just connect the packs as previously? (2 10s4p packs and another 2 in parallel and those groups in series).Each pack will have its own bms and they will not be able to cut-off while riding!This will make a 20s8p pack but the individual 4p and the other 4p groups will not be connected with each other.Just the whole 10s4p packs positive and negative will be connected together!
 
yes, you can do that. but generally not recommended. its better to just slap them together and make them all into 1 happy pack.
 
A pack-voltage LVC can work but should be adjustable

both based on the C-rate (higher amps means lower voltage)

and on how well-matched or relatively healthy the cells are.

Best used with bottom balancing, and intimate familiarity so intuition kicks in.

Otherwise really best for the LVC to be based directly off the weakest 1S group, first cell level to hit the low point
 
HI! Thanks for your reply!
To avoid connecting each individual group of each pack together, Will there be an issue if just bypass all the bms'es and just connect the packs as previously? (2 10s4p packs and another 2 in parallel and those groups in series).Each pack will have its own bms and they will not be able to cut-off while riding!This will make a 20s8p pack but the individual 4p and the other 4p groups will not be connected with each other.Just the whole 10s4p packs positive and negative will be connected together!

Never connect packs in series. The bms MOSFETs that switch them off in event of over current, over discharge etc are rated to break the voltage of 1 pack. 2 packs requires twice the voltage rating FETs, there's a 90% chance they will instantly blow as soon as one of the BMSs in series try to switch off.

You can always just rip the BMSs off and put a new BMS on.

Combining in parallel is normally ok.
 
virturally all mosfets used in cheap chinese bms can handle 100V.
 
mxlemming said:
HI! Thanks for your reply!
To avoid connecting each individual group of each pack together, Will there be an issue if just bypass all the bms'es and just connect the packs as previously? (2 10s4p packs and another 2 in parallel and those groups in series).Each pack will have its own bms and they will not be able to cut-off while riding!This will make a 20s8p pack but the individual 4p and the other 4p groups will not be connected with each other.Just the whole 10s4p packs positive and negative will be connected together!

Never connect packs in series. The bms MOSFETs that switch them off in event of over current, over discharge etc are rated to break the voltage of 1 pack. 2 packs requires twice the voltage rating FETs, there's a 90% chance they will instantly blow as soon as one of the BMSs in series try to switch off.

You can always just rip the BMSs off and put a new BMS on.

Combining in parallel is normally ok.

Thanks for your reply!
I forgot to mention that each 10s4p pack will have its own bms! I made a schematic so it is a lot easier to see everything Screenshot_20210418-124544_Electric Circuit Studio.jpg

I finished the wiring yesterday and tested around the block.It seems fine to me and it even gained a little more top speed(because of the bypassed bms'es)!
I will do a long test today and report the results here!

In summer after my exams are finished,I will disassemble everything and add more cells in parallel with a decent 20s BMS so it will be correctly made!For now I just want this to work without overheating or other issues! I think that it doesn't need any alterations to work without issues(except that in case something happens the pack will not cut-off so I will be watching for that).
 
if your controller has a LVC set to ~60V you only need a bms during charging. so you can bypass the bms during discharge and only connect the bms to charge it to protect and balance the packs.
 
flippy said:
if your controller has a LVC set to ~60V you only need a bms during charging. so you can bypass the bms during discharge and only connect the bms to charge it to protect and balance the packs.
My controller is just a cheap Chinese 48v-72v so I suppose the LVC is around 39v(3 volts per cell for a 13s(48v) battery).
With my current setup the BMS'es are only for charging and balancing.
I don't need the lvc, as my speedometer also shows battery voltage so I will not let it drop below 60v!
Having taken all these into consideration, I think that it is going to work okay for now!
If you have any concerns about running this setup, I would like to know!
 
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