21s bestech bms on a 14s pack

[Invisionelite]

I have a 21s bestech bms the d131b and a 14s pack with no bms, would it be possible to use these together and just not hook up the last 7 sense wires?

 

[Ykick]

Try and see. I ran 16S using 20S D167 as you describe. Don't connect upper channels.

 

[jonescg]

You might need to tie all of the other channels together... I know the EV-Power and ZEVA BMS modules were like that.

 

[Invisionelite]

I hooked it up last night and left the upper channels disconnected, and the board just started heating up. All the connections were correct I double checked them the only thing different from their diagram was the upper channels not connected.

 

[Ykick]

What part of the board heated up, balance bleed resistors?

 

[Invisionelite]

Yea it was the lower level of resistors near channel one of the sense wires. Before I built the pack I charged every cell individual on a foxnovo intelligent charger so I started with all cells at equal voltage. Could it be trying to balance the cells that are connected with the upper channels that are disconnected, balancing the 14s cells at 4.2v with the upper 7s at 0v due to the disconnected wires? Or is it just bleeding the minor variation In voltage of cells that are connected and I should just let it do it's thing

 

[Invisionelite]

It wasn't just warm, it was hot, like a 1 sec touch then damn somethings not right

 

[Ykick]

Or is it just bleeding the minor variation In voltage of cells that are connected and I should just let it do it's thing

This IMO. They can get very HOT. I guess I would let it finish any balancing that's taking place? But, I really don't like this sort of "balancing" - bleeding off higher voltage cells is so damn inefficient and as you can see, creates a LOT of heat. Not good if BMS packed away inside an enclosure.

And I've had several of the bleed balance transistors fail short circuit and drain cells. If/when that starts happening I simply lift the bleed resistors from circuit and effectively disable balance/equilibrium function.

LV/HV protection is still there, I just balance manually if ever needed. Ironically, that's only been after a balance transistor shorted and knocked the pack way out of balance. RC Lipo here, btw.

Keep an eye on cell voltages and bleed resistor heat. You might already have shorted bleed transistor(s)?

 

[Invisionelite]

That's kinda what I was thinking, I have never run a higher cell count bms on a lower cell pack, so if it's just balancing that's fine, I just want to see if I was doing the right thing with the extra sense wires by leaving them open, or like the previous poster said they may need to be connected together

 

[Ykick]

That's kinda what I was thinking, I have never run a higher cell count bms on a lower cell pack, so if it's just balancing that's fine, I just want to see if I was doing the right thing with the extra sense wires by leaving them open, or like the previous poster said they may need to be connected together

Less chance of fouling things up leaving them open IMO. Previous poster is very knowledgable but not sure if he's ever used BestTech Power BMS? I have although not the same model you're using.

Keep an eye on that bleed shit and if the cell channel keeps dropping voltage and resistor heating, it's likely a shorted transistor on the channel. Remove the resistor to stop the carnage.

 

[Invisionelite]

Sounds like a plan, what I was thinking was running a couple miles on the pack to drain it a little then reconnect the bms and charge that way it's at a happy voltage with all the components. If it still heats I'll burn everything ro the ground and start over lol... or just check and replace transistors

 

[Invisionelite]

In the meantime I will proceed with the upper 7 open, unless someone can say definitely something else should be done. I'll post my findings tonite