can 2 lipo packs and 2 bms ever get along?

[Bill Nye the science guy]

Good to see all my old ebike friends are still up to their old tricks.

I am building a new battery pack for my bigger frame and moving all the parts to the new frame. I was going to use lithium ion 15 ah pouch cells from bmsbattery.

I couldn't find the answer to this problem, lets say 2 60volt 15ah packs with 2 separate bms's feeding into one controller.

I remember hearing that this will make a kaboom because any unbalance will cause one battery to charge the other when load is low maybe over heating the bms?? could it be done with two diodes, one from each bms before they T ? would lose any regen that way.

or is there already a solution? would be 2p 16s Li-NiCoMn

Thanks

 

[teslanv]

Don't do it.

Yes, connecting two different voltage packs in parallel will make lots of heat and screw up both batteries and possible explode.

Connecting them in Series would not hurth them, unless you started discharging them. If they are not the exact same amp-hour capacity, the smaller one would drop in voltage faster, and eventaully reach LVC. Not the end of the world, but not good either. - This would of course increase the voltage significantly, and probably destroy your controller or motor.

Bottom line is...

Don't do it.

 

[dnmun]

if you build it as 2P then you only have one BMS. but you can connect a second BMS in parallel if it has the sense wires connected to the pack too.

you do not use diodes when the paralleled packs are identical in series count and chemistry.

you can also connect two separate 16S packs together in parallel at their P- spot and B- of each BMS goes to a separate pack that is not connected in parallel at the cell level.

to charge this type of pack paralleled at the P- spot means you have to pay attention to how the individual BMS charges. if they have a separate charging mosfet and circuit from the output mosfet then they have to be separated at the P- spot while charging.

if there is not a separate charging mosfet then they do not have to be isolated at P- while charging.

if you plug a fully charged battery into a discharged battery then the BMS on the fully charged battery will turn off the output mosfets for overcurrent when the batteries are shorted together. if you bypass the BMS at this time when they are connected the the wire will overheat and maybe melt. do not ask me how i know. only that all the insulation on a 10AWG wire melted off and the copper got so hot it turned black with oxidation before i could cut it.

 

[teslanv]

Sorry about the previous post. I misunderstood. Thought you wanted to parallel two packs of different voltages.

 

[dnmun]

Sorry about the previous post. I misunderstood. Thought you wanted to parallel two packs of different voltages.

you were right. you cannot connect two different voltages together without having the huge current surge. but the BMS will shut down the discharge immediately so it won't damage them, but they will not be connected either since the BMSs are now isolated by the output mosfets that turned off for overcurrent.

at this point you would charge the lower voltage or more discharged pack up to match the voltage of the pack that had turned off. when the voltage is equal then the output mosfets will turn back on again and the packs will work in tandem.

the reason to isolate the packs at the P- or drain leg of the mosfets is so that the battery which shuts off charging for HVC on one channel will continue charging through the BMS of the other pack, when they are connected at P-. the charging current can bypass the charging mosfets that are turned off and continue charging and overcharge the pack trying to shut off the charge for HVC.

if the BMS does not have a separate mosfet for charging then the BMS shuts off all of the mosfets for HVC so there is no risk of the charging current bypassing the BMS of the pack at HVC.

 

[Bill Nye the science guy]

Sorry for the delay, Thanks for the replies.

Question 1.

Is this cell from bmsbattery good or should I look at a different source? or a different setup altogether?



I am not afraid of anything complicated just want a good battery that will have good performance and last at least 500 charges.



What should I look for on the circuit board for a separate charging mosfet, should it specify in details or does anyone have a good bms source?

 

[dnmun]

how will you make the connections if you buy separate pouches?
if the seller offers to sell you a 60V BMS and claims it is 16S but shows a 12S BMS then you should doubt their ability to determine right from wrong.

 

[Bill Nye the science guy]

Ya the picture is more for reference, I am sure they are just too lazy to take another picture.

I think I will stick with a single pack and up the voltage as high as I can go. switch to a 72 volt controller.

***If I were to join 2 I would connect the 2nd battery to

battery positive to battery positive.

p- to p-

c- to c- during charge only, using the charger connector to make the connection ( only thinking there might be some wacky charging or balancing between the pack under no load)




I'm not really seeing any real complaints about bmsbattery so i guess they are ok, normal Chinese tech support charm.

anyway wanted to write this up cause I could not find much discussion about 2 bms's.

 

[amberwolf]

I'm not really seeing any real complaints about bmsbattery so i guess they are ok, normal Chinese tech support charm.

I'm guessing you haven't done a search on ES or Google/etc to find all of the many problems, then. :wink:

 

[Bill Nye the science guy]

Thanks Amberwolf, I didnt see many bad threads on here but on youtube they are getting torn a new one.

I am at a bit of a loss as to what to do for a battery. Im back to hobby king again.

 

[amberwolf]

They're no better.

If you want good, with warranty you know will be honored, wihtout exorbitant shipping costs taht could double the cost (especially depending on customs), you should probably check out some of the well-known and respected vendors, like Grin, Ebike-kit.com, Allcell, EM3EV (shipping still killer on that one though), etc. There's some threads about good and bad vendors.

 

[Bill Nye the science guy]

Thanks for the info Amberwolf, I appreciate people looking out for others on this forum. so ended up buying 16 of SPC 8200 mAh 2s. Going to run 2 16s packs in parallel with diodes to separate the output to the controller. A charging plug that will connect and power the two bms's for charging.



I just tested them and they held 60.7 volts @ 47.5 amps with 16 cells in series, already way better than my Nickle metal hydride setup.


and two of these BMS's from Bestechpower



Wanted to post my solution anyway for anybody searching in the future.